[Kirby64]

What's your math on this? I don't understand how you're getting to 37A.

A 50 amp rated outlet (backed by a 50 amp rated breaker, and wire capable of handling 50 amps) is fully NEC compliant if you run it for 40 amps continuously (80% derating) or 50 amps temporarily (which NEC defines as a load expected to continue for 3 hours or more). EV chargers fall under the 80% derating. I'm not aware of any EV charger that uses 37A. They either support 40A (80% of 50A), 32A (80% of 40A), or 48A (80% of 60A).

[egberts1]

I did take liberty here in saying 37A-ish (40A is code) because that's what some national fire marshalls are reporting about hot garage and reuse of range-type wiring for new purpose of EV charging.

It's the wire material and size issue in hot area.

[Kirby64]

But, if it supports 40A charging it'll charge at 40 amps unless you back off the charging rate (which nobody does, typically). You'd always assume 40 amps for circuit purposes.

[egberts1]

See @quickthrowman's comment for detailed breakdown on high-temperature and copper wire.

[quickthrowman]

> What's your math on this? I don't understand how you're getting to 37A

You have to derate conductors installed in ambient temperatures greater than 86F. I would definitely use NM-B 6/2G (or 6/2G MC cable) for a NEMA 14-50R in a garage, just to account for derating since you get to start at 75A for #6 rated for 90C.

The first derating factor is .94 for ambient temps between 87-95F, .94 times 40A is 37.6A if you use the 75C column. I think NM cable is rated for 90C, so if you derate #8 from 55A at .67 to account for a much higher temp of 120-130F, you get 36.85A.

310.15 (B)(1)

Ambient Temperature Correction Factors Based on 30°C (86°F)

For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities specified in the ampacity tables by the appropriate correction factor shown below.

https://conduit.site/tables/table-310.15-b-1.php

[Kirby64]

I’m not disputing that you have to derate conductors based on excess temperature according to NEC. I’m saying that no EVSE exists that will pull 37A. It’ll pull either 32A or 40A and needs wiring and a breaker backing it that either is 40 or 50 amp rated, respectively.

Also, Romex (NM-B) is always considered to be 60C rated, and you cannot assume 90C rated conductors (in the case of a properly rated 90C conductor like THHN) because the outlet and breaker must also be matched and capable of 90C. No 40-50A breaker I’m aware of is 90C rated. They’re typically 75C, and sometimes 60. Likewise, no NEMA 14-50 outlet I’ve ever seen is 90C rated. They’re all 75 or 60C. The crappy Leviton one mentioned in the article is 60C. That means everything is essentially treated as 60C for derating purposes besides derating factors for things like bundled conductors.

In comparison, Bryant and Hubbell outlets (the more expensive ones EV forums tend to recommend) are 75C rated, so you can usually use the 75C derating tables for everything, since breakers are THHN are typically capable of 75C. MC cable would also be 75C rated, as well.

[egberts1]

Thanks! The further clarification and use of terminology is much needed here. Plus I needed that citation!