Dimmer capacity

[SalvatoreDelorean]

I'm working in a college black box theater, and I have 24 dimmers to use to design an entire (reasonably large and complex) show. As you can imagine, it's pretty limiting and I'm hoping I can get creative with twofers. I have 2.4k Berkey Colortran dimmers and 1.8k circuits, and I'm working mainly with a bunch of Source Fours and 575w Fresnels. Is there any way to tell how many instruments I can twofer into a circuit before my dimmers burst into flames?
Thanks!

 

[chausman]

What are the S4s lamped at?


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[SalvatoreDelorean]

What are the S4s lamped at?


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575w/120v

 

[Musicman56]

It would appear that your circuit is going to be the limiter here. It can only hold 1800 watts. I've never personally seen a situation where a circuit can't hold as much as a dimmer, but it appears to be possible. Regardless, here's the math that you need to do to solve your problem. You have 1800 watts to work with and 575 watts per fixture (you can round to 600 for ease and as a threshold). 1800 divided by 600 is 3. So three 575 units is the max load you can put into your circuits, with 1725 watts being used. Now, I know a lot of electricians who use the 80% rule and only use 80% of their available wattage as a safety. Along that reasoning, you would use 2 units. The theory being that getting too close to the limit will trip the dimmers even though you don't use all of their potential. By that same reasoning, your 2.4k dimmers could run 4 lights technically and 3 by the 80% rule. But your circuits can't do 2.4k, so they are the limiting factor.

Hope that was helpful!

 

[ptero]

Can you patch two circuits to a single dimmer? Then you could go to 4 lamps on a dimmer, by way of using two circuits plus twofers. We are tight on dimmers (who isn't?) and routinely load our 2.4k dimmers w/4x575w. These are old Strand CD80 2.4k 12-paks and they have a pair of 'outs' for each dimmer. It pushes beyond the 80% concept but has worked for years. Breakers don't trip and if it is shortening dimmer life that's over a long timeline. These paks have been at it since 1984 iirc.

Good luck...

 

[Musicman56]

Ah, I was assuming a 1 to 1 hard patch, but if you can play with the hard patch then...

Can you patch two circuits to a single dimmer? Then you could go to 4 lamps on a dimmer, by way of using two circuits plus twofers.

 

[SalvatoreDelorean]

It would appear that your circuit is going to be the limiter here. It can only hold 1800 watts. I've never personally seen a situation where a circuit can't hold as much as a dimmer, but it appears to be possible. Regardless, here's the math that you need to do to solve your problem. You have 1800 watts to work with and 575 watts per fixture (you can round to 600 for ease and as a threshold). 1800 divided by 600 is 3. So three 575 units is the max load you can put into your circuits, with 1725 watts being used. Now, I know a lot of electricians who use the 80% rule and only use 80% of their available wattage as a safety. Along that reasoning, you would use 2 units. The theory being that getting too close to the limit will trip the dimmers even though you don't use all of their potential. By that same reasoning, your 2.4k dimmers could run 4 lights technically and 3 by the 80% rule. But your circuits can't do 2.4k, so they are the limiting factor.

Hope that was helpful!

Thank you! That's a huge help and that makes a lot of sense. If I overloaded the circuits but kept it under 2400 watts, would I just trip a breaker or would I damage the circuits/dimmers? And yeah, it's 24 circuits hard-patched into 24 dimmers.

 

[Musicman56]

You don't want to overload the circuits. That's bad news (that's how electrical fires are caused). If you overload them you'll screw up the cabling before it gets to the dimmers. You are sure that they are rated less than the dimmers though? Could the circuits be older than the dimmers? Anything under 2.4k isn't gonna hurt the dimmers though. In this configuration, you max is gonna be 1.8k no matter how powerful your dimmers are.

Could you maybe see how others have managed to do it in the past? If you really end up needing more power than you think your space can safely provide you with, I would suggest looking into renting dimmer packs from your local shop.

 

[ruinexplorer]

Are you positive that the lamps are 575w/120v? HPLs are often 575w/115v. This will alter the number of lamps on a circuit. Also, take into account that lamp filaments are a non-Ohmic load and will thus act differently depending on how warm the filaments are.

 

[JD]

Just to be clear, what is the limitations on the circuits caused by? A 2.4kw dimmer will have a 20 amp breaker on it and the circuit should be 12 gauge wire (or better if there is cable de-rating involved.) 1.8kw implies a 15 amp limit. This can be due to one or more of the following:

Secondary 15 amp breakers.
Insufficient wire gauge for the application.
Connectors rated below 20 amps.
Total system de-rate based on available supply.

Depending on why there is a de-rate on the circuits, you may be able to get an electrician to find a work around.

 

[hobbsies]

I'd imagine it's from using edison plugs.

 

[SalvatoreDelorean]

Thanks, that makes sense. I figured overloading the circuits couldn't be a good thing. I have no idea why or how our circuits are rated less than the dimmers, it does seem incredibly stupid. It's a pretty sketchy converted space that has all sorts of weird quirks like that, and it seems to have been built by people who had no idea what they were doing. I'll track down the old Master Electrician and ask about the limiting factor of the circuits. They're twist-lock, not Edison, so I'd guess it's either insufficient wire gauge or lack of power supply. Like I said, it's a sketchy system that doesn't make a lot of sense.

 

[TimMiller]

Then how do you know you are limited at 1.7k? If the connectors and wire are rated for 20a you should be ok. Next you have to find out how many amps you have to play with feeding the rack you may have to play with your cueing so that you do not trip the main. I worked at a venue that had an array of 2.4k 6k and 12k dimmers but all of the larger dimmers were split down into 2.4k circuits. Everything was hard patched. It was also possible to overload the mains so you had to keep in mind what your loads were and what was on each leg.

 

[hobbsies]

I would just confirm you're using 12/3 circuits. If you look at the tiny writing on the cable, look for something that says 12/3 or 14/3. Should also say something like 600v soow. The EEs on this forum can tell you what it should say in completion.

 

[Clarkwg3]

This thread has my head spinning, and I deal with just about every issue discussed on a daily basis. I'm gonna try to clean-up the previous posts to help make sense of it. I realize I'm opening my work to constructive criticism.

2.4Kw Dimmer = 20Amp = 12AWG(American Wire Gauge) SO, SOO, SOOW(standard) cabling = 10AWG circuit wiring for full load at 20A outlet; if the circuit wiring is 12AWG then 80% de-rating MUST apply(It's National Electrical Code Requirement based on length and gauge).

I think this is what your situation has been discribed as:

2.4Kw Dimmers, 12AWG circuit wiring = 15A per circuit; hard patch spider board allows twofering multple Stage circuits to be supplied from a single dimmer. If correct; then you have two places to watch your wattage. First place is the wattage per stage circuit(1750w) The second is 2400w per dimmer at the Spider patch(make sure to use 12AWG twofers, 10AWG would be better here)

Has the squiggly line been straightened?

 

[derekleffew]

...2.4Kw Dimmer = 20Amp = 12AWG(American Wire Gauge) SO, SOO, SOOW(standard) cabling = 10AWG circuit wiring for full load at 20A outlet; if the circuit wiring is 12AWG then 80% de-rating MUST apply(It's National Electrical Code Requirement based on length and gauge). ...

Not exactly. Article 310 of the 2011 NEC specifies exact conductor ampacity, which is dependent on many factors, including: ambient temperature, temperature rating of conductors, number of current carrying conductors, pipe fill, etc. I don't see anything in that section regarding conductor length or voltage drop. Can you cite the "80% de-rating factor"?

...(make sure to use 12AWG twofers, 10AWG would be better here)...

I don't think I've ever seen a #10 AWG tufer in my life. We discussed under-sized two-fers here: http://www.controlbooth.com/forums/lighting-electrics/22174-awg-ampacity-two-fers.html .

 

[SalvatoreDelorean]

2.4Kw Dimmers, 12AWG circuit wiring = 15A per circuit; hard patch spider board allows twofering multple Stage circuits to be supplied from a single dimmer. If correct; then you have two places to watch your wattage. First place is the wattage per stage circuit(1750w) The second is 2400w per dimmer at the Spider patch(make sure to use 12AWG twofers, 10AWG would be better here)

Has the squiggly line been straightened?

Do you mean I can bypass the stage circuits and patch directly into the dimmers and get more wattage out of them?

 

[epimetheus]

2.4Kw Dimmer = 20Amp = 12AWG(American Wire Gauge) SO, SOO, SOOW(standard) cabling = 10AWG circuit wiring for full load at 20A outlet; if the circuit wiring is 12AWG then 80% de-rating MUST apply(It's National Electrical Code Requirement based on length and gauge).

I think this is what your situation has been discribed as:

2.4Kw Dimmers, 12AWG circuit wiring = 15A per circuit; hard patch spider board allows twofering multple Stage circuits to be supplied from a single dimmer. If correct; then you have two places to watch your wattage. First place is the wattage per stage circuit(1750w) The second is 2400w per dimmer at the Spider patch(make sure to use 12AWG twofers, 10AWG would be better here)

Has the squiggly line been straightened?

No. Not sure where you're getting this 15A stuff or the mandatory 80% derating, as it's not in the NEC. A 2.4kW dimmer is good for outputting 2.4kW. Dimmer racks use 100% rated breakers, so the 80% derating applied of normal thermal breakers does not apply. 12AWG circuits are good for 20A in most situations, with the exception of the rating factors Derek mentioned.

 

[Clarkwg3]

When I stated the mandatory 80% derating I was thinking standard breaker, not full load breakers.

What is the derated load at the end of the 12AWG stage circuit(100 feet plug to dimmer)? Is not 2400watts. Next lowest device rating is 15A or 1750watt. I suspose one could use 20 amp plugs and label them for the maximum allowable wattage at that plug.

 

[kicknargel]

Wow, this thread has some bad, some good, and some overly complicated and confusing information. I don't think we're all talking about the same things. Sometimes CB discussions digress into abstract code arguments rather than trying to solve the OP's questions.

First, when you say "circuit," I take it to mean hard-wired permanent circuits from the dimmers through conduit, etc, to some kind of outlets or pigtail in the grid. Or are we talking about SOOW or other temporary cabling? Second, on what information are you basing the 15a circuit rating? Third, photos always help us stay on track.