Romex and Wall Charger revisited @wwhitney

[Jschoef]

@wwhitney ,
I have seen your posts in various forums and you seem like the best person to talk to about the Tesla Wall Connector. I installed one with 30ft of 6-2 NM cable. It is exposed. Without talking about code, I am wondering if it is safe? Running at 48amps to a 60amp breaker, it is barely getting warm.

Now talking about code, if I run the charger at 44amps, does that make it code compliant being it is rated for only 60deg? The manual states 90deg if running it at maximum power. 44amps would make it at the 20% rule of 55amps.

The other idea I had was to strip the jacket off the NM and run 3/4 flex conduit. Specs state it is THHN OR THWN-2…unmarked though.

Please let me know what you think?

 

[wwhitney]

NM cable is limited to the 60C ampacity, so #6 Cu has an ampacity of 55A. Stripping the jacket and replacing it with a conduit method doesn't help you, as the inner conductors are unmarked and not suitable for running in conduit. Switching to a conduit method (other than LFNC) and using #6 stranded THHN/THWN would allow you to use the 75C ampacity of 65A.

For a continuous load, the NEC requires both a breaker and conductor ampacity of at least 125% of the load. The breaker, however, is allowed to be sized up to the next standard size when the ampacity does not correspond to a standard size. So with 55A ampacity, you can use a 60A breaker and set the EVSE to 44A (or the next lowest setting).

Now, as it happens, I think it is unnecessary for the NEC to require a 125% factor on the conductor ampacity, as the ampacity is already a continuous rating. [And when a special 100% rated breaker is used, there's no need to upsize either breaker or wire, which shows that the wire does not need upsizing on account of the load itself.] The breaker still needs the 125% factor because of the limitations of breakers, and the ampacity still needs to be above the next lower size of breaker. The case of 6/2 NM to an EVSE is actually the smallest example where this distinction matters.

So I think it should be allowed to run your EVSE at 48A, but it is not. Set it to 44A.

Cheers, Wayne

 

[Jschoef]

@wwhitney, Is it safer to run the internal wire in conduit being it is rated for THHN/THWN without taking code into account? Does running the NM 6-2 at 44amps make the 90deg spec on Teslas instructions go away as it’s being underpowered?

Your knowledge seems to be above most out on the net regarding these things…are you an electrician?

 

[wwhitney]

Modern NM has conductors with a 90C rating inside of it, so even though it is limited to a 60C ampacity, I would say it complies with the TWC's "requirement" for 90C conductors.

Also, my knowledge is principally about the NEC, so I'm not in a position to comment without taking code into account.

Cheers, Wayne

 

[Jschoef]

Modern NM has conductors with a 90C rating inside of it, so even though it is limited to a 60C ampacity, I would say it complies with the TWC's "requirement" for 90C conductors.

Also, my knowledge is principally about the NEC, so I'm not in a position to comment without taking code into account.

Cheers, Wayne

Quoted from NM-B spec sheet: “Type NM-B ampacity limitation shall be in accordance with the 60C conductor temperature rating, as specified in the NEC.” Does this contradict your opinion on the 90C “requirement”?

Does this mean you are an electrician or are you trying not to divulge this info about yourself on the net ?

 

[wwhitney]

Quoted from NM-B spec sheet: “Type NM-B ampacity limitation shall be in accordance with the 60C conductor temperature rating, as specified in the NEC.” Does this contradict your opinion on the 90C “requirement”?

Nope. From 334.80 of the 2017 NEC:

"334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final calculated ampacity does not exceed that of a 60°C (140°F) rated conductor."

The conductors inside are 90C conductors just like most modern wiring methods; the NEC just restricts the ampacity to the 60C rating instead of the usual 75C rating, out of an abundance of caution, or because NM doesn't get any respect, or whatever reason.

Cheers, Wayne

 

[Rocky_H]

the NEC just restricts the ampacity to the 60C rating instead of the usual 75C rating, out of an abundance of caution, or because NM doesn't get any respect, or whatever reason.

The "whatever reason" is the same reason why you stay warmer when you wrap your body in a coat rather than not having a coat. NM-B has a tight rubber insulating sheath wrapped around that bundle of wires that traps the heat in, making them warm up more, so they have to reduce the current rating.

 

[wwhitney]

The "whatever reason" is the same reason why you stay warmer when you wrap your body in a coat rather than not having a coat.

While I'm sure there's an effect, I expect that the magnitude is much less than 15C. SER has an outer covering, for example, and it's not subject to the same limitation.

It's either a bias against NM or an extra safety margin given how widely used NM is. And the latter goal would be accomplished by restricting NM to the 60C rating only for #10 and smaller, say.

Cheers, Wayne

 

[Rocky_H]

It's either a bias against NM

Stop with the bias conspiracy theory bull$#&^!! It's an object. National Electric Code board would not have some angry emotional bias against an object!

While I'm sure there's an effect, I expect that the magnitude is much less than 15C. SER has an outer covering, for example, and it's not subject to the same limitation.

Fine--here's the other thing. It's where Romex is intended to be used. It is meant to be run through the inside of finished walls, trapped between sheetrock with fiberglass insulation all around it, which is intended to trap heat and with the air mostly trapped in there too. So it's not just the rubber sheath. This is placed in areas with trapped air that heats up and can't let it escape easily, so it is more subject to heat build-up. Conduit is mostly run in more open spaces--not inside walls--so it is exposed to outside ambient air, where that heat can be transferred away by convection and air movement. Satisfied now?

 

[wwhitney]

Stop with the bias theory

Look, there may be technical reasons to be biased against NM and require a greater safety margin. But the NEC is also written by humans, so there may be human factors incorporated into the requirements, not just technical ones.

For example, prior to the 2002 NEC there was a 3-story limit on the use of NM cable. My understanding is that the 3-story limit was a compromise during an earlier code making process between those who wanted to restrict NM cable to residential construction, and those who wanted to allow it in most uses without a height limit.

As to your factors about where NM cable is run, all of them apply to other cable methods not subject to a 60C limitation, and some of them apply to conduit as well.

Cheers, Wayne

 

[ChrisMPK]

NM cable is usually fastened with hammered in staples, which often pinch the conductors more closely together than any other type of MC or raceway run cables. This is one of the primary reason it is idiot-proofed with a 60C amperage rating. Building codes are meant to protect you from the dumbest, sloppiest trades-person. Not the best.

 

[Jschoef]

I couldn’t take the uncertainty and I bought 3/4” flex conduit and 3x 6awg. So I essentially made a code compliant cable and it wasn’t that hard to do.

 

[SilverString]

Generally speaking, questions of this type lead to contentious replies and are akin to stepping into a pile of dog poop. It's not that the replies are wrong or incorrect, it's that some people think like engineers and others think like electricians. Electricians tend to stick to code and there are good reasons for that as there are potential liability issues, however remote they may sometimes be. Engineers tend to look at specifications as they apply to the task at hand. Both views will get you lambasted from one side or the other.

If you understand that most all of the concerns are based on heat build up, then you can safely use specification combined with specific knowledge of the situation at hand. But if you are not inclined or interested in doing your own wiring, you should listen to the electricians and follow code, remembering that code tends to be black and white in nature, and is designed to address all situations in a safe manner.

Happy Holidays everyone!

 

[Jschoef]

So I do think my previous Romex installation was safe…cable barely got warm. I want to point out that getting 3/4” flex water tight conduit and 3 6awg THHN wires were the same cost. I thought the conduit setup was easier to install using pvc fasteners rather then hammer staples for Romex.

On a side note, the electrical supply house sold me the wrong wire twice. I asked for 90deg, 60+ amp rated cables. First they sold me temporary welding cable rated 55amps, then the Romex and told me it was rated 75amps, then I made my own. It was the principal of it all to make them make it right also.

 

[wwhitney]

S 3/4” flex water tight conduit

LFNC or LFMC? All indoors? UL listed?

Cheers, Wayne

 

[wwhitney]

LFNC or LFMC? All indoors? UL listed?

To expand on this, some (all? most?) LFNC and LFMC is only rated to 60C in wet locations or locations exposed to Oils, 80C in dry locations. And some is not UL listed, intended for use by power companies, telephone companies, etc. But as long as the install is indoors and you got a UL listed product, you should have a valid 75C rated wiring method.

Cheers, Wayne

 

[Jschoef]

To expand on this, some (all? most?) LFNC and LFMC is only rated to 60C in wet locations or locations exposed to Oils, 80C in dry locations. And some is not UL listed, intended for use by power companies, telephone companies, etc. But as long as the install is indoors and you got a UL listed product, you should have a valid 75C rated wiring method.

Cheers, Wayne

Honestly not sure…it’s indoors, exposed and sold by an electrical supply house. It looks like it is Southwires NM UL conduit.

 

[wwhitney]

indoors . . . Southwires NM UL conduit.

Then you're good, it should be a compliant 75C wiring method.

Cheers, Wayne