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
- Location
- San Jose, CA
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- Electrical Contractor
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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- Grass Valley, Ca
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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- EC
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
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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
If your looking at the UL standards look at the ones for for SE, MC and Tray cable I believe they all reference UL 44 but I have not checked in years, basically it says 'use a NEC wire type in your cable'.
The UL standard for NM requires a very basic PVC that does not go thru all the tests for THHN or TW, the requirements of UL 44 are not met. NM is good only short-term exposures to 90°C , definitely not 75°C terminations, unless they do happen to use real THHN as @CoolWill suggests, but there is no way to rely on that.
NM is not required to have any of the additional plasticizers that real THHN has to withstand 75°C terminations.
There are some promising new forms of tray cable that were targeting residential they were approved as 'joist pull', so installers can use it without a cable tray. I am not sure if its listed for 75°C but it should be if its not, as try cable is usually THHN/THWN-2.
Stranding is nicer than NM, for example you can get more sizes of Joist pull tray cable in stranded just like you can with MC.
I just checked and apparently the type TC-ER-JP cable is only 75°C terminations on generators, which is bizarre. That would be low hanging fruit for a PI, I mean if its rated 75°C its rated 75°C generator or not.
If manufacturers are making a cable targeting just generators I am sure they would make a 6/3 TC-ER-JP for EV charging.
If manufacturers are making a cable targeting just generators I am sure they would make a 6/3 TC-ER-JP for EV charging.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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With a neutral, I hope.
sunbear
Member
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- New Jersey
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So is it safe to use #6 Romex for an EV Charger (or for any other continuous load device like a heater for that matter)?
What is the worst that could happen? Wondering if the PVC could melt and the wires could short and start a fire inside the wall.
An electrician is installing the wiring to support a 48A Tesla charger today at my friend's house. They are using Southwire "6-3 CU NM-B WITH GROUND" behind the sheetrock in the basement, from the main electrical panel to a service box on the outside of the house. Presumably this makes the job a lot easier (cheaper) for the electrician than using 4/3 NM or 6/3 MC behind existing sheetrock.
FYI - I'm interested in looking out for my friend's (the homeowner's) best interest here rather than the electrician.
The specification PDF (https://cabletechsupport.southwire.com/en/cablespec/download_cable/?cable=26863&country=US) mentions both 60C and 90C. Does this mean that the insulation around the wires themselves (not the sheathing that wraps the 3 wires plus ground together) will be able to withstand 90C without melting but the sheathing could melt because it's rated for a lower temperature?
What is the worst that could happen? Wondering if the PVC could melt and the wires could short and start a fire inside the wall.
An electrician is installing the wiring to support a 48A Tesla charger today at my friend's house. They are using Southwire "6-3 CU NM-B WITH GROUND" behind the sheetrock in the basement, from the main electrical panel to a service box on the outside of the house. Presumably this makes the job a lot easier (cheaper) for the electrician than using 4/3 NM or 6/3 MC behind existing sheetrock.
FYI - I'm interested in looking out for my friend's (the homeowner's) best interest here rather than the electrician.
The specification PDF (https://cabletechsupport.southwire.com/en/cablespec/download_cable/?cable=26863&country=US) mentions both 60C and 90C. Does this mean that the insulation around the wires themselves (not the sheathing that wraps the 3 wires plus ground together) will be able to withstand 90C without melting but the sheathing could melt because it's rated for a lower temperature?
If your friend cares about legality, warranty or the validity of their insurance policy the EVSE should be adjusted accordingly to not excede the 80% of the NEC ampacity of the wire, anything other than 'code' is mere speculation it might be fine it might not, but if there is an issue that results in a claim you can bet the insurance will try to weasel and blame the electrician and go after their insurance. Other than that were just some people on the internet.
The Tesla charger is capable of being deprecated to match the circuit supplying it. In this case, it should be set to 40A and the circuit breaker should be 50A.
The worst that happens is some guys on the internet talk about the finer points of PVC granules.
kwired
Electron manager
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- NE Nebraska
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- EC
How easy is it for the user to set it higher afterwards? Guess putting a 50 amp breaker on it does give it a certain amount of limitation but if the max it will draw is 48 amps, you probably won't trip a 50 amp breaker very often or maybe never will at all.
This is the thought process...
The calculated answer that requires no Tesla restrictions or NEC exceptions is no. Since the 48A charger is a continuous load, that must be on a circuit that can handle 60A (48A * 125%). 6 AWG NM-B is rated for 55A. So, the largest breaker that can be installed on this is 55A.
But, there is an NEC exception that says if you calculate, and end up with an odd size breaker, you can move up to the next size breaker. So, if you consider 55A an odd size, you can move to 60A. A 55A breaker is considered an odd size (even though they are available for most common residental panels). My guess is this is what the install will look like. BTW, this is essentially what the State of NJ has said is ok which started this thread.
You can always program the Tesla charger to not exceed 40A and use a 50A breaker (40A * 125% = 50A). If you do this, you have not used the odd size rule to bump up the breaker.
Greentagger
Senior Member
- Location
- Texas
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- Master Electrician, Electrical Inspector
Just curious, how is an inspector to determine that XLPE is actually RHW when it is not marked or identified on conductor as required per 310.8?