wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
This PI is more substantive, it proposes to allow the use of 240.4(B) to achieve the 125% OCPD size required for continuous loads without upsizing the conductors all the way to 125%, seeing as how the conductor ampacity is already a continuous rating.
I would definitely appreciate any comments on the validity of this change, or the wording proposed, or the argument I made for it. On the technical side, the only reason I can see not to allow this is if part of the reason for upsizing the conductor to the full 125% is to provide increased heat sinking for the upsized OCPD.
Obviously that will have some effect, but the question is whether it is significant. I would think that just the 125% OCPD oversizing is enough to avoid nuisance trips. Any direct experience with continuous load nuisance tripping would be of interest.
Cheers, Wayne
(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:
Statement of Problem and Substantiation for Public Input
Recall that the 125% continuous use factor exists in the NEC solely due to the limitation of an overcurrent device installed in an enclosure which may allow heat buildup greater than would occur in the free air testing conditions of the applicable UL standard, possibly resulting in nuisance tripping when the overcurrent device is loaded continuously at its rating. In particular, there is no need to upsize the conductor itself based solely on the continuous loading; the ampacity is by the Article 100 definition a continuous rating. Any need to upsize the conductor derives from the need to upsize the overcurrent device and then to ensure than the conductor is still adequately protected under 240.4.
This amendment proposes to allow the use of 240.4(B) as indicated, which use would otherwise be circumvented by 210.19(A)(1). To illustrate the effect, consider a 48A continuous load (such as an EVSE, an increasingly common new installation) installed with a 60A overcurrent device and possibly supplied by 6/2 NM cable. NM cable is limited to the 60C ampacity column, so before adjustment and correction 6/2 NM has an ampacity of 55A.
Now the 55A rating is a continuous rating, and greater than the 48A continuous load, so the cable will not be overloaded during normal operating conditions. And 60A is 125% of the 48A continuous load, so the overcurrent device rating complies with 210.20(A) and should not lead to nuisance tripping. The only remaining question as far as the safety of the installation is whether a 60A overcurrent device can protect the 55A ampacity conductor with a 48A continuous load during abnormal conditions.
For the case of a non-continuous load of 55A, 240.4(B) does allow a 60A overcurrent device to protect a 55A ampacity conductor. The difference in loading conditions is not material to whether or not the 60A overcurrent device can properly protect a 55A ampacity conductor. That is, for the 55A non-continuous load case, 240.4(B) tells us that the overcurrent device's protection curve is suitably more conservative than the 55A ampacity conductor's damage curve, so that the 55A ampacity conductor is protected. The same confidence about abnormal conditions applies regardless of normal loading conditions, so the 55A ampacity conductor is protected by a 60A overcurrent device for the 48A continuous load case as well.
As such, since the non-continuous configuration discussed is allowed under 240.4(B), the continuous configuration should also be allowed. It is currently disallowed only due to the requirement in 210.19(A)(1) for the 125% continuous use factor. The new exception provides the narrowly tailored relief necessary to apply 240.4(B) to continuous loads.
I would definitely appreciate any comments on the validity of this change, or the wording proposed, or the argument I made for it. On the technical side, the only reason I can see not to allow this is if part of the reason for upsizing the conductor to the full 125% is to provide increased heat sinking for the upsized OCPD.
Obviously that will have some effect, but the question is whether it is significant. I would think that just the 125% OCPD oversizing is enough to avoid nuisance trips. Any direct experience with continuous load nuisance tripping would be of interest.
Cheers, Wayne
(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:
- 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 in accordance with 310.14.
Exception No. 1 to (1): If the assembly, including the overcurrent devices protecting the branch circuits, is listed for operation at 100 percent of its rating, the ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load in accordance with 110.14(C).
Exception No. 2 to (1): Where the overcurrent device is sized per 210.20(A) and does not exceed 800A, the ampacity of the branch-circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load, provided the ampacity is more than the next lower standard rating of overcurrent device in accordance with 240.4(B). - The minimum branch-circuit conductor size shall have an ampacity not less than the maximum load to be served after the application of any adjustment or correction factors in accordance with 310.15.
Statement of Problem and Substantiation for Public Input
Recall that the 125% continuous use factor exists in the NEC solely due to the limitation of an overcurrent device installed in an enclosure which may allow heat buildup greater than would occur in the free air testing conditions of the applicable UL standard, possibly resulting in nuisance tripping when the overcurrent device is loaded continuously at its rating. In particular, there is no need to upsize the conductor itself based solely on the continuous loading; the ampacity is by the Article 100 definition a continuous rating. Any need to upsize the conductor derives from the need to upsize the overcurrent device and then to ensure than the conductor is still adequately protected under 240.4.
This amendment proposes to allow the use of 240.4(B) as indicated, which use would otherwise be circumvented by 210.19(A)(1). To illustrate the effect, consider a 48A continuous load (such as an EVSE, an increasingly common new installation) installed with a 60A overcurrent device and possibly supplied by 6/2 NM cable. NM cable is limited to the 60C ampacity column, so before adjustment and correction 6/2 NM has an ampacity of 55A.
Now the 55A rating is a continuous rating, and greater than the 48A continuous load, so the cable will not be overloaded during normal operating conditions. And 60A is 125% of the 48A continuous load, so the overcurrent device rating complies with 210.20(A) and should not lead to nuisance tripping. The only remaining question as far as the safety of the installation is whether a 60A overcurrent device can protect the 55A ampacity conductor with a 48A continuous load during abnormal conditions.
For the case of a non-continuous load of 55A, 240.4(B) does allow a 60A overcurrent device to protect a 55A ampacity conductor. The difference in loading conditions is not material to whether or not the 60A overcurrent device can properly protect a 55A ampacity conductor. That is, for the 55A non-continuous load case, 240.4(B) tells us that the overcurrent device's protection curve is suitably more conservative than the 55A ampacity conductor's damage curve, so that the 55A ampacity conductor is protected. The same confidence about abnormal conditions applies regardless of normal loading conditions, so the 55A ampacity conductor is protected by a 60A overcurrent device for the 48A continuous load case as well.
As such, since the non-continuous configuration discussed is allowed under 240.4(B), the continuous configuration should also be allowed. It is currently disallowed only due to the requirement in 210.19(A)(1) for the 125% continuous use factor. The new exception provides the narrowly tailored relief necessary to apply 240.4(B) to continuous loads.
