Home low voltage problem

[ship]

So I have four of my 4.5" Plano Convex 1920's fixtures lighting my work home shop as re-lamped and bench focused for 35w/12v MR-16 lamp. I run them off a wire rope type track lighting 12v. transformer rated at 15A in center circuit breaker. Only four of the fixtures no more than 25' away from the transformer. 14ga wire feeding the individual fixture feeders and 14 to 16ga. wire feeding the transformer.


Load side breaker was 15A and one would think as opposed to wire rope feeding the fixtures, I’m now copper in direct, it should be sufficient to power up four 35w fixtures. Not persay in question of the breakers in going thru many of them, more just a question of wire gauge or non-sufficient breaker somewhere? Where did I go wrong in this system working last year but now problematic this year after changing breakers?

14ga cable at up to 25' given 35w/12v lamps as split into four fixtures, or more about the transformer or power supply to it from switch which is 16 ga. at about ten foot in 16/7 multi-cable? Why are these load side thermal breakers overheating a year or more after the install given the above assuming the load side breaker is doing its job at 15A for blowing in output for the 12v/35A load side?



Feeder to this and another circuit is 16/7 multi-cable. Goes up from the switch for the low voltage switched circuit and line voltage dimmer circuit up a few feet to a patch box for total length of 16/7 cable to about 10'. From there it is 14/3 to the tranformer in feeding it about five feet away.

Feeder wire to the transformer too small or transformer too small in being huge? Thought against many lines but yet to solve the problem. Mostly I think it adds up in should work... where did I calculate wrong?

 

[ajb]

Neither undersized wires nor undersized transformer would cause nuisance tripping. Smaller wire -> higher resistance -> less current flows. Smaller transformer -> voltage sags more for a given load -> less current flows (given a linear load). Yes, overloaded conductors and transformers are no good from a fire prevention standpoint, but that's why we have overcurrent protection.

Just to be clear, is it the 15A breaker on the transformer secondary that's tripping?

Have you checked to output voltage of the transformer? If it's too high you'll get more current drawn by the fixtures (V=IR and all). Or has the breaker been subjected to excessive ambient heat (ie too close to one of those fixtures)? That plus age could do it.

 

[ship]

Neither undersized wires nor undersized transformer would cause nuisance tripping. Smaller wire -> higher resistance -> less current flows. Smaller transformer -> voltage sags more for a given load -> less current flows (given a linear load). Yes, overloaded conductors and transformers are no good from a fire prevention standpoint, but that's why we have overcurrent protection.

Just to be clear, is it the 15A breaker on the transformer secondary that's tripping?

Have you checked to output voltage of the transformer? If it's too high you'll get more current drawn by the fixtures (V=IR and all). Or has the breaker been subjected to excessive ambient heat (ie too close to one of those fixtures)? That plus age could do it.

Thanks... do this kind of thing for a living but the Mechanic's garage type of thing with his own car perhaps in me figuring out the problem. Focused on carpenter mode in making a swing arm clamp holder and not on electric's mode. Still though, Origional breaker should have been fine and on the third replacement now. Three lamps, works fine for a while but it's also blown the breaker at times. Each breaker at three or four lamps works fine for a while or a few months but than causes problems later.

This also tonight after about two hours blinking out again and I caught it before the thermal push to reset type breaker tripped. Made a bit of a noise and lamps went out than on and I hit the switch.

Yes 15A circuit breaker on the secondary that's tripping. The primary side - at the wall panel feeding 400w wash light for the line voltage dimmed circuit in combination with this low voltage transformer has never blown its fuse. This is line side before the 16/7 or various 14/3 cable.

There is no fixture anywhere near the transformer also. If of help, salvaged or new (don't know where I got them from) and probably 20+ year old Wood Electric Corp. MFD. 0868 or P/N375-215-101 breakers are currently in use to replace the origional I think Tyco thermo push to reset breaker that went bad a year or more later.

Hmm, thanks in checking the output voltage.. Wifie's Craftsman clamp meter used at home says 360v on the load side of the transformer and it's absolutely reading that voltage on the only 600v AC reader scale for the meter.

Fascinating and not possible as were such voltage seen by the 12v lamps - it would pop them instantantly. The lamps seem to last a nomal 2,000 hour lamp life given two years of constant use.

Fascinating this voltage reading but also not possible. I'll bring my true RMS meter home tomarrow.

Just re-checked that reading and it's 10 volts - yea, I got no idea of the 360v reading in not being able to reproduce it. Amp clamp of the cable feeding the transformer short of re-opening the transforme housing shows nothing other than a definate noticable humm from it. This humm has also become more noticable over time.

I'll open it up after work tomarrow and get source amperage and voltage. Still though, I have not changed anything in over a year - thinkin about adding a few banks of fluorescents, but in general have not changed anything. A year later given all good lamp sockets and lamps at the fixtures...

 

[Sony]

the only thing I can think of is the duty cycle of the second breaker is really low. If you have 4 12v 35w lamps then that is about 3 amps per fixture and 12 amps which is right at the 80% mark out of 15 amps. Knowing that common thermal-magnetic breakers are only rated for 80% duty cycle you could be right on the edge of that secondary circuit breaker. It's unlikely, but just a thought.

 

[Morpheus]

Amp clamp of the cable feeding the transformer short of re-opening the transforme housing shows nothing other than a definate noticable humm from it. This humm has also become more noticable over time.

I wonder if this is the problem, or a least a clue.

 

[JD]

Regarding voltage reading: If the secondary is floated, and the meter is high enough in ohms per volt, you can get all sorts of strange readings if you happen to read between the output and ground (or neutral.) I have an old telephone "stop light" (see below) which is like a high voltage probe, and I've had that trip off on low voltage isolated circuits before.
Fluke C9970 High Voltage Detector

 

[FMEng]

Ship,

You are asking for too much precision from a cheap, thermal type breaker. If it is mounted in an enclosure next to a heat producing transformer, it is likely to trip at a current well below 15 Amps, because they are affected by ambient temperature. Besides that, they just aren't made to tight tolerances.

Yank the breaker and replace it with a beefy, time delay fuse. Maybe something like a Littelfuse SLC-15, but whatever you pick will be governed by what kind of fuse holder you can find to fit.

There are small, magnetic type breakers that would work in that application. They tend to have more accurate trip points and are not affected by ambient temperature. But, you do need one that has a little delay to it so that it won't falsely trip at turn on, due to cold filament inrush.

 

[ship]

Re: Home low voltage problem & FOH Ethernet DMX

Thanks all in the help but had more thoughts - 50w lamps added to the three lamp as opposed to four lamp system in causing the breaker to fail and really the telling 10v voltage reading from the power supply was really important and thanks for reminding me to read such a thing as the major source I think. In not even considering measured voltage of 10v=14a + line voltage loss = 15a breaker failing cause I just couldn't get it. Were I at work perhaps, but it's my garage and the work hat is off - I'm into carpentry mode at home. My swing arm bar clamp holder is going to be briliant in design and add to that the now only three 35w fixtures have not killed the breaker so far tonight as perhaps a trueum in concept of given 10v from the transformer instead of the expected 12v - it's a world of difference given low voltage lighting.

I was waking up this morning and did some quick math... 35w x four fixtures... 10 Volts... given around an amp of line lossage, should be simple enough to figure out - 10V out of a 12v transformer is a problem and causing these thermal breakers to blow. Three of them isn’t so far tonight blowing - though it has in the past but that could be back when I had some 50w lamps thrown into the mix before re-lamping.

Slap myself upside the head and thanks for the help in getting me to measure the output voltage instead of cursing it for understanding why this was a problem.

Next step for me is to research the vendor of the transformer in getting its actual rating. Than in say installing a 20w lamp in one out of the four fixtures if not in more rated installing a larger circuit breaker in it. Thanks all, this in also TBA soon going fluorescent but fluorescents of at least 85 in CRI... I do hate compact fluorescents and normal fluorescent fixtures. Still though as opposed to a multitude of filament lamps in lighting the shop in a focused and washed way with still shadows... the fluorescent are necessary. About to run Ethernet to the garage, perhaps I’ll pull another 20A to the garage also at that point.

 

[ajb]

Re: Home low voltage problem & FOH Ethernet DMX

Anyway, back to the above “you will feel foolish” comment, I was waking up this morning and did some quick math... 35w x four fixtures... 10 Volts... given around an amp of line lossage, should be simple enough to figure out - 10V out of a 12v transformer is a problem and causing these thermal breakers to blow.

Sorry, but if you're suggesting that 35W * 4 Fixtures / 10V = 14 A, then you're not quite there.

If the lamps are rated at 35W & 12V, then that means they draw 2.9A at 12V, so using Ohm's law (V / I = R), the effective filament resistance is 12V / 2.9A = 4.1Ω

Hook them up to a 10V supply, and using Ohm's law another way round (V / R = I ), they will each draw 10V / 4.1Ω = 2.4A, for 9.6A total.

Also, the only way you can 'lose' current is through leakage from one conductor to another, and if you've got an amp of leakage current in that small of a system you've got something seriously wrong. You can lose voltage due to conductor resistance, ie a too-long run of undersized wire, but a linear load will draw less current as the supply voltage drops, not more.

 

[ship]

Re: Home low voltage problem & FOH Ethernet DMX

Yep, I feel foolish in all ways and thanks all above for the help - it was very needed.

Salvaged say 40 year old thermal breakers of unknown (normal) make in an enclosure to replace the origional one also of uknown age and quality that perhaps just went bad..

Me also in adding 35+35+35+35 Watts to get 140w and dividing it by 10 Volts. This gives me 14 Amps and with theoretical line lossage an easy 15 Amps.

I was raised on Volts x Amps = Watts and never got into studying the proper Ohm's law = I very much should have learned. I remember a question on the Certified Electrician's test published as a Pre-test and an article in PLSN all about it which I a "Lamp guy" will have gotten wrongiin me not learning' even given the easy access to the info to learn from.

Thanks to all and especilly AJB for helping solve this problem and especially reinforcing this need for me to start hitting the books again so as to study what I don't yet know.

For me the next step is to learn this Ohm's Law method, obviously fix the transformers' breaker and figure out without help what the maximum amount of wattage is that I can power up from this transformer. I would love to not go fluorescent in my garage and instead add more fixtures, but probably will anyway. Still though upping the wattage to 50w/12v... that would be nice. Don't tell me - something I need to learn...

Thanks.

 

[Chris15]

I'm going to complicate things and suggest Ohm's law does not apply here...
Lamps are non ohmic, we know this because of their cold filament resistance characteristics being different to their operating point...

Now given we are talking about halogen lamps, I know enough to know the chemistry is more complex than a basic lamp, but not much more beyond that.

But what I'm thinking is perhaps that the filaments aren't getting heated enough given they are under voltage and thus aren't reaching their normal operating temperature and so have a lower resistance and thus the extra current draw.

This being mostly postulation, so feel free to show how boring things like physics mean otherwise to what I've just said...