wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
The reason for the 125% multiple on continuous currents in the termination checks (without ampacity correction or adjustment, see Example D3(a) in the Annex) of 210.19(A)(1)(a) and 215.2(A)(1)(a) has eluded me for a long time. But I now recognize that it is a side effect of the UL listing standards for non-100% rated equipment (like most circuit breakers): those testing standards are based on the upsized conductors required by those sections, and the temperature rise is checked with that heat sinking effect. So it is another part of the equipment limitation of non-100% rated equipment.
However, in the case of NM cable, 334.80 as currently written unnecessarily limits us to 60C termination ampacities, even though for the "run of the conductor" ampacity checks it recognizes that the 90C ampacity may be safely used, given the requirement for 90C rated insulation in NM-B. Therefore this PC is an attempt to allow the use of the 90C base ampacity for termination checks; the final ampacity would still be limited to the 60C base ampacity as always.
Time is tight, and I'm not sure my argument is laid out as clearly as it could be, so edit suggestions would be appreciated.
Cheers, Wayne
334.80(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.
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.
However, in the case of NM cable, 334.80 as currently written unnecessarily limits us to 60C termination ampacities, even though for the "run of the conductor" ampacity checks it recognizes that the 90C ampacity may be safely used, given the requirement for 90C rated insulation in NM-B. Therefore this PC is an attempt to allow the use of the 90C base ampacity for termination checks; the final ampacity would still be limited to the 60C base ampacity as always.
Time is tight, and I'm not sure my argument is laid out as clearly as it could be, so edit suggestions would be appreciated.
Cheers, Wayne
334.80(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.
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.