NEC 2017

[tom baker]

Got it. Many thanks!

Ron

This section has been changed for the last couple of code cycles. But now the CMP seems to get that if you increase the size of the UG conductors, that can deliver more short circuit current and the EGC needs to be larger as pointed out. If you want a good understanding of the issue, download this program, GEMI https://steeltubeinstitute.org/reso...nd-simplifies-protective-power-system-design/

 

[tom baker]

any resemblance between any of sting's first post and the actual "situation" is purely accidental

Yep if we had all the info these posts would have one or two answers and be done.

 

[rc/retired]

This section has been changed for the last couple of code cycles. But now the CMP seems to get that if you increase the size of the UG conductors, that can deliver more short circuit current and the EGC needs to be larger as pointed out. If you want a good understanding of the issue, download this program, GEMI https://steeltubeinstitute.org/reso...nd-simplifies-protective-power-system-design/

I appreciate that. Thank you

Ron

 

[hhsting]

In addition to what augie said, note the difference in 210.19(A) between (1) and (2). Part (2) is explicitly after derating for conductor count and temperature, so implicitly part (1) is before derating. For part (1) you'd compare 70A (if 60C terminals in the EVSE) (and no 70% factor) against 125% * 48A = 60A, which is fine. And for part (2) you'd compare 70A (if 60C insulation, which would be unusual) * 70% = 49A against 48A (no 125% factor), and that's also fine. For this scenario, with 60C terminals and 60C insulation temperature, #4 would be the smallest size allowed, so 250.122(B) would not require upsizing the EGC; you could use a #10 EGC.

However, if you have 75C terminals (more likely) and 90C temperature insulation (quite likely), then #6 Cu would be sufficient: for part (1) 65A (75C ampacity) > 125% (continuous load) * 48A = 60A. And for part (2) 75A (90C ampacity) * 70% (conductor count derating) = 52.5A > 48A. So in this case, since there are #4 ungrounded conductors instead #6 ungrounded, the minimum EGC size is #8 instead of #10, per 250.122(B).

But you listed a #6 EGC, so that's certainly large enough.

BTW, the information about 8 CCCs in one conduit would have been helpful to include in the first post.

Cheers, Wayne

So whats the deal with part 2 you dont do 125%. Conductor count derating and temperature does not apply to continuous load? Whats the idea behind that

 

[wwhitney]

So whats the deal with part 2 you dont do 125%. Conductor count derating and temperature does not apply to continuous load? Whats the idea behind that

Basically, the 125% factor only applies at the terminations. Ampacity is a continuous rating (see the Article 100 definition), so the wire itself doesn't need the 125% factor. It's just the equipment that may need it, as most breakers do.

In contrast, derating for the number of CCCs doesn't apply at the terminations, perhaps on the assumption that terminations are in an enclosure where the conductor heat can dissipate more easily. Likewise for ambient temperature.

So you do two separate checks, one with the 125% factor for continuous loads and no derating (and based on the termination temperature limitation), and one without the 125% with derating (and based on the insulation temperature rating). Thus never the 125% factor and derating at the same time.

Cheers, Wayne