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All of my heating is on one breaker, 240V on a double pole breaker, each marked 20A (it's a small condo). I wonder how I can determine the maximum wattage I can hook up to that breaker.

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1 Answer

up vote 13 down vote accepted

Power (Wattage) = Current x Voltage (P=I*V)

20A X 240V = 4800W

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Thus 40A * 240V = 9600W? It's been a while since I had science classes... and physics never was my strongest suit :( –  MPelletier Jan 25 '11 at 2:31
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If a double pole breaker is marked as 20A then its 20A. Its not doubled because its a double pole. –  Web Jan 25 '11 at 2:33
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That formula is for DC. For AC you also need to consider power factor, although for heating alone it's usually not a real concern. Just be aware that as a general rule it's not the correct formula for AC. –  John Gardeniers Jan 25 '11 at 8:00
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+1, Although that will give an estimate only - the breaker will not have exactly 20A cutoff, just very close to that, so it might happen that it breaks the circuit on slightly lower or slightly higher current. –  sharptooth Jan 25 '11 at 9:43
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not really 4800W. The reality it's that P=U*I and this gives you VA, in order to get W you need to multiply this with cosPhi, that you can take arround 0.8 to be safe (so it shoud be just a shade under 4000W) –  s.mihai Jan 25 '11 at 11:26
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