Derating / breaker size

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nietzj

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My neighbor has an existing EV charger (48a max), it's landed on a 2-pole 60a breaker, fed with 2# 6's THHN and a #8 ground (no neutral needed) through a 3/4 pvc. Perfect. He asked if he could just add another 20A receptacle using the same raceway. The #6's just pass through the deep 1900 box (no splices) and then on to the fixed wall charger so volume shouldn't be an issue. This would be a single receptacle and there would now be a total of 4 current carrying conductors so he would have to derated to 80%. My question is would he be able to use the existing 60a breaker and land the receptacle on a new 20a breaker?
 
As long as you are using THHN (or any 90°C conductor) after derating you can still use your 60 amp and 20 amp breakers.
(#6 90°C ampacity = 75 amps X .8 = 60 amps)
 
Augie has it exactly right. But this is a good example to illustrate a further point:

The #6 THHN would be fine for the application with up to 9 CCCs in the conduit. That gives a 70% derating factor, so at first you might think that as the 48A is a continuous load, it's a problem that 70% * 75A (the 90C ampacity of #6) = 52.5A is smaller than 60A = 48A * 125% (continuous use factor). But we are never required to apply both the "conditions of use" factors and the continuous use factor at the same time. Rather the necessary checks would be

1) Termination: 65A (75C ampacity at terminations) >= 48A * 125% (continuous use factor)
2) Run: 75A (90C ampacity) * 70% (CCCs) = 52.5A >= 48A
3) OCPD: 52.5A ampacity may be protected by 60A OCPD per 240.4(B)

Cheers, Wayne
 
wwhitney said:
Augie has it exactly right. But this is a good example to illustrate a further point:

The #6 THHN would be fine for the application with up to 9 CCCs in the conduit. That gives a 70% derating factor, so at first you might think that as the 48A is a continuous load, it's a problem that 70% * 75A (the 90C ampacity of #6) = 52.5A is smaller than 60A = 48A * 125% (continuous use factor). But we are never required to apply both the "conditions of use" factors and the continuous use factor at the same time. Rather the necessary checks would be

1) Termination: 65A (75C ampacity at terminations) >= 48A * 125% (continuous use factor)
2) Run: 75A (90C ampacity) * 70% (CCCs) = 52.5A >= 48A
3) OCPD: 52.5A ampacity may be protected by 60A OCPD per 240.4(B)

Cheers, Wayne
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I have encountered inspectors for PV systems who just cannot seem to grasp this.
 
ggunn said:
I have encountered inspectors for PV systems who just cannot seem to grasp this.
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2020 NEC 690.8(B) does a good job of elaborating the distinction between what I called the Termination and Run checks. I'm surprised that's not clear enough. I have several PIs for the 2026 NEC to use the 690.8(B) terminology/format in articles 210, 215, and 230 as well.

Cheers, Wayne
 
Put me on the list of inspectors who fail to grasp the concept.
210.19(A)(1)(b) states the conductor shall have an ampacity 125% of the continuous load which leads me to believe a 48a "continuous" load would require a conductor with an ampacity of 60 amps.
Is it due to the wording that "the overcurrent device shall be sized for continuous" as opposed that the load is actually continuous ?
 
augie47 said:
Put me on the list of inspectors who fail to grasp the concept.
210.19(A)(1)(b) states the conductor shall have an ampacity 125% of the continuous load
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I think you mean 210.19(A)(1)(a) in the above. Here's both (a) and (b), from the 2017 NEC (easiest to copy and paste):

(a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
(b) The minimum branch-circuit conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.
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A strict reading of the above, since the term ampacity as defined in Article 100 includes any adjustment or correction factors, yields two oddities: (1) part (b) is totally redundant, as 125% of the continuous load, plus 100% of the noncontinuous load, will always be greater than 100% of the load and (2) the phrase "after the application of any adjustment or correction factors" in part (b) is totally redundant, since the word ampacity implies that already.

Those are both clues that (a) is using the word ampacity improperly, and what it really means is "ampacity before application of any adjustment or correction factors." That should be written explicitly, but it is still implicit, given the use of the phrase in part (b) but not part (a). That understanding makes the two requirements distinct.

The intended difference is clearer in the version of the above requirement that appears in 690.8(B), see it for comparison. Apparently the CMP for Article 690 are better language crafters than the CMP for Article 210.

Don and I both submitted PIs for the 2023 NEC to fix this misuse of terminology in 210.19(A)(1)(a), but the CMP response was basically "not required, it already says that." Even though that is clearly wrong.

Cheers, Wayne
 
wwhitney said:
Don and I both submitted PIs for the 2023 NEC to fix this misuse of terminology in 210.19(A)(1)(a), but the CMP response was basically "not required, it already says that." Even though that is clearly wrong.
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They should put it back to how it used to read. It made more sense.

The conductor shall have an ampacity for the load served, and the minimum conductor size shall be X based on continuous loads.

Don't know why they ever changed it.
 
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