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continuous load circuit capacity

Posted by dan1554 (My Page) on
Fri, Feb 15, 08 at 23:36

Here's a question that I am unclear on. I have an electric heat circuit (240 volt) in a house I recently bought. It is wired with 12 guage wire, and is on a 20 amp breaker. This is an original circuit & wiring, circa 1973 when the house was built. There are 3 heaters, different rooms, 2000 watt, 1500 watt, and 1000 watt. All 3 are on individual thermostats. Does this count as a fixed load, for the 80% rule ? (240 volts x 80% = 3840 watts), or is this ok since each one has a seperate thermostat, and it's unlikely that all 3 would be in operation simultaneously for several hours ?


Follow-Up Postings:

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RE: continuous load circuit capacity

Here is the technical answer:

NEC 2005
ARTICLE 424 Fixed Electric Space-Heating Equipment
424.3(B) Branch-Circuit Sizing
Fixed electric space heating equipment shall be considered continuous load.

In today's world this installation would not be legal.
I do not know what the code on this was in 1973 though.
I will say, whether or not this was ever legal, they do change these things for a reason.


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RE: continuous load circuit capacity

" is this ok since each one has a seperate thermostat, and it's unlikely that all 3 would be in operation simultaneously for several hours ?"
I guess that's for you to decide:

By code you need a 30A #10 cc't for this load.
4500W/240= 18.75A/80%= 23.4A


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RE: continuous load circuit capacity

1971 NEC 424.3(b) "The size of branch-circuit conductors and overcurrent protective devices supplying fixed electric space heating equipment consisting of resistive elements --- shall be calculated on the basis of 125% of the total load---."


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RE: continuous load circuit capacity

Thanks for the prompt follow ups. That's what I was thinking also, because of course there will be times when all 3 heaters are on. I have easy access underneath, and plenty of room in the 200 amp breaker box, I think the first project will be to put the 2000 watt heater on it's own 20 amp circuit. Thanks for the help !


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