EV Charger: #8 Romex on 50a Breaker

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BarklieEstes

Member
Location
Richmond, VA
Occupation
Master Electrician
Greetings,

I've heard "you can never put #8 romex on a 50a breaker", but my reading of the 2017 NEC suggests that it can be allowed. What am I missing?

The situation is a 40a EV Charger with #8 romex hooked to a 50a breaker.

334.80 says "The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor" (40a). I don't need the cable to allow more than 40amps though since it's a 40a charger. Nor does it seem like the words "allowable ampacity" refer only to non-continuous loads since the definition of Ampacity says "The maximum current, in amperes, that a conductor can carry continuously...". I am asking a #8 NM Cable to continuously carry 40a and 334.80 says that I am allowed to ask that of it.

210.19(A)(1)(b) says "the minimum branch-circuit conductor size shall have an allowable ampacity not less than...125 percent of the continuous load" (40 * 1.25 = 50a). The allowable ampacity of a #8 sized copper conductor is 50a when terminating to 75° lugs [Table 310.15(B)(16)]. The calculated current of 50a is not greater than the 50a ampacity of #8 copper.

Only if I read the words "allowable ampacity" in 334.80 to mean the calculated ampacity from 210.19 would this application be disallowed. Nowhere in the Code does it say to intermingle the calculated current vs conductor size ampacity test from 210.19 with the actual current vs reduced-rating ampacity test from 334.80. The Code reads these as two separate tests and this application passes both.

210.20(A) says that OCPD needs to be no less than 125% of my continuous loads (40 * 1.25 = 50a). Therefore, I have selected a 50a breaker.

I know I can't put a 50a non-continuous load like a stove on #8 romex, but it seems like I can put a 40a continuous load like an EV charger on it.

Regards,
Barklie Estes
 
R

retirede

Senior Member
Location
Illinois
BarklieEstes said:
210.19(A)(1)(b) says "the minimum branch-circuit conductor size shall have an allowable ampacity not less than...125 percent of the continuous load" (40 * 1.25 = 50a). The allowable ampacity of a #8 sized copper conductor is 50a when terminating to 75° lugs [Table 310.15(B)(16)]. The calculated current of 50a is not greater than the 50a ampacity of #8 copper.
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This is where you went astray. NM must be limited to 60°C ampacity regardless of the 75° lug rating.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
BarklieEstes said:
334.80 says "The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor" (40a). . .

210.19(A)(1)(b) says "the minimum branch-circuit conductor size shall have an allowable ampacity not less than...125 percent of the continuous load" (40 * 1.25 = 50a).
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When NM is the wiring method, the language in 334.80 modifies the meaning of the phrase "allowable ampacity" everywhere it appears in the code, including 210.19(A)(1)(b). So your proposed installation is a violation of 210.19(A)(1)(b), as you are only providing a 40A allowable ampacity branch-circuit conductor.

Cheers, Wayne
 
tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
Can you just set some dip switches or otherwise program the charger to only put out 32 Amps?
 
B

BarklieEstes

Member
Location
Richmond, VA
Occupation
Master Electrician
wwhitney said:
When NM is the wiring method, the language in 334.80 modifies the meaning of the phrase "allowable ampacity" everywhere it appears in the code, including 210.19(A)(1)(b). So your proposed installation is a violation of 210.19(A)(1)(b), as you are only providing a 40A allowable ampacity branch-circuit conductor.

Cheers, Wayne
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Thanks Wayne. That's the answer I've been given most often- that you are supposed to intertwine the two tests together. I just can't find where the Code says to do that. It also seems like doing so contradicts the definition of allowable ampacity, which says already clarifies that it's talking about a continuous circuit.

You can account for the circuit being continuous by reducing the ampacity of the cable (40a actual current =< 40a adjusted ampacity) or increasing the current of the load (50a calculated current =< 50a actual capacity), but why would you account for it twice?
 
B

BarklieEstes

Member
Location
Richmond, VA
Occupation
Master Electrician
tortuga said:
Can you just set some dip switches or otherwise program the charger to only put out 32 Amps?
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It hasn't been installed yet. I'll probably just use #6 like always. This is just a question that's always bugged me because I can't find where in the Code it says to mix these two tests together. It's always seemed like an obvious case of double counting to me.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
BarklieEstes said:
Thanks Wayne. That's the answer I've been given most often- that you are supposed to intertwine the two tests together. I just can't find where the Code says to do that.
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Needing to do that is a logical consequence of the phrase "allowable ampacity" being single-valued. 334.80 affect the value of "allowable ampacity" for NM for all cases.

BarklieEstes said:
It also seems like doing so contradicts the definition of allowable ampacity, which says already clarifies that it's talking about a continuous circuit.
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This is not specific to NM cable. It is odd that for a 40A continuous load, you must use a wire that is rated for 50A continuous, and that a wire rated for 40A continuous does not suffice.

The oddity is attributable to the limitations of OCPD. If you had a 40A 100% rated breaker, then you could supply a 40A continuous load with a 40A continuous rated wire. But 40A 100% rated breakers don't exist. A regular 40A breaker may nuisance trip on 40A continuous (the breaker is tested to hold in free air at 40C on 40A continuous; but once you put it in an enclosure with other breakers, you have other sources of heat which will affect the breaker performance). So you are required to use a 50A breaker.

And now your 40A continuous rated wire is too small to be properly protected by a 50A breaker (you would have a 240.4 violation). So you are required to upsize your wire to 50A. [I actually think you should only be required to upsize your wire to 46A, which per 240.4(B) could be protected at 50A. I submitted a PI for the 2026 NEC to that effected.]

Cheers, Wayne
 
tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
wwhitney said:
[I actually think you should only be required to upsize your wire to 46A, which per 240.4(B) could be protected at 50A. I submitted a PI for the 2026 NEC to that effected.]
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Wyane how many PI's did you submit?
Is the 46 amps because 45 is a standard size breaker?
Seems like that would already be allowed no?
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
tortuga said:
Wyane how many PI's did you submit?
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Hmm, I hadn't counted before, but it comes to 35 PIs. I think I posted them all to the PIs forum here (although some are the same idea across multiple articles, which I only posted once).
tortuga said:
Is the 46 amps because 45 is a standard size breaker?
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Precisely.
tortuga said:
Seems like that would already be allowed no?
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No, not per the text of 210.19(A)(1)(b). The ampacity is required to be at least 125% * the continuous load, which is completely unnecessary. It should suffice that the ampacity be large enough for the conductor to be protected by the required 125%-sized OCPD, and that the ampacity be at least equal to the load.

Cheers, Wayne
 
B

Birken Vogt

Senior Member
Location
Grass Valley, Ca
How did it come about that the 60C NM limitation was removed from SE cable about 10 years ago?

My supply house lists 8 Cu SER in the catalog, but it is not in stock - have to order 1000 feet.
 
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