wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
My PI 494-NFPA 70-2023 about adding an exception to 210.19(A)(1) to allow the use of 240.4(B) with continuous loads was rejected, with a substantive response: "The 125% continuous use factor does not exist in the NEC solely due to the limitation of an overcurrent device installed in an enclosure. Heat rise tests performed on equipment address more than just the OCPDs located within. These tests are also addressing other components installed within the assembly. The increase in conductor size by application of 125% of continuous loads helps to provide conductor material to act as a heat sink for achieving a lower impact on temperature rise."
So this made me realize that 210.19(A)(1) does have a real purpose on its own, rather than just being a typical consequence of 240.4 and 210.20, namely the heat sinking effect of the increased conductor size. However, that heat sinking effect does not depend on conductor insulation temperature rating, just on the conductor's physical size. Therefore basing 210.19(A)(1) compliance on a conductor's insulation temperature rating, via the reference to ampacity, is off the mark.
Thus I came up with the Public Comment below.
Cheers, Wayne
Public Comment No. 747-NFPA 70-2024
210.19(A) General.
Branch-circuit conductors shall have an ampacity not less than the larger of the following and comply with 110.14(C) for equipment terminations:
(1) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous loadin accordance with 310.14. For the purposes of this requirement only, ampacity values shall be taken directly from Table 310.16, without regard to the table notes and without ampacity adjustment or correction, and the temperature column selected shall be based on the termination temperature rating of the conductor's terminations without regard to the conductor's insulation temperature rating.
Statement of Problem and Substantiation for Public Comment
To begin, here is an example of an installation that the new wording would allow, and which the current wording needlessly disallows: a 60A branch circuit with 60A 75C rated OCPD supplying #6 copper NM cable supplying a 48A EVSE, with conditions of use that require no ampacity adjustment or correction. #6 copper has a 60C ampacity of 55A, while 334.80 limits NM cable to the 60C ampacity; but the current wording of 210.19(A)(1) requires a termination ampacity of at least 60A, so #6 copper NM is disallowed. Let us consider the various reasons behind the NEC rules governing this installation, and whether the above configuration should be allowed or disallowed.
First, 210.19(A)(1) has an exception for 100% rated OCPD. If we could find a 100% rated 60A branch circuit OCPD, then our 55A ampacity #6 copper NM would comply with 210.19(A)(1). This shows that the reason for 210.19(A)(1) is solely related to performance of OCPD (or other equipment; I will stick to OCPD as the typical case); it is not related to conductor adequacy as far as the conductor itself. Indeed, a 55A ampacity means that the conductor may carry 55A continuously, and the load proposed is only 48A continuously.
As discussed in PI 494-NFPA 70-2023, the 125% factor in 210.19(A)(1) is related to the requirement to upsize non-100% rated OCPD for continuous loads as per 210.20(A). This is due to the congested thermal environment of an enclosure with multiple OCPDs, each with their own I^2*R heating, which differs from the isolated, free air testing conditions of UL 489, under which every breaker is tested to hold continuously at 100% of its rating. As per the committee response to PI 494-NFPA 70-2023, part of this upsizing includes an upsizing of the conductors on the OCPD, which will provide a heat sinking effect to further dissipate the heat production from the continuous current.
However, this heat sinking effect depends only on the physical size of the conductor used, and not on the conductor's insulation temperature rating. Moreover, UL 489 6.1.4.2.1 specifies the size of conductor used for OCPD testing, which depends on the temperature rating of the OCPD's terminals, either 60C or 75C. This table matches Table 310.16 in the NEC. So a 60A OCPD with 75C rated terminals will be UL tested with #6 copper conductors, and the level of heat sinking expected is that of #6 copper conductors, not #4 copper conductors. Or to put it differently, #6 copper NM cable, limited to the 60C ampacity, provides the same heat sinking as #6 MC cable, which may use the 75C ampacity.
Note that there is no concern that the smaller #6 copper conductor will have an excessive temperature rise due to this heat sinking effect, due to the requirements of 210.19(A)(2).
Thus we see that heat sinking capability has no dependence on the conductor insulation temperature rating, and therefore it is not appropriate for 210.19(A)(1) to depend on the conductor insulation temperature rating. The conclusion is that in our example installation, #6 NM cable should be allowed rather than disallowed. The proposed language reflects this analysis.
To summarize, the various sections of the NEC that govern the various necessities of this installation:
- 210.19(A)(2) ensures that the conductor will not be damaged by the current required for the load
- 210.20(A) ensures that the OCPD will not nuisance trip from a continuous load.
- 240.4 ensures that the OCPD will adequately protect the conductor
- And 210.19(A)(1) ensures that the conductor will be upsized to provide an enhanced heat sinking effect when there is a continuous load. This heat sinking effect does not depend on conductor insulation temperature, only conductor size.
Related Item
• Public Input No. 494-NFPA 70-2023
So this made me realize that 210.19(A)(1) does have a real purpose on its own, rather than just being a typical consequence of 240.4 and 210.20, namely the heat sinking effect of the increased conductor size. However, that heat sinking effect does not depend on conductor insulation temperature rating, just on the conductor's physical size. Therefore basing 210.19(A)(1) compliance on a conductor's insulation temperature rating, via the reference to ampacity, is off the mark.
Thus I came up with the Public Comment below.
Cheers, Wayne
Public Comment No. 747-NFPA 70-2024
210.19(A) General.
Branch-circuit conductors shall have an ampacity not less than the larger of the following and comply with 110.14(C) for equipment terminations:
(1) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load
Statement of Problem and Substantiation for Public Comment
To begin, here is an example of an installation that the new wording would allow, and which the current wording needlessly disallows: a 60A branch circuit with 60A 75C rated OCPD supplying #6 copper NM cable supplying a 48A EVSE, with conditions of use that require no ampacity adjustment or correction. #6 copper has a 60C ampacity of 55A, while 334.80 limits NM cable to the 60C ampacity; but the current wording of 210.19(A)(1) requires a termination ampacity of at least 60A, so #6 copper NM is disallowed. Let us consider the various reasons behind the NEC rules governing this installation, and whether the above configuration should be allowed or disallowed.
First, 210.19(A)(1) has an exception for 100% rated OCPD. If we could find a 100% rated 60A branch circuit OCPD, then our 55A ampacity #6 copper NM would comply with 210.19(A)(1). This shows that the reason for 210.19(A)(1) is solely related to performance of OCPD (or other equipment; I will stick to OCPD as the typical case); it is not related to conductor adequacy as far as the conductor itself. Indeed, a 55A ampacity means that the conductor may carry 55A continuously, and the load proposed is only 48A continuously.
As discussed in PI 494-NFPA 70-2023, the 125% factor in 210.19(A)(1) is related to the requirement to upsize non-100% rated OCPD for continuous loads as per 210.20(A). This is due to the congested thermal environment of an enclosure with multiple OCPDs, each with their own I^2*R heating, which differs from the isolated, free air testing conditions of UL 489, under which every breaker is tested to hold continuously at 100% of its rating. As per the committee response to PI 494-NFPA 70-2023, part of this upsizing includes an upsizing of the conductors on the OCPD, which will provide a heat sinking effect to further dissipate the heat production from the continuous current.
However, this heat sinking effect depends only on the physical size of the conductor used, and not on the conductor's insulation temperature rating. Moreover, UL 489 6.1.4.2.1 specifies the size of conductor used for OCPD testing, which depends on the temperature rating of the OCPD's terminals, either 60C or 75C. This table matches Table 310.16 in the NEC. So a 60A OCPD with 75C rated terminals will be UL tested with #6 copper conductors, and the level of heat sinking expected is that of #6 copper conductors, not #4 copper conductors. Or to put it differently, #6 copper NM cable, limited to the 60C ampacity, provides the same heat sinking as #6 MC cable, which may use the 75C ampacity.
Note that there is no concern that the smaller #6 copper conductor will have an excessive temperature rise due to this heat sinking effect, due to the requirements of 210.19(A)(2).
Thus we see that heat sinking capability has no dependence on the conductor insulation temperature rating, and therefore it is not appropriate for 210.19(A)(1) to depend on the conductor insulation temperature rating. The conclusion is that in our example installation, #6 NM cable should be allowed rather than disallowed. The proposed language reflects this analysis.
To summarize, the various sections of the NEC that govern the various necessities of this installation:
- 210.19(A)(2) ensures that the conductor will not be damaged by the current required for the load
- 210.20(A) ensures that the OCPD will not nuisance trip from a continuous load.
- 240.4 ensures that the OCPD will adequately protect the conductor
- And 210.19(A)(1) ensures that the conductor will be upsized to provide an enhanced heat sinking effect when there is a continuous load. This heat sinking effect does not depend on conductor insulation temperature, only conductor size.
Related Item
• Public Input No. 494-NFPA 70-2023
