Calculating Safe Circuit Capacity for Baseboard Heaters


Old 11-16-06, 12:39 PM
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Join Date: Nov 2006
Posts: 1
Calculating Safe Circuit Capacity for Baseboard Heaters

First of all, I am not an electrician. Secondly, thank-you for this type of forum - to allow more serious DIY's to complete reasonably simple tasks.
I am going to be installing several baseboard heaters into a newly renovated home. I have done as much reading as needed and currently have a paid permit from our local electricity company. I have read your post and firmly agree with what you have stated, however, the inspectors from this local electric authority have stated that such use of 12/2 wire appears to be "excessive" (i.e. they feel that 14/2 wire would be adequate using a 20A double pole breaker).
I have several circuits - all are 240V.
Circuit A = 1250W + 750W = 2000W total (14/2wire&15Abreaker)
Circuit B = 1250W (14/2wire&15Abreaker)
Circuit C = 1250W (14/2wire&15Abreaker)
Circuit D = 1500W (14/2wire&15Abreaker)
Circuit E = 750W * 3heaters = 2250W (14/2wire&15Abreaker)
Circuit F = 750W * 4heaters = 3000W (14/2wire&20Abreaker)
Circuit G = 500W * 2heaters = 1000W (14/2wire&15Abreaker)
Circuit H = 500W (14/2wire&15Abreaker)
The choice of guage of wire and size of breakers are as recommended to me by this local authority.
They mentioned something about the fact that baseboard heaters are considered continuous loads and that in this case the wire can be 100% and the breakers 80%??? For 14 guage wire the amperage capacity should be 15A. So, at 240V the max wattage should be 3600W and I thought a safe load would be 80% or 2880W. I thought that, for example, circuit F above would have to be on 12/2 wire and a 20A double pole breaker for sure. Since 3000/240=12.5Amp multiplied by 1.25=15.625A which is too high for 14guage wire? Sure, the 20A breaker should not trip off at 15.625A but wouldn't my wire "cook" or overheat?
I know that different jurisdictions have different electrical codes, however these basic electrical principals should apply universally regardless of the country.
Sorry for the length of the post! But I'd like a little more clarification before trusting what they have suggested.

Originally Posted by thinman
2000 watts + 1500 watts = 3500 watts.

3500 watts divided by 240 volts = 15 amps (rounded off to the next higher number).

15 amps X 1.25% = 19 amps.

Use #12 wire and a 20 amp two-pole circuit breaker.
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Old 11-16-06, 02:04 PM
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Location: Near Lansing, Michigan
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> the inspectors from this local electric authority have stated
> that such use of 12/2 wire appears to be "excessive" (i.e.
> they feel that 14/2 wire would be adequate using a 20A
> double pole breaker

I don't have my code book handy to cite an article, but I am sure their suggestion violates code. A 20A breaker should never be used on #14 wire to power a space heater (which is a continuous load). Both the conductors and the breaker must be oversized to 125% of the heater amperage.

240V, 15A breaker, #14 cable = 2,880 W max heater
240V, 20A breaker, #12 cable = 3,840 W max heater
240V, 30A breaker, #10 cable = 5,760 W max heater

To address the rest of your concerns, I can say that it is never wrong to use a larger size wire than what is required. If they only are going to require you to use #14, they cannot penalize you for upsizing to #12 or even #10 if you wanted to. Therefore if you feel it is safer to use #12 (which I think it is), then do so; you'll sleep better knowing you did the right thing.

A quick comment on your design. Have you considered combining some of these 8 circuits into 2 or 3 30A circuits? Sixteen breaker slots and 8 home runs of cable is a lot of potentially wasted materials.

Last edited by ibpooks; 11-16-06 at 02:41 PM.
Old 11-16-06, 02:08 PM
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
Art. 424.3 (B) reads------- "Fixed electric space heating shall be considered "continual load"

Art 210.20, (A) reads---- " (for) continual loads, the rating of the ( circuit-breaker ) shall be 125% of the continual load"

Art 240.4 (D) reads--- " the ( rating of the ) over-current protection shall not exceed 15 amps for #14 wire and 20 amps for #12 wire " ( copper conductors)

With "X" = max circuit-load in amps ,We have -- 1.25 x X = 20 amps ( C-B rating )

X ( max load in amps) = 20 / 1.25

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