Voltage Drop and other Math

[zac850]

Hi all,
I was wondering if I could get some help with a math problem that is facing me.

My school is having a massive outdoor concert, and since my job is dependent on electricity, I want to get some out there. I am getting a 60Amp breaker box, but I need more then 60 amps.

So I am going to be running power cables out from outlets of my two theater spaces that I am near. Distance from the outlets to the stage is about 250 feet. I just bought three 250 foot cables, 14-3 AWG and some edison plugs to wire on them. I know 12 gauge cable would be better, but the budget wasn't there, and I ran the idea by an industry professional who said it should be fine.

However, just for my mental rest, in practice, am I running into any problems with running 15 to 20 amps through this cable?

Also, what kind of voltage drop would I be getting? I'm in the US, so its around 120 volts at the outlet.

Thanks,
Zac

[kingfisher1]

Resistance is equally to p, the resistivity of the wire times teh quantity Length of the wire in square meters times the cross section area

[kingfisher1]

14 gauge wire has a resistance of about 2.5 omhs per 1000ft, or some wher about 1/4 of and ohm for a 250 foot run.

[kingfisher1]

i just realized this doesn't say much about votage drop now does it?
can we applie the lovely Ohm's law to these lovely ohms?
R=V/I

[Radman]

resistivity, that'll be on the vocab quiz friday... lol

[Chris15]

Using Ohm's Law, V=IR, so the voltage drop = 20*0.25 = 5 volts. Now we must consider that the electricity has to flow up the cable and back again, so in fact you have a voltage drop of 10 volts. You are looking at loosing 200 watts of power in each cable to heat caused by resistance.

I am concerned about the statement "I am getting a 60Amp breaker box, but I need more then 60 amps." If you are overdrawing your breakers, then you are likely to have one of two results, a] breaker tripping or b] fire. Fire being caused by overheating.

I don't know what the regulations are over in the US, but here in Australia, you cannot have an extension lead longer than 30 metres (100'), irrelevant of the cable gauge, at least with single phase, three phase has different regulations. So the 250' extension here would be illegal. To comply with legislation, you would have to put two joins in the cable, I would think that less safe. What I am saying is that your NEC may have something to say about maximum length of an extension cable.

[fosstech]

Over 60 amps on 3 14/3 cables? :shock:

And why do you need a breaker box if you're running just three separate circuits out of the main building? Those circuits are already breakered for probably 20A already.

[Chris15]

I was more concerned about the running more than 20 amps through a 20 amp connector and breaker. You would need to ensure that they are plugged into three different circuits and that on the breaker panel they are fed from, that the breakers are not next to each other. This being because a breaker can handle less overload when the one next to it is also being overloaded. Think heat buildup.

[Mayhem]

Relax guys – he said that in addition to the 60A breaker box, he intends on running 3 additional circuits from an adjacent building. Providing he monitors the draw on each lead he should have no problems. The one thing that needs to be considered in that as the voltage drops, the current draw will increase to compensate.

A 600W lamp on a 120V supply will draw 5A but the same lamp on a 100V supply will pull 6A.

A general rule of thumb is to load your circuits to 80% to account for fluctuations in the power supply.

Remember that 120V power supply will vary in actual voltage output. I know that our 240V supply is often up as high is 250V (have seen it exceed this level) and as low as 220V.

I would also stick a meter into each lead to visually see what you have at your disposal.

[Chris15]

I see that I misread the situation. I agree that a meter into the lead would be the best idea, but that would only work if you meter it whilst fully loaded. Given that the current will alter, so too will the voltage drop. The voltage available that you measure unloaded will likely change when you meter with the full load applied.

The voltage fluctuations are generally caused by changes in loading to the power grid. So at times when people are using a lot of power for cooking, heating or cooling. So unfortunately if you are having this in the early evening, then you will probably be getting a lower voltage from the outlet to start with.

[zac850]

Yes, thank you Mayhem. I am getting a 60 Amp box brought out and in addition need more power.

I have been playing with Lightwrite for a while, patching dimmers and making sure I keep individual runs below around 17 amps, just in case.

So its possible that I will have voltage drop of 20 volts.

Thanks guys,
Zac

[CURLS]

In addition to all this on a sidenote a really great program that can teach you a lot whether you are a sound guy or a lighting guy is LD Calculator Lite. It's relatively easy to find on the interet and a free download. Download it and I am sure you will fall in love with it!

[Chris15]

Yes, thank you Mayhem. I am getting a 60 Amp box brought out and in addition need more power.

I have been playing with Lightwrite for a while, patching dimmers and making sure I keep individual runs below around 17 amps, just in case.

So its possible that I will have voltage drop of 20 volts.

Thanks guys,
Zac

My calculations would indicate that it will be 5 volts each way for a total of 10 volts

[tenor_singer]

Is this a high school event?

Who is doing the wiring of the 60-amp breaker?

[zac850]

Oh, I run a mac and I forgot that I have LD Calculator Lite in Virtural PC. Forgot about that!

Wiring will be done by an electrician, a lighting designer/rigger/electrican who designs the lighting for clubs all around the world. Shortly, I trust his wiring.

[tenor_singer]

Oh, I run a mac and I forgot that I have LD Calculator Lite in Virtural PC. Forgot about that!

Wiring will be done by an electrician, a lighting designer/rigger/electrican who designs the lighting for clubs all around the world. Shortly, I trust his wiring.

I just wanted to be sure that your high school wasn't making you do the wiring.

As my drama students would say... I am fanatically obsessive about high school students' safety (considering that I was almost killed in high school working with electricity because the school wouldn't step in to fix things).

BTW... I visited your site the other day. Great work. I think you'll do an amazing job in college .

[kingfisher1]

just as an update were running and extra run for lights to stay on the safe side, and maybe even to more if we can scrowng up the cable. hopeflly we won't be stuck patch edison-pin converters tryihng to get some extra five feet

[AVGuyAndy]

You should be getting about 30v of voltage drop, which is unnaceptable.

20amps over 250 feet is alot to ask out of 14awg. I would go up to 10awg. With 10 you would only have a 12v drop.

[n1ist]

14ga cable is only rated for 15A, even if you ignore the voltage drop.
/mike

[kingfisher1]

we ended up spliting up the runns to reduce volt drop, so we ended up only pulling about 11 amps per run

[ship]

In addition to all this on a sidenote a really great program that can teach you a lot whether you are a sound guy or a lighting guy is LD Calculator Lite. It's relatively easy to find on the interet and a free download. Download it and I am sure you will fall in love with it!

Very useful, used it tonight as opposed to the tables I'm working on for work that I didn't complete yet by way of having trouble figuring out which formula to use. There is quite a few formulas to use dependant upon what type of wire you are using. But still for a rough guestiment, it gave me what I needed. Good stuff this download. Thanks for the info.

Still, it's not a perfect formula as it only does a "in general" voltage drop. THHN verses SC verses SJ cable or wire will all have different voltage drops, much less it's only a figure of outlet to fixture as it were. Total voltage drop as might become a factor in going back to the service panel drop, might get more complex than this.

Still a good website for the calculation. Given time, I'll have to post the why this came up in another section. Very curious in I'm going to kick someone's rear once I figure out if it was a bad GFCI by way of lag time now tripping or in fact voltage drop causing me to do a third day of waking up at 4AM to get to the install on time for a third day.... grumble grumble grumble. Was all done, all worked, than I get this call, the lights shut down and the on-site people keep re-setting the breaker only for it to trip again - this four to six hours after I got the system working... The heck... it was working, why did it start tripping now? So much for my ten day vacation starting tonight. Could be the 20A 120 to 24v transformer causing a lag time on a bad GFCI at this point or could be the 250' run of 12/3 cable feeding the thing. I didn't know how long this power cable was, I was doing the install not the power feeding it. Idiot... only an estimated 16.66 amps, gee add a say 2% voltage drop from the transformer and one might see a slight wee problem on a 20A breaker powering a GFCI outlet. This granted that "breaker tripping" does not for sure mean to me by way of description if it was the GFCI or actual breaker tripping at this point. Gee, Friday morning I get to re-run the power cord to #8 in size, much less re-wire some electrical stuff in not knowing yet how or what I'm doing but bringing lots of parts to do about anything. grumble grumble, that's why I don't do shows or installs....

[SHARYNF]

Obviously I am telling you Ship something that you already know, but breakers should not be used for power switches, and I have found that over time the breaker weakens and that they trip and lower and lower amp's. I have seem many situations where people simply think they are just switches in a design. First thing an electrician probably would do is replace the breaker, since the 20 amp verison are not only inexpensive but cheap.



OTHER thing to look for is that something has the ground and the neutral swapped. I have found this quite a number of times even some of the cheap testers don't pick this up, depending on how far you are from the ground point in your service feed.

Just a thought

Sharyn

[ship]

Thanks, yep, know about the circuit breakers as switches - the shop light banks are circuit breakers and already going snap and crackle. Neutral/hot in this case also should not matter, both feed into a transformer. This other than in the line in at the transformer being breakered that than would be breakering the neutral. In this case, it wasn't the transformer tripping, it was the house service panel or GFCI still unknown.

This install wasn't turning on or off, it was going to be always on. It's the archway Christmas lighting at a very high profile store down town. Just has to last a month.

Theory is one of two things. Either the voltage drop from the 250' run of 12/3 at 16.66 amps when added to a theorized up to 2% loss from the transformer caused the circuit breaker to heat up over time due to over amperaging, than late was still hot enough not to reset due to the over amperaging, or the GFCI was old and if the transformer was inducing a lag time between hot and neutral, it could have been what was tripping. Theory being that an old GFCI might not trip immediately. Second part is less likely but possible thus I'll bypass it. Lights are in a coved arch and the system up to the low voltage lighting is grounded so I'll punt.

Plan at this point is to replace the 12/3 Edison with 8/5 SO that has L21-30 plugs on it. (It's the only stuff I had available in the shop which was longer than 100' on last minute notice.) Install a Kellems grip on the cable and do a wire rope tie off to somewhere on the roof. Drop the cable down from the roof, cut the 12/3 feeding the transformer and install a L21-30 plug on it in using it single phase and adding a insert.

Found a Bell box outdoor cover at the shop tonight that's designed for a L21-30 outlet - or at least after I made the hole slightly larger. OK, so it was designed for a L5-20 and I made it fit the L21-30. Punch a hole in the exterior service panel feeding the GFCI, disconnect the GFCI outlet from the service panel, install my own Bell Box and wire in the three phase outlet as single phase. Plug the drop line in, add silicone, tape up plugs etc.

The other option I'm ready for is punching the panel and adding a set of tails to it for my 8/5 cable.

Both possibilities for what caused this are than solved. That's the theory, two hours work and I'm out of there. Granted it's a 30+ foot arch I'll have to be connecting into at the transformer which means a very tall geni lift trip to do the final connection.

[ship]

Still not working and I’m befuttled.

Original install (now that I have been up to the roof) had a Home Line exterior two space breaker panel fed by #8 wire and two phases measuring 123v and 213v between them. There was two dual 20A breakers in the two slots making four circuits in the panel. All is fine so far. Below the panel is two outdoor outlet (bell boxes) that have two each un-GFCI mounted duplex receptacles. The Edison receptacles are split wired for individual 20A circuits. All seems fine with the wiring so far.

Our original installer a few days before hand ran a 250' Edison cable of 12/3 from one of the roof top outlets above (none of the other outlets are in use) down from the roof and the side of the building to the cove I’m lighting. From there, I hooked the 12/3 directly hot directly into the line side of a 20A compact magnetic circuit breaker. Load side of the breaker along with neutral went into their terminals on a 500 KVA 120v to 24v transformer. This transformer was metered out in the shop and tested out in outputting 24v. Even had the other ME where I work verify that what I was thinking with the inadequate instructions for what to tap on the transformer and which was the hot/neutral was which, much less in general how I was doing it and got the thumbs up from him. My transformer was good and should work. Tested good all was good. From the output side of the transformer another 20A circuit breaker was installed as per the transformer’s instructions in me replacing a fuse with circuit breaker but otherwise following the plan for what to tap, which than went to a Marithon block and barrier strips for the 24v power.

Box was grounded and installed about 34 high in the peak of the concave arch. This as opposed to on the roof a few stories above where we will have had to run #1 wire size to do 20A at 24v given a 250' run - once we started to do the math. Urr, I started to do the math given it was someone else’s project I was only providing the boxes for and I was now stuck with it. Sure, it will have been nice to park the transformer on the roof - easy access to it’s circuit breakers... than the voltage drop comes in.

About 320 5w/24v lamps were divided fairly evenly into five circuits which ran all over the 30' archway in chains of belt lights that had various taps all over the place for branches of each circuit. Sunburst effect for the most part in following the windows. All taps were weather resistant and it has not rained in the past couple of days. Various taps and all belt light circuits were of 16/2 SPT-2 wire (though marked that way, in stripping it, it more seemed like #12 wire than #16 wire in use for the belt lights), of the taps some needed short extension cables added on to make the tap longer. 16/2 outdoor cable (similar to the SPT zip cord but made for outdoors) was used for the short no longer than 5' and normally 6" extensions where needed between say branches or from the end of a string to the transformer.


First attempt to power the thing up on Wednesday Morning was disastrous. Line side of the transformer breaker tripped. Ok fine, it’s and inductive load going thru a transformer, I try it again and the load side circuit breaker trips. Line side stays on - great... Re-set the load side of the transformer breaker and it trips again and again instantly. That’s bad. (By the way as I write this, I’m adding to my 4AM at the shop pull list for what to bring so it’s good in a way at least in thinking out the problem to describe so far what I have observed.)

So, we have the load side of the transformer compact breaker now tripping and unlike the line breaker, it won’t stay on after being re-set un-like the line side breaker. Believe that initially, when I powered up the transformer as per the instructions on some past transformers I wired, I had the load side breaker already on, but I could be mistaken on the order of which was already on first and or what some past instructions said to do in powering it up. Anyway, it seems to me that each time I was leaving the load side breaker on in re-setting and powering up the line side only for after the initial line breaker again tripping each time. After the innitial tripping of the line breaker, it stayed on and has always worked. At some point no doubt I will have had the line side on and re-set the load side also only for it to instantly trip.

By this time, it was fairly well understood that it was either something in the belt lights causing the load side breaker to trip, or a bad circuit breaker. Both breakers I am fairly well certain were both 20A, and connected in the proper line/load wiring but I saw some rubbed away red paint on the lever of the load side breaker meaning it was not a new one - it was a used circuit breaker. This circuit breaker had a some time in the past life as a phase Y to something so potentially in not being new, perhaps it was a bad circuit breaker.

I confirmed at least at the time this assumption that I had a bad circuit breaker on the load side by way of one by one of the five circuits - while 30+ feet in the air and kind of leaning back at a bad angle to get into the box, that it was the breaker that was at fault - I thought. Out of five circuits, I removed one by one from the barrier strip they were plugged into and each time under power from now four circuits, the load side breaker was still instantly tripping. So, given by process of elimination in out of five circuits, removing any one of them did not stop the tripping of the breaker, it had to be the circuit breaker. I bypassed the wiring to and from the used and potentially bad circuit breaker and powered up the lights now by way of only the line side circuit breaker.

All circuits worked now. Lots of not screwed in all the way lamps or lamp base taps that didn’t pierce a conductor to troubleshoot, but an hour later afterwards everything worked and all circuits were up and running with all lamps working now. Problem solved in near miss, bad circuit breaker on the load side. This I thought - project done time for my 10 day vacation.

About four hours later I got the call that the belt lights went out and in attempting to re-set the circuit breaker from the roof, it just kept re-tripping. Up to now, the circuit breaker sub-panel on the roof has not been a problem. That was the original post on this subject about voltage drop as the assumption of what now was going wrong. Since the still connected line side breaker was not tripping, and it was nearest to any potential fault, and it might take a while for a circuit breaker powering up a voltage drop problem to heat up enough to trip, this was very much assumed to be the problem. Simple answer or if in the roof they were using a GFCI and who ever kept trying to re-set the thing was calling it a breaker, that was also simple enough still under this explination of the problem or perhaps a lag time on a bad GFCI that took a while to trip. Certainly if the breaker nearest the transformer on it’s line side was not tripping, one of the two possibilities with the house power or the cable drop was the answer and easy to solve so I thought at 10:30 PM at night as I was leaving the shop after two and a half hours there spent packing my tool kit for the day and finding what outlets to use. Lucked out on the outdoor twistlock outdoor cover, it went right into the already installed Bell box - simple as anything to install a single phase three phase thirty amp plug into the already installed box. Pull their wiring and outlet, install mine.

Given this assumption of it being the wiring and not the transformer or the belt lights the solution was easy. Could have been out of there in two hours given my assistant for the day or someone after him didn’t grab one of our 100' pieces of 8/5 cable. This much less for the 100' run he did get on the truck, it was not one of ours - didn’t have any of our markings on it and seemed to be 10AWG instead of 8AWG. By chance he managed to find a cable that never should have been put into stock and was not the right wire gauge, than somehow between him being sure he pulled two 100' cables and a 50' one, lost the other 100' cable. No problem, it’s 6AM on a holiday from work morning - who can I call to Un-lock the shop, grab a cable and drive 60 miles in running out another cable for us... Ah’ a salary staff sucker with keys to the shop person like me born every minute, or at least one that woke up to my ringing their phone finally. Of like six or seven people with keys, I got lucky.

So under today’s assumption, doing 12ga wire for 250' even if at only 16.66 amps roughly, once you added in voltage drop or extra loading due to inefficiencies of the transformer, you now were over 20Amps assuming a 120v power supply that in theory could be less in making the amperage at the house breaker end go up some. Since it was the house breaker or GFCI receptacle at that point still unknown, and the remaining line side of the transformer circuit breaker, other than in initial power up was not blowing first as it should, it was assumed that this was a voltage drop issue. Such a breaker in having a few amps more than it was rated for would take a bit of time to trip given it won’t trip under a other than abnormal load before it reaches the 127% ratio for the circuit breaker. In other words, it sounded perfectly reasonable that in an inductive incandescant loading, that it potentially in out door freezing conditions could take like four hours for a circuit breaker to trip and once heated, no longer re-set once warm given say a 22 Amp loading on the circuit. There was no GFCI in use so once I got up to the roof power supply it was definitely not some bad GFCI having problems with the lag time generated between hot and neutral due to the transformer that powers up from both the hot and neutral. Simple and should have done the trip. #8 wire feeding a 16.66 amp load should have been more than sufficient.

Going back to what I crossed out in the voltage drop assumption, given it was a bad circuit breaker on the load side and bypassing it solved the problem in that the line side breaker was not tripping, I just wrote this off as a fluke occurrence and it still might be where you might have a bad circuit breaker initially and the voltage drop was a secondary problem. I at least A.S.S.U.M.E.D this. Little did I know that perhaps that load side breaker in tripping than being bypassed was working properly and the line side circuit breaker perhaps is the bad one now in having tripped I believe once initially and now becoming just a switch. These are expensive magnetic mini-breakers, one would assume that if they go bad, they do so in the tripped position and stay off but who knows. This morning’s disappointment of it still tripping the house power circuit breaker after I switched to a now combination of (by accident 100' or 75' unknown of) 10/5 and 8/5 cable unsettles me in that above problem of voltage drop having been solved as theory I was satisfied with and not related to why it’s still tripping now.

Seemingly it was nothing about voltage drop. To test this and given it was #8 wire feeding the dual circuit breaker, I woke the project manager for this install up again at 7AM. He needed to get his rear over to the job site and bring with him a 30A home line single pole breaker. Dutifully, he got there by 9:30 with the store opening at 10AM and no longer a possibility of setting up a large Geni lift in front of the main doors.

I installed the 30 Amp circuit breaker to test a theory anyway from the roof. Either we will be golden in my voltage drop calculations having been still off - even if way outside of theory, or at least it would power up the transformer and work or trip the transformer’s remaining line side 20A breaker. This or at least pop the prospectively 20A or unknown main service panel’s breaker. The wire feeding this sub-panel should be safe for 30A as with all the cable now feeding the transformer.

Popped the brand new 30A circuit breaker in that sub panel on a constant attempt basis. My world just sank. We have 123v, a good neutral, I detect no short between hot and ground and no short between neutral and ground between here and the lights. I do detect a continuity between hot and neutral between the sub-panel and the lights, but given it was still hooked up to the transformer and it’s load, I should. Once un-plugged, all jumpers on the roof - like 200' of them showed good lack of continuity between conductors. It of course was too late to set up the lift and un-plug the transformer and test the dropping down the side of the wall cable. Could be that given this in theory at least 100' cable that didn’t belong to us yet was pulled for the show, and is 10/5 and not 8/5 but the longest length I had to work with had a short on it’s female end. The male end once opened looked fine but the female end I did not inspect. One would also assume that should there be a short between hot and neutral on it, such conductors will have burned up given the 30A circuit breaker took a few moments to trip in giving me enough time by way of hearing the breaker thinking about it, say some stuff not public to repeat. Snap crackle and pop was not in my lingo at the moment. That brand new 30A breaker tripped, than upon re-setting instantly started tripping.

So, it was 9:45 AM of the biggest shopping day of the year and we in theory could set up a 40' Geni Personal Man Hoist on center in front of the main doors, which would tend to make them not work, to the very high profile store, or we could put our tails between our legs - project manager, truck driver/assistant who was no slouch by way of help (and at double overtime in pay) and I the guy (on vacation now) that had it all figured out and apologize that the store would now three days late not get their Christmas lighting effect again today. Possibly in the morning or by Tuesday six days late if I have to order a new transformer and mount it into a new box. Four window every morning between 6AM when security gets there and 10AM in the store opening, should have been long done by now - but these are new lights and there is some wee... trouble shooting to do in the system - like it’s blowing the main breaker.

I’m about ready to rip this whole thing down and lay it down on the shop floor so as to re-do it all and re-wire it under factory/shop conditions. Already an assistant at double time pay scale (me being salary and on vacation in losing money), a two day rental of a lift at $200.00 a day now being rented for a week given a two day expected install, gas, $30.00 per car parking fees per day etc. this project just lost money and is still not working. Luckily as long as he doesn’t have to do his re-assigned new show the original ME for this project in the morning again, he might be on-site to help me troubleshoot. Guy is salary also but I don’t feel bad for him in comparison.

We will at 6AM after I’m at the shop at 4AM, the project manager up on the roof in reading a book and waiting to flip on and off the circuit breaker, the #4 electrician at our shop and I troubleshooting the problems 30' up in the or around the 30' dia. arch. #3 has also been consulted and doesn’t know. We don’t want to bother #1 electrician as it would take hours and get nowhere, much less he would necessitate being on the time clock to offer advice for as long as we could bear a non-decision.

Chatted over the phone with the other electrician, we have our theories. His first question was did I follow his layout of the lighting plan to the letter? Yep, unfortnately the project manager left that plan locked up inside an electrical closet while the store was closed, but to the best of my knowledge, I did follow the plan in circuiting it. Still, it’s not very likely that one circuit has too much amperage, first these are 5w lamps in say 20 to 50' sections at 24v. Should be sufficient not to blow a 30 amp circuit breaker even given 16ga short jumpers at times used.

Almost cross out wrong circuiting amongst the five circuits of 320 lamps. Could be however in that like #34 electrician at the shop was wiring part of it, and I did the later splices, perhaps and very possible, that we have two separate circuits linked into a parallel situation by way of splicing. Would not be bad unless we now had a hot/neutral or as other ME at the shop corrected me on, Hot/return given it’s following the transformer, shorting situation. Don’t believe it’s this but such is possible and would trip a breaker.

One would think it would both trip the line side of the transformer breaker and do so of either it or the sub-panel breaker immediately, but given the size of the transformer, perhaps it did take a while to get overt enough to hours after it all worked, cause real problems.

Could be that some lamp base is shorting between conductors, could be that more than one lamp base on a number of circuits is doing so. This would as with the above if in combination with and or the cause explain why by way of individually testing each circuit, why the load side breaker kept tripping. By way of last minute fixes to individual lamps not working, I certainly saw enough lamp sockets that had their cable piercing sockets that were bent thus not conducting. One would think that such sockets that pierced but shorted might be dim at least or not work due to the short by way of lamp, but who knows. No lamps seemed dim overall but it was daylight and who knows what equates to a 30a but high resistance short.

Gonna be more than one problem if that’s the case - I hate it when there is more than one problem to troubleshoot at a time. One thing I did not do in troubleshooting that bad circuit breaker was to troubleshoot one circuit powered up at a time. I was powering up four out of five in tripping the breaker but did not attempt one at a time. This will have shown much thus keeps alive the above as theory and real pain in the rear to troubleshoot.

I was amazed enough that the splices all worked, now I get to look at 320 lamp bases for what is the short?

Another possibility is that the transformer while outputting 24v and not tripping my bench test circuit was bad. It otherwise went bad by way of the above short in circuits which could take four hours to go bad, or that the 320 5w/24v lamps loading on it were more than 500KVA of the transformer rating - my mistake, or that it’s really a lot more than 320 lamps and the transformer was overloaded than melted down internally. This transformer if melted down and shorting, while confusing that the line side breaker is still yet to trip under a 30A load, would also easily explain the 4 hours of it working and now tripping.

Bad transformer by way of some short still in the system we would also have to find - could be metal to cable or lamp base or bad circuiting, but this would be hard to figure out, much less still require fixing now both transformer - now shorted at the line end and at the load end trouble shooting.

Thanks again McMaster Carr. Just put on Will Call pickup for the morning after I if necessary leave the store again with tail between my legs a 30A transformer 750KVA *given I understand my math, and all parts required for a Sunday- going Sunday trouble shooting four hour period of troubleshooting. Once I again wire the thing, it should in replacement solve the problem of either over wattaging of the lamps or should we have shorted the transformer, it’s direct replacement once we figure out what shorted it.

Perhaps I’ll park the project manager outside the store initially first to count lamps so as to figure that out or cross out that first. Project manager is slave labor on this project at this point - you chose this system of low voltage lights, you are my ***** until they work and remain working.

That’s where I am at now. Still don’t work and I’m seriously fustrated by way of I know my connections are good in theory at least. If they are not, it’s going to be a huge problem to troubleshoot this 1' on center 320 5w/24v lamps, circuited into five circuits over a 30+' dia. One cannot even imagine the amount of things to check here.

[lightbyfire]

Just wondering KingFisher, but did you check the voltage under load? I would be interested to see what the actual drop was.

[hmspe]

I only skimmed through your post, but I'm seeing several issues. First, voltage drop starts essentially at the main switchboard for the building. If you're taking power at the roof receptacle you need to take the conductors from the store's panel to the roof receptacle into account. Second, is the transformer really 500KVA (5th from last paragraph)? Your load is 1600VA total. That's 67 amps at 24 volts. A 500KVA transformer would put out almost 21,000 amps at 24 volts. You need something more on the order of a 2KVA or 2.5KVA transformer if you use a single transformer. 500VA (not KVA) or 750VA (not KVA) isn't enough for this load. Third, [mostly from curiosity] what type and what gauge wire did you use for the lighting runs? Assuming an even division of 320 lamps on 5 circuits, the load per circuit would be 13.3A.

Are you trying to start all 320 lamps at once? If so, you have a pretty hard short circuit on the transformer secondary. Filament resistance is extremely low until the filaments heat up. I'd suggest trying to start only one string. If that works, you might be able to start the strings sequentially. Of course, that would take time delay relays or something similar.

If I was doing this I'd be looking for a different power supply point. 1600VA is only 13.3A at 120 volts, so just about any available receptacle in a commercial building should feed this just fine. Isn't there a landscaping receptacle closer?

Martin

[kingfisher1]

Just wondering KingFisher, but did you check the voltage under load? I would be interested to see what the actual drop was.

that show is done and fogoten lol. went off without a glitch, and even looked good

[zac850]

Otherwise known as no, we did not. Everything worked, and we barely had enough time to set up and start, no extra time to do anything extra.

[lightbyfire]

Gotcha,

glad it went well, i was just interested in the difference between theory and actual. I am sure we will run into something like that at some point so it would be good to know what to expect.

[ship]

I only skimmed through your post, but I'm seeing several issues. First, voltage drop starts essentially at the main switchboard for the building. If you're taking power at the roof receptacle you need to take the conductors from the store's panel to the roof receptacle into account. Second, is the transformer really 500KVA (5th from last paragraph)? Your load is 1600VA total. That's 67 amps at 24 volts. A 500KVA transformer would put out almost 21,000 amps at 24 volts. You need something more on the order of a 2KVA or 2.5KVA transformer if you use a single transformer. 500VA (not KVA) or 750VA (not KVA) isn't enough for this load. Third, [mostly from curiosity] what type and what gauge wire did you use for the lighting runs? Assuming an even division of 320 lamps on 5 circuits, the load per circuit would be 13.3A.
Are you trying to start all 320 lamps at once? If so, you have a pretty hard short circuit on the transformer secondary. Filament resistance is extremely low until the filaments heat up. I'd suggest trying to start only one string. If that works, you might be able to start the strings sequentially. Of course, that would take time delay relays or something similar.
If I was doing this I'd be looking for a different power supply point. 1600VA is only 13.3A at 120 volts, so just about any available receptacle in a commercial building should feed this just fine. Isn't there a landscaping receptacle closer?
Martin

Hmm, 2KVA transformer..., that's skipping the rest of the troubleshooting in me not checking other people's math.

Man, you are good, I put an amp clamp on the zip cord while it was lasting on the 30A breaker and got 60+ amps for as long as it lasted. Gee, had I only done the math instead of trusting what I was given for what to order the parts. Gee, yea, I suppose that even a 30A/750KVA transformer wouldn't work so well. Metered out house power to 123v, at the transformer, 25v. Amazing that the 500KVA transformer, once I removed the circuit protection from it, lasted four hours...

Did our math later after metering out and detecting a slight wee problem in 60 Amps load on the transformer in =- you told me to get a 16.66 Amp transformer to power this up... at no point did while this was still something I was buying parts to and not doing the install, did I hear any mention of 60 to 80 amp transformer needed... So much for trusting someone else's math - this given I was while ordering parts working on quite a few other projects and not engineering this system, just making the parts.

And than I got sucked in.

As of today the system works. 150' of #8 wire to the 2KVA transformer. #2 feeder cable dropped down the last like 75' of cable to the patch panel wiring up the 24v system. Good all around now on voltage drop, and finally sufficient in the properly sized transformer for the application.

I was noting as I was replacing the 16ga zip cord with #12 ga zip cord today, that some of the belt lights are #12 wire and some are #16 wire. No doubt when I ordered the #16 wire, I had seen a belt light with the #16 wire feeding it.

Yep lots of errors in this system. Works now even with a few past errors that are not so good.

Start to finish in it taking a week longer than it should have was all about engineering sepcification of the system and mistakes based upon mistakes. Any number of the mistakes will have caused a problem, them combined took a week to figure out.

Works now, this as opposed to what I recommended to using 6w 130v lamps in bypassing the transformer and not changing anything else. How long, don't know - it's a real cluster screw up of a system.

But in the end in me buying now two transformers with already knowing about the number of lamps, their wattage and voltage, yet not checking the math of what size transformer to buy... yep. egg on me. Silly me line verses load side - I just didn't want to think at the time in assuming the same or pairity. Big expensive mistake on my part in at least not taking some time to sit down and re-do the math.

Works now, done deal until such a thing comes back to the shop and I re-do lots about it. Now where were you like five days ago = this before I was troubleshooting other things such as a short in the system?

[hmspe]

Glad it's working. Electrical design is my day job.

Five days ago I was trying to survive having 4 grandkids under 9 in my house for several days. That, and trying to get an NSI IF501 to talk to a CD80.

Martin

[ship]

Brings up a very serious professional thing for me to work on. Granted I was very busy in lots of other projects and told those requesting the parts and components to use, I am the person they look to in getting not just the parts, but the right parts. I was provided innitially a request for a spool of 12/3 wire to power the thing up, how many 24v/5w lamps, and I should have taken the time to figure out what was needed for the project. That I thought myself too busy to do my job, much less later, also didn’t sit down and do it, was not me doing my job and instead deflecting that part of it onto others to do for me. My response given the info I was provided of “tell me what you want and I will get it - I’m a bit slammed at the moment” didn’t help the situation and was in part the cause of this cursed problem. My seeing a 16ga belt light, than ordering that same gauge for it’s jumpers and not checking the math on it also compounded this issue. Me not looking to overall amperage also didn’t make me suspicious of the use of a long length of 12/3 cable to feed it all.

One by one, once the project became mine I did figure out the problem, problem is that it became once mis-estimated a huge money loss project. Could have solved it days ago also, this had I only also sat down and re-calculated the math from start to finish on the entire project.

Turned out that all the splices were good, there was no short to frame of the building or mis-connected circuits that had shorting of hot and return. Feeding cable also was good as a supply cable. By Sunday when I was checking amps on the 750KVA transformer in the limited time between it clicking off and my clamp metering it, I got 8.3, 11.8 and 13.5 amps on the first three circuits by way of further confirming the over 60A of draw. At that point I knew I had a problem, and knew what the problem was.

Monday I ordered the new parts, made new boxes for them etc. Worked 23 hours straight on the project than between showing up and figuring out the new transformer, getting it same day, than installing it in it’s new service panel. Two new lamps that decided not to work and needed fixing but overall, it was simple as to cause - all about doing the math. No matter how busy I might have been, I should have done so and at that point will have had this project done days ago.

Learned some off this project, mostly that of doing my job in specifying a system and making time to do so as opposed to just tell me what you want and I'll get it thus it being your problem which became my problem. Seven days on this project including 2.1/2 hours at the shop during Thanksgiving day by way of prepping for the next day's 4AM wake up call. 100% of the problems will have been solved by laying out the system and doing all the math for it in all components.

[ship]

Glad it's working. Electrical design is my day job.
Five days ago I was trying to survive having 4 grandkids under 9 in my house for several days. That, and trying to get an NSI IF501 to talk to a CD80.
Martin

Electrical design by way of wiring also is my day job by way of solving probelms but not with low voltage normally. Still can't figure out the advantage of the 24v system, over making it 120 volt, but had the project been mine and had I time, I will have wrapped myself about it and figured it out I would hope. Developed a tunnel vision in this project by way of assuming what I knew more about, and accepting what when the math was telling me the wrong figures to be having calculated it the wrong way I expect. Just not the simple of this is the calculation you should see thus what I would buy components for. Will have been much more cost effective if a 130v system.

That last day of wiring was a 23 hour day for me. Told those who chose to buy the new transformers as opposed to buying the 130v lamps that it would take me 12 hours to make the components for the new transformers - beyond wiring them in. Guess that was the easy for them in that they didn't have to stay up all night in making the replacement parts and whole new system work. Took instead 14 hours as I ran into problems, but boy did it feel good to get the project finally done - even if a week later.

As for grand kids... while an uncle, luckily at this point work is very seperate from family. Done is good in the end.