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Old 06-29-2009, 07:01 PM   #1
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New circuit?

How do you calculate, figure out load? I need to figure out for a additon how many circuits I need. There will be 5 ceiling mounted light and 6 otlet in one room and walk in closet. One circuit ok or I need 2?


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Old 06-29-2009, 07:18 PM   #2
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One is fine as long as there isn't a bathroom involved.


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Old 06-29-2009, 07:51 PM   #3
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Thanks, bathroom will have its own circuit. Allso smoke detectors too. I have to install them in every room.
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Old 06-29-2009, 08:42 PM   #4
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Depends what you're going to have plugged in those outlets. The total amount of power (wattage) a circuit carries is equal to the volts (always 120 here in the US; actually it ranges give or take around 115, but for calculation purposes, assume 120) multiplied by amps, or Watts = Volts * Amps.

If you use a 15A breaker, then your circuit can provide total power of 1,800 (120v * 15A) watts at a maximum before it blows the circuit. 20A circuit would be 2,400 watts. So you need to figure out the total wattage of everything you plan on putting on that circuit to figure out how many circuits are adequate.

If you have 5 ceiling lights and use 60 watt bulbs in each, then that is a total of 300 watts used up on overhead light. You can google to see what other common appliances use in terms of watts. I think TVs are a few hundred or so, give or take depending on size, type, etc. Even computers are only a couple of hundred watts at most. Appliance that really suck up wattage are things with heating elements. Hair dryers, irons, toasters, etc all use up a thousand watts or more. Power tools use up a lot too, but presumably you won't be doing that inside!

Anyway, I don't remember what the NEC actually says a circuit has to accommodate. It is something like the total expected maximum wattage of everything on the circuit shouldn't exceed 75% of the total wattage...or something like that (I'm sure you can find the specifics pretty easily online). Long story short, once you know the total wattage of everything you might have in that room, round up. If it exceeds about 2000 watts or so, go for two separate circuits. If your total wattage will be comfortably under 2,400, a single 20A circuit will suffice. For rooms that aren't kitchens or bathrooms, there isn't a minimum number of circuits, so long as you have enough wattage to power what is needed, with some cushion left over.
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