Proper wire for 12v fixtures


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Old 03-29-14, 03:46 PM
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Proper wire for 12v fixtures

In my living room there's a set of three pendant lights that use 12v halogen bulbs. They're rated for 35w bulbs, but I noticed that the wire leaving the transformer felt warm with 35w bulbs, so I downgraded to 20w bulbs. The wiring appears to be 18g doorbell wire. The transformer is designed for 12v halogen bulbs, maximum 150 watts. The total run from the transformer to the end light in the series is about 10 ft.

I'd like to replace the wiring with something heavy enough to use with 35w bulbs. Would 14g stranded work ok?

Thanks for advice.
 
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Old 03-29-14, 04:22 PM
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Just to make things clear here, your talking about the wires that came installed from the factory, correct?

If so, I would not change anything. Doing so would violate the UL listing of the light fixture. While the wire may be small, fixture wires are commonly smaller than "standard" building wire and also could have a higher temperature rating than the THHN wire you would install, which is 90 degree C. Wires do get warm, however that does not mean there is a danger.
 
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Old 03-29-14, 05:17 PM
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No, I'm referring to the house wiring to which the light fixtures are connected.
 
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Old 03-29-14, 06:36 PM
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You have three fixtures with a separate, remotely mounted transformer.... correct ?

#18 wire is definitely undersized for that load. #14 stranded should work fine in that application.
 
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Old 03-30-14, 05:54 AM
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Yes. The three fixtures are connected in series via 18g wire to the transformer, and the transformer is connected to the regular 12g house wiring.

The only 14g stranded wire I can find is speaker wire. Is there any reason that wouldn't be appropriate?
 
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Old 03-30-14, 12:42 PM
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The house wiring is to the transformer, you said it is #12 which is good.

The wires from the transformer to the light fixtures is #18. Allowable ampacity of a #18 fixture wire is 6 amps (NEC 402.5). 35watts / 12 volts = is just under 3 amps.

I still say you are fine with the #18 wire, however if you want to use larger wire, that is fine too. The speaker wire is fine as long it is not installed in wall, and it insulation is rated for 12 volts or higher.
 
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Old 03-30-14, 01:02 PM
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I'm picturing the lights as three in parallel on a single cable.
At the transformer end you'd have 3x3=9A.
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Old 03-30-14, 04:24 PM
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Funny, I'm picturing a separate run to each fixture.
 
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Old 03-31-14, 12:35 PM
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Depending on the length of the wires and whether two or all three lights share part of the wiring (see above diagram) you may find that the lights further away from the transformer are dimmer. In this case heavier wires would help.

The consequences of voltage drop in long wires are more significant at 12 volts compared with at 120 volts. It is the number of volts, not the percentage of voltage, that is lost for a given number of amperes and a given piece of wire regardless of the voltage.

If you experience non-uniform brightness then you will want to compute the wire size taking into account voltage drop. Compare that with the wire size taking into account just amperes (say, from the NEC table). Choose the larger of the two.
 

Last edited by AllanJ; 03-31-14 at 01:02 PM.
 

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