Is #6 NM on a 60A breaker code compliance for a Tesla 48A (continuous load) EV charger?

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sunbear

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I had the following useful discussion a couple of years back on a similar topic where the conclusion was that #6 NM on a 60A breaker was not code compliant

Tesla charging station

Thanks for everyone's helpful advice so far. Does anyone know any electricians in the Essex County, New Jersey area who would be willing and capable of running conduit through studs? If so, please can you send a direct message to me with the contact details? The electricians I've spoken to so...
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I was today sent this note from the NJ which comes to the opposite conclusion
One of the more frequent questions we get in the Code Assistance Unit relates to the use of Non-metallic sheath (NM) cable for EV chargers. Specifically, the use of #6 CU on a 48-amp load. There appears to be a significant portion of code officials with a difference in opinions on how we should apply the requirements found in Article 625 of the National Electrical Code (NEC), with continuous load designation and NM cable. First let’s start with Article 625; Article 625.41 requires the overcurrent protection device (OCPD) to be sized for “continuous duty,” thus applying a factor of 125% to the full load of the equipment. “Continuous Duty” is a term used to alert installers that considerations must be made to help dissipate excessive heat when a substantially constant load is present for an indefinitely long period of time. This increase is a “cushion” to help the circuit components deal with this heat. There is NO additional current on this circuit.

Table 310.16 of the 2020 NEC shows 6 AWG having an ampacity of 55 amps at 60°C (See 334.80 for ampacity limitations). "Ampacity," as defined by the NEC, is “The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.” Based upon table #6, NM can safely carry 55 amps WITHOUT exceeding the temperature rating of the conductor at 60°C. Considering most devices and OCPD’s have dual rated lugs with temperature limitations of 60°C/75°C, limiting the ampacity to 55 amps or less will inherently limit the temperature of any device to be well within its listed operating temperature. Which means, applying the 125% to a conductor which is already limited by 60°C does nothing more to protect the equipment or the conductors in the circuit. In the 2020 NEC, a change to Article 334.80 was made from “allowable ampacity” to just “ampacity." This change seems to imply that the ampacity safely carried by NM cable is dictated by the actual load and not OCPD. Furthermore, the provisions of Article 240.4 for applications under 800 amps do not exclude NM cable. Therefore, the opinion of the Code Assistance Unit is that #6 NM cable should be permitted to be utilized for EV chargers with loads not exceeding 48 amps. Source: Scott Borsos Code Assistance Unit (609) 984-7609
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What do people think about this?
 
Below is my PC on the question for the 2026 NEC Second Draft, along with the commitee's response. Seems to me the comittee did not address the technical merits of my proposal, so I doubt they properly considered it.

Cheers, Wayne

Public Comment No. 1609-NFPA 70-2024 [ Section No. 334.80(A) ]

(A) General.
The ampacity of Types NM and NMC cable shall be determined in accordance with 310.14. The 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, or for termination requirements, provided the final calculated ampacity does not exceed that of a 60°C (140°F) rated conductor. The ampacity of Types NM and NMC cable installed in cable trays shall be determined in accordance with 392.80(A).

Statement of Problem and Substantiation for Public Comment

The CMP's informative response to PI 494-NFPA 70-2023 has led me to the understanding that the issue raised in that PI is best addressed by amending this section of Article 334. The purpose of this change is to allow the use of NM with equipment with a termination temperature requirement above 60C, in recognition of the 90C rated conductors within the NM cable, without changing the overall limit of NM cable to the 60C ampacity.

So consider a 48A continuous load (such as an EVSE, an increasingly common new installation) installed with a 60A overcurrent device with 60C/75C rated terminations, and supplied by NM cable. What size NM cable is required?

The conductor size selected is driven by the requirements of 210.19(A)(1)(a) and (b), as well as 240.4. According to 334.80(A), the calculated ampacity of a 6/2 NM cable is limited to 55A, and that of a 4/2 NM cable is limited to 70A, per the 60C column of Table 310.16. Whenever this limit controls, rather than the ampacity being even lower due to the necessary adjustment and correction starting with the 90C Table 310.16 ampacity, 6/2 NM cable complies with 210.19(A)(1)(b), as its 55A ampacity exceeds the load of 48A. In such circumstances it also complies with 240.4(B) with respect to the 60A OCPD required by 210.20, as that is next standard OCPD size larger than its ampacity of 55A.

But as currently understood, 6/2 NM would never comply with 210.19(A)(1)(a), which calls for a conductor with an ampacity of 60A before ampacity adjustment and correction. This causes such installations to require 4/2 NM cable.

However, what is the idea behind 210.19(A)(1)(a)? This is a termination requirement, and it recognizes that equipment terminations may rely on the heat-sinking effect of the connected conductors to control termination temperature. The listing standard for equipment, such as UL 489 for circuit breakers, specifies the size of conductor to use in the heat rise test based on the termination temperature rating of the equipment. For a 60A OCPD, if the termination is rated 60C, it requires testing with #4 copper conductors, while if the termination is rated 75C, it requires testing with #6 copper conductors.

So a breaker with 60C/75C terminations has been tested with #6 copper conductors not to overheat. The terminations may rise in temperature to 75C, but as per the allowance for NM ampacity adjustment and correction based on the 90C rating, NM and its conductors are rated for such a temperature. [In a sufficiently hot attic, where the 90C ampacity after adjustment and correction controls, if the conductor is loaded to its full ampacity, the conductor temperature is allowed to rise to 90C.]

Thus we see that using 6/2 NM cable on a 60A breaker rated 60C/75C with a 48A continuous load will provide the necessary level of heat sinking at the breaker termination as per the UL testing standard and will not cause the NM conductors to exceed their rated temperature. Moreover, the load is still less than the 60C ampacity as required by 334.80(A).

Therefore this installation with a 60C/75C rated breaker and 6/2 NM cable should not be prohibited by the combination of 210.19(A)(1)(a) and 334.80(A); the prohibition is not supported by the physics or by testing limitations. Given the reasons behind 210.19(A)(1)(a) the best way to allow this installation is to adjust 334.80(A) to allow the higher temperature rating to be used for termination requirements. Note that the proposed change would still require 4/2 NM cable to be used where the equipment termination temperature rating is 60C.

With the ongoing electrification of the US automotive fleet over the next two decades, literally hundreds of thousands of 48A EVSEs will be installed on 60A circuits, many of which will be supplied by NM cable. Enacting this change will provide significant economic savings and a reduction in unnecessary resource utilization.

Committee Action: Rejected

Resolution: The relationship between the cable/conductor temperature and the termination temperature is addressed in section 110.14(C). Section 310.14 governs ampacities and does not have an adjustment for termination requirements. See Informational Note No. 1 of section 310.14(A)(3).
 
The conductor insulation on NM cable is the lowest grade PVC that can pass, basically the cheapest plastic blend that can pass UL 719 as electrical insulation. In older NM cable you can sometimes see it marked as type 'TW' . If I recall correctly the PVC blend is only good for 65°C, with short-term exposures to 90°C tested. It does not have any of the additional plasticizers that real THHN / TWHN-2 has to withstand 75°C terminations.
 
tortuga said:
The conductor insulation on NM cable is the lowest grade PVC that can pass, basically the cheapest plastic blend that can pass UL 719 as electrical insulation. In older NM cable you can sometimes see it marked as type 'TW' . If I recall correctly the PVC blend is only good for 65°C, with short-term exposures to 90°C tested. It does not have any of the additional plasticizers that real THHN / TWHN-2 has to withstand 75°C terminations.
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I don't believe it. Besides being told otherwise by a Southside insider, you can see it with the naked eye that it is identical to the stuff on the reels. It even has the nylon condom that THWN has. Economically speaking, it just makes sense that they manufacture one type of wire. Some gets marked and goes on a reel and some gets put into NM.
 
Would it be fair to say that it is at the very least THN?

I think NM being in attics, embedded in insulation, etc. in residences are the main reasons.

1746854189711.jpeg
 
wwhitney said:
Below is my PC on the question for the 2026 NEC Second Draft, along with the commitee's response. Seems to me the comittee did not address the technical merits of my proposal, so I doubt they properly considered it.

Cheers, Wayne

Public Comment No. 1609-NFPA 70-2024 [ Section No. 334.80(A) ]

(A) General.
The ampacity of Types NM and NMC cable shall be determined in accordance with 310.14. The 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, or for termination requirements, provided the final calculated ampacity does not exceed that of a 60°C (140°F) rated conductor. The ampacity of Types NM and NMC cable installed in cable trays shall be determined in accordance with 392.80(A).

Statement of Problem and Substantiation for Public Comment

The CMP's informative response to PI 494-NFPA 70-2023 has led me to the understanding that the issue raised in that PI is best addressed by amending this section of Article 334. The purpose of this change is to allow the use of NM with equipment with a termination temperature requirement above 60C, in recognition of the 90C rated conductors within the NM cable, without changing the overall limit of NM cable to the 60C ampacity.

So consider a 48A continuous load (such as an EVSE, an increasingly common new installation) installed with a 60A overcurrent device with 60C/75C rated terminations, and supplied by NM cable. What size NM cable is required?

The conductor size selected is driven by the requirements of 210.19(A)(1)(a) and (b), as well as 240.4. According to 334.80(A), the calculated ampacity of a 6/2 NM cable is limited to 55A, and that of a 4/2 NM cable is limited to 70A, per the 60C column of Table 310.16. Whenever this limit controls, rather than the ampacity being even lower due to the necessary adjustment and correction starting with the 90C Table 310.16 ampacity, 6/2 NM cable complies with 210.19(A)(1)(b), as its 55A ampacity exceeds the load of 48A. In such circumstances it also complies with 240.4(B) with respect to the 60A OCPD required by 210.20, as that is next standard OCPD size larger than its ampacity of 55A.

But as currently understood, 6/2 NM would never comply with 210.19(A)(1)(a), which calls for a conductor with an ampacity of 60A before ampacity adjustment and correction. This causes such installations to require 4/2 NM cable.

However, what is the idea behind 210.19(A)(1)(a)? This is a termination requirement, and it recognizes that equipment terminations may rely on the heat-sinking effect of the connected conductors to control termination temperature. The listing standard for equipment, such as UL 489 for circuit breakers, specifies the size of conductor to use in the heat rise test based on the termination temperature rating of the equipment. For a 60A OCPD, if the termination is rated 60C, it requires testing with #4 copper conductors, while if the termination is rated 75C, it requires testing with #6 copper conductors.

So a breaker with 60C/75C terminations has been tested with #6 copper conductors not to overheat. The terminations may rise in temperature to 75C, but as per the allowance for NM ampacity adjustment and correction based on the 90C rating, NM and its conductors are rated for such a temperature. [In a sufficiently hot attic, where the 90C ampacity after adjustment and correction controls, if the conductor is loaded to its full ampacity, the conductor temperature is allowed to rise to 90C.]

Thus we see that using 6/2 NM cable on a 60A breaker rated 60C/75C with a 48A continuous load will provide the necessary level of heat sinking at the breaker termination as per the UL testing standard and will not cause the NM conductors to exceed their rated temperature. Moreover, the load is still less than the 60C ampacity as required by 334.80(A).

Therefore this installation with a 60C/75C rated breaker and 6/2 NM cable should not be prohibited by the combination of 210.19(A)(1)(a) and 334.80(A); the prohibition is not supported by the physics or by testing limitations. Given the reasons behind 210.19(A)(1)(a) the best way to allow this installation is to adjust 334.80(A) to allow the higher temperature rating to be used for termination requirements. Note that the proposed change would still require 4/2 NM cable to be used where the equipment termination temperature rating is 60C.

With the ongoing electrification of the US automotive fleet over the next two decades, literally hundreds of thousands of 48A EVSEs will be installed on 60A circuits, many of which will be supplied by NM cable. Enacting this change will provide significant economic savings and a reduction in unnecessary resource utilization.

Committee Action: Rejected

Resolution: The relationship between the cable/conductor temperature and the termination temperature is addressed in section 110.14(C). Section 310.14 governs ampacities and does not have an adjustment for termination requirements. See Informational Note No. 1 of section 310.14(A)(3).
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My state amended 334.80 and nm cable is only restricted to 60c ampacity when it’s run in contact with thermal insulation which in residential ends up being most cases , but the amendment is beneficial allot of the time when wiring evse with panels in unfinished basements/garages and hardwired evse located in those garages or on the outside wall of garage , and since most if not all hardwired evse is marked 75c ,so we can terminate 6-2 nm at its 75c ampacity in accordance with 110.14(c)(1)(a)(3) and it’s compliant for 60 amp circuit for hardwired evse
 
Birken Vogt said:
You can get copper SER 6-6-6-6, it is less costly than 4/3 NM, but a lot more than 6/3 NM would be.
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You would only need SEU for hardwired evse, they don’t need a neutral
My state puts the same restrictions on se cable as nm though when run in contact with thermal insulation
338.10(B)(4)(a)(3) is amended and all SE cable is restricted to terminate at 60c ampacity when run in contact with thermal insulation.
So if it’s passes through a insulated wall SE cable would not be a option here for a 48 amp hardwired evse
Would need to be # 4 nm cable or a #6 75c wiring method (mc/thhn)
 
CoolWill said:
Some gets marked and goes on a reel and some gets put into NM.
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NM cable plants that I know of take raw copper rod and PVC pellets as inputs and only make NM cable.
Even having to do something like print 'THHN' or anything on the cable would involve re-tooling the production line.
Per the UL spec they are allowed lots of flexibility, the only tests it has to pass is the insulation test which it does on the production line.
Here is a video of a NM cable plant
 
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Jaybone812 said:
My state puts the same restrictions on se cable as nm though when run in contact with thermal insulation
338.10(B)(4)(a)(3) is amended and all SE cable is restricted to terminate at 60c ampacity when run in contact with thermal insulation.
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I should take whomever voted for that amendment on a tour of a SE cable pant, SE cable is made with cross-linked polyethylene (XLPE) insulation type RHW per UL 1581 which has including superior heat and chemical resistance, high insulation resistance, and lower dielectric losses its nothing like the cheap PVC used in NM cable.
SE cable is definitely designed and tested for 75C even when in contact with run in contact with thermal insulation.
 
tortuga said:
I should take whomever voted for that amendment on a tour of a SE cable pant, SE cable is made with cross-linked polyethylene (XLPE) insulation type RHW per UL 1581 which has including superior heat and chemical resistance, high insulation resistance, and lower dielectric losses its nothing like the cheap PVC used in NM cable.
SE cable is definitely designed and tested for 75C even when in contact with run in contact with thermal insulation.
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Yea man you’re correct it definitely is I’m not a fan of the amendment either , they remove the NEC language of ungrounded conductors #10 and smaller and make it a general rule for se cable , although i will say I’ve never seen it enforced and I tend to ignore it as well
 

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