Why is a 240 Volt 20 Amp circuit limited to 3840 Watts (20 A x 240 V x 80%) when it could be 4800 Watts (20 A x 240 V)?

I learned about this limit when installing baseboard heaters. I understand that I'm limited to 3840 Watts on a 240 Volt 20 Amp circuit because 240 V x 20 A x 80% = 3840 Watt but I'm wondering why I have to use 80% of the maximum 4800 Watts. Is it "just in case"?

Also, I've read on the internet (so it must be true) that I can swap the 20 amp fuse for a 30 amp fuse with 12 gauge wiring to increase permissible wattage but that doesn't sound right. My understanding, after reading more, is that doing this would exceed the ampacity of the wire. I know the wire coming out of the fuse box is 10 gauge but somewhere along the way, after a thermostat, it changes to 12.

You are watching: Maximum wattage on 20 amp circuit 240 volts

Can someone clear that up for me? I'm in Canada, if that matters.


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level 1
· 6y · edited 6y

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Supply
CEC 8-104 (6) Where a fused switch or circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices, the continuous load as determined from the calculated load shall not exceed (a) 80% of the rating of the circuit where the ampacity of the conductors is based on Column 2, 3, or 4 of Table 2 or 4;

The reason you can't put the full wattage on is because our breakers are only designed to handle 80% of the load continuously.

As for your conductor size you would need number 10's if you installed a 30 Amp breaker but again you would need to derate by 80%

CEC 14-104 (2) Except as provided for by Subrule (1)(c), the rating of overcurrent protection shall not exceed (a) 15 A for No. 14 AWG copper conductors; (b) 20 A for No. 12 AWG copper conductors; (c) 30 A for No. 10 AWG copper conductors;

EDIT: Read my reply for fixed resistance heat......It's been a while since code class


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level 2
· 6y
Journeyman

^ This is the right answer.


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level 2
Op · 6y

Thank you for the explanation. I understand.

So the 80% is because of the design of the breaker, not because the wire can't handle the load.

Is the 80% firm or does going to 4000 Watts on a circuit limited to 3840 watts OK?


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level 1
· 6y
Spend the extra money to get 10 gauge wire. The potential headaches and troubles that may occur from saving a small amount of money isn't worth it. How many far away are your heaters from your breaker? Also since he's in canada shouldn't the OP go ahead with 10 gauge since they may run quite regularly?


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level 2
Op · 6y

It seems like the gauge of the wire varies depending on its location. From the box to the first thermostat is 12 but after some of the thermostats, I've noticed it's 14 gauge romex to the heater.

In Montreal, Canada, heaters can run from Mid-September to Mid-May.

See more: How Much Is A 1963 Silver Quarter Worth, 1963 Quarter Value


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