MikeD
Posts: 616
Joined: Thu Jun 24, 2010 6:14 am
Delivery Date: 12 May 2011
Leaf Number: 592

Re: Nissan finally gets it with new 240V portable EVSE

Thu Jan 18, 2018 9:52 pm

wwhitney: The following manufacturer's data sheet lists allowable ampacities for NM-B 8/2 and NM-B 8/3 as 40a, not 50a, together with the statement "Type NM-B ampacity limitation shall be in accordance with the 60ºC conductor temperature rating, as specified in the NEC":

https://www.encorewire.com/wp-content/u ... re-NMB.pdf

Am I misunderstanding something? Thanks for all your valuable and informative posts here over the years, btw!

wwhitney
Posts: 715
Joined: Sat Jul 17, 2010 10:10 am
Delivery Date: 01 Apr 2011
Location: Berkeley, CA

Re: Nissan finally gets it with new 240V portable EVSE

Thu Jan 18, 2018 11:42 pm

No, you're not missing anything, did something earlier in the thread give you a different impression?

NM cable cable is limited to the 60C ampacity for historical/political/robustness issues. Other wiring methods can typically use the 75C ampacity if used with 75C rated terminations. For #8 copper, the 60C ampacity is 40A, and the 75C ampacity is 50A.

Cheers, Wayne

MikeD
Posts: 616
Joined: Thu Jun 24, 2010 6:14 am
Delivery Date: 12 May 2011
Leaf Number: 592

Re: Nissan finally gets it with new 240V portable EVSE

Fri Jan 19, 2018 4:42 am

wwhitney: Yes, your insistence that "The only part of a (properly installed) 40 amp circuit that isn't rated for 40 amps continuous is the circuit breaker itself.".

I did not want readers of this thread to get the mistaken impression that that the NEC currently supports your position that the very commonly used NM-B 8/x cable was "rated 40 amps continuous" when it is appears evident (from NEC article 334.80 "Nonmetallic-Sheathed Cable Types NM, NMC, and NMS/Ampacity" etc) that it does not.

Now, the reason for this (as is the case for many rules in the NEC) is not clear, certainly not to me. I usually assume that it is due to discovery from experimental data and/or experience data over time that there there is a substantial risk of fire and/or shock hazard to rule otherwise.

wwhitney
Posts: 715
Joined: Sat Jul 17, 2010 10:10 am
Delivery Date: 01 Apr 2011
Location: Berkeley, CA

Re: Nissan finally gets it with new 240V portable EVSE

Fri Jan 19, 2018 7:16 am

MikeD wrote:I did not want readers of this thread to get the mistaken impression that that the NEC currently supports your position that the very commonly used NM-B 8/x cable was "rated 40 amps continuous" when it is appears evident (from NEC article 334.80 "Nonmetallic-Sheathed Cable Types NM, NMC, and NMS/Ampacity" etc) that it does not.

Ah, you're still missing the point. When a conductor has a 40 amp rating, that is a continuous rating. Again, it is only the circuit breaker whose rating is not continuous, and that is the reason everything gets upsized by a factor of 125%.

Cheers, Wayne

goldbrick
Posts: 93
Joined: Tue Aug 01, 2017 3:33 pm
Delivery Date: 01 Aug 2017
Leaf Number: 311806
Location: Colorado front range

Re: Nissan finally gets it with new 240V portable EVSE

Fri Jan 19, 2018 12:25 pm

MikeD wrote:wwhitney: Thanks for all your valuable and informative posts here over the years, btw!


+1

MikeD
Posts: 616
Joined: Thu Jun 24, 2010 6:14 am
Delivery Date: 12 May 2011
Leaf Number: 592

Re: Nissan finally gets it with new 240V portable EVSE

Sat Jan 20, 2018 3:59 pm

wwhitney: Yes, I forgot that the basic NEC definition of ampacity involves maximum CONTINUOUS current not just maximum current (more specifically "Ampacity: The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."), so thank you for being patient in reminding me of that.

I read an explanation of some of the reasoning behind Article 210.10 (A)(1) (...the minimum branch-circuit conductor size...shall have an allowable ampacity not less than ...125 percent of the continuous load...), which readers of this thread might be interested as well, although I am not certain that it applies with a circuit load connected via a receptacle.

Briefly, "The manufacturer [of a connected electrical device] ... [may] count on a relatively cool conductor to function as a heat sink for heat generated within the device under these continuous operating conditions.".

This is a subtlety that I, for one, rarely think about. However, I am all for better understanding of reasons behind important safety rules.

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