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Old 09-23-2010, 09:55 AM   #1
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under cabinet lighting


I want to add a dedicated 20A circuit for under cabinet lighting.
I plan to run 12/2 romex from the circuit breaker to three 12 volt transformers located within the kitchen cabinets. From each transformer, I will run the low voltage cable from the manufacturer (Kichler) and install a total of 30 xenon lampholders.

At line voltage of 120, a 20A circuit should handle 2400 amps, but am I safe at 12 volts (12v times 20A = 240 amps)? This 240A is less than the 300 watts I plan to install. Am I missing something?

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Old 09-23-2010, 03:31 PM   #2
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Sorry, the last line should read "12v times 20A = 240 WATTS. This 240 watts is less than the 300 watts I plan to install. Am I missing something?"

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Old 09-23-2010, 05:06 PM   #3
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Quote:
At line voltage of 120, a 20A circuit should handle 2400 amps,
I assume, also, that you meant to say 2400 watts.

So, it sounds like you want to install 300watts worth of lights? You did not say it, but you are installing 30 lights at 10W each?

This should not be a problem for your circuit. A 20 amp circuit (at 120V) can provide 2400 watts of power. You only want to use 300 watts. This sounds well within the capacity you are adding.

Perhaps the confusion is that it appears that you are assuming your transformers can only output 20 amps in total. This is not necessarily the case. Your limiting factor here is power (watts). This circuit could, theoretically, provide enough power for a 12V transformer to generate up to 200amps (2400watts = 12V x 200A). Of course, I don't suggest trying this at home. That would be a pretty large cable.

There are other factors, such as conversion losses, but I don't see anything here to worry about.
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Old 09-23-2010, 05:22 PM   #4
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Oberkc, thanks for your input!

Yes, I did mean 2400 watts.....sorry.
Yes, 30 lights (10 watt/each).
Each of the three transformers are 150 watt electronic units, and each transformer will control 7 to 14 lights.
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Old 09-23-2010, 09:09 PM   #5
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sounds like no problem to me. 150 watts is not enough to cause any problem. Even three of them.

150 watts at 12 volts is just over 12 amps. Cable size is typically driven by current, rather than power. Hopefully, the cable coming from your transformers is large enough. Based upon my landscape lighting experience, you should see something no smaller than 14 gage wire for these short distances. I expect that you will.

Like 12V landscape lighting, it is not uncommon to use 20W MR-16 or wedge-base lamps. While total power is often only a few hundred watts, one must use 10 or 12 gauge wire to handle the current and minimize voltage drop.

On the other end, electrical transmission lines are at much higher voltages than household 120 or 240. This allows greater amounts of power transmitted at relatively low current.
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Old 09-23-2010, 09:32 PM   #6
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Again, I appreciate your reply.

The spec sheet calls for low voltage SPT cable (10 gauge) and that seems even better than you expected.

So I should not be concerned that my 300 watts / 12 volts = 25 amps, even though the circuit will be on a 20 amp breaker? Just trying to be safe!!!
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Old 09-24-2010, 10:07 AM   #7
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no worries. A 15 amp circuit could handle this. You will find that the transformer for these lights will require around 3 amps in total for all three.

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