Load calculation for low & high voltage circuit?

Question

To my understanding that a commercial circuit breaker has 20 amp load or approx 2200w load. I have a circuit breaker that is shared both 110V light fixtures and 12v light fixtures

One (1) flourescent fixture with 4 x 32w bulbs = 128w
One (1) 110v tracklight with 3 x 50w halogen bulbs = 150w
Three (3) 12v tracklight with 4 x 20w halogen bulbs = 240w

Total watt is 518w which is below the wattage limit.

But by the equation Watt / Volt = Amp,
The total AMP load for the three 12v tracklights with 12 halogen bulbs: 12 * (20w / 12v) = 20 amp

I know there is something wrong here that does not add up. Please help. I am planning to add one more 110v tracklight fixture with 3 x 50w halogen bulbs if it is possible.

thanks

Answer 1

You've neglected one thing: At some point (after the 20A breaker) the 110V goes through a step down transformer to bring it to 12V.

Yes, your 12V lights are drawing 20A (at 12V) but the draw off the 110V circuit is approx 2.6A (assuming your transformer is running 80% efficient).

Answer 2

First, household wiring is 120 volts not 110. Your 20 amp breaker provides 2400 watts. Your flouresent fixture has a ballast that steps up the voltage. Figure around 90 watts for that fixture. The 12 volt lighting is provided by a step down transformer from the 120 volt circuit. You are using just over 2 amps for the low voltage lamps. You were close until you got to the low voltage lamps. You were correct with the 240 watts. But you need to divide that by 120, not 12. So your circuit is around 480 watts or 4 amps. The new fixture would take another 150 watts or 1.25 amps. So your load would be just over 5 amps on a 20 amp circuit. No problem at all.