That's fair. I should say that Romex brand NM from Southwire uses THHN/THWN. Other brands may do differently. The Southswire engineer I talked to said, almost exactly, "It's THWN today, but tomorrow it could be something else because the standard is so broad. But at Southwire right now, it's the same stuff you pull in conduit." He made very sure to let me know not to assume it was THHN/THWN, but at the time of our conversation, it was.
Is #6 NM on a 60A breaker code compliance for a Tesla 48A (continuous load) EV charger?
- Thread starter sunbear
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curt swartz
Electrical Contractor - San Jose, CA
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- San Jose, CA
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Are you referring to USE cable?
SEU and SER use XHHW or THHN/THWN, definetly not RHW.
XHHW was the standard for a long time but in recent years some manufactures have changed to THHN/THWN.
Birken Vogt
Senior Member
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I have seen some marked XHHW, and some THWN-2, and some unmarked, within the past year or so.
Right my bad , I just took a look at UL 854 section 14.1 it appears to allow SE cable to be a few NEC wire types, THHN / THWN-2 XHHW or RHH/RHW-2 it does not appear to allow the wiggle room that NM cable has, it has to fully comply with UL 44.
kwired
Electron manager
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Yet we can start ampacity adjustments with 90C ampacity as long as it is type NM-B which is all that has been made for 35-40 years now so is only a true 60C conductor for older cables when you encounter those.
"IF" we allow #6 NM cable to be used in this manner for EVSE, then, would we be able to apply this same kind of thinking for any other continuous loads such as an electric water heater? The resistive elements in a water heater will never draw more than they are actually designed for. This seems to be the apparent logic from DCA.
kwired
Electron manager
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IMO if they allow it for one thing they basically are saying we no longer have a 60C conductor and therefore should allow it for any other load of similar amp draw. One common place that could get you over the years that many didn't realize is air handlers with heat strips. Most of what were nominally called 10 kW were actually 9.6 kW, and adding a blower motor to the mix usually still left you with MCA below 55 which allowed you to have 6 AWG @ 60C on a 60 amp breaker. Bump the amps up just a little bit by either the blower or the heat strip and it put you over 55 but under 60, still good if using 75C ampacity table but not enough if you had NM cable and had to use 60C ampacity table.
NM cable seems to be the wrong tool for the job, its meant to be low cost, every day typical light duty residential installs, nothing wrong with that and I'd imagine changing the PVC formula any higher than 60C (140F) ampacity to be a unnecessary added manufacturing cost that would raise the price of NM across the board.
EV's and other larger continuous residential type loads that need a cable wiring method should look at SE, Tray or MC cable.
I have proposed before that copper 6 SE cable would be best as it would have a 65A rating, good for any 50A continuous load, but some manufacturers don't make it, and the ones that do have small production runs, due to lack of demand.
So if we all go out there and demand it they will start making more of it.
EV's and other larger continuous residential type loads that need a cable wiring method should look at SE, Tray or MC cable.
I have proposed before that copper 6 SE cable would be best as it would have a 65A rating, good for any 50A continuous load, but some manufacturers don't make it, and the ones that do have small production runs, due to lack of demand.
So if we all go out there and demand it they will start making more of it.
wwhitney
Senior Member
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Eh, the listing standard for NM cable requires an insulation formulation rated for 90C. See this post for an excerpt of UL 719 on NM cable:
Type NM in EMT
I believe they are approved but aren't labeled or marked. There is a difference IMO My recollection is the inner conductors are not a defined wire type. If you look at the product standard (UL 719) it gives several options for the insulation but dances around calling then "THHN"
forums.mikeholt.com
The lack of separate listing and labeling of the inner conductors is purely a cost savings measure. The only justification I see for the NEC treating NM cable differently from SER cable would be a claim that its overall durability is lower, and therefore its ampacity should be reduced.
Would be interested to know if the outer jacket of SER cable is thicker/tougher than NM cable. If so, it's a matter of judgement whether that reduced thickness/toughness justifies a reduction in ampacity.
Cheers, Wayne
wwhitney
Senior Member
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P.S. It is odd that UL 719 4.4.1 prohibits marking the inner conductors if they are made to THHN standards, or made to TW standards using PVC. If a manufacturer wants to mark their NM inners, they must use an insulation that is to TW standards and not PVC. I wonder if anyone does that.
Cheers, Wayne
Cheers, Wayne