Sizing of Service Conductors for Dwellings with Temperature Correction Factor

[jar546]

Service conductors and feeders for certain dwellings are permitted to be sized in accordance with 310.12. How often do you apply the correction factor?

With No Required Adjustment or Correction Factors. If a 175-ampere service rating is selected, a service conductor is then sized as follows:
175 amperes × 0.83 = 145.25 amperes per 310.12.
If no other adjustments or corrections are required for the installation, then, in accordance with Table 310.16, a 1/0 AWG Cu or a 3/0 AWG Al meets this rating at 75°C (167°F).

With Required Temperature Correction Factor.
If a 175-ampere service rating is selected, a service conductor is then
175 amperes × 0.83 = 145.25 amperes per 310.12.
If the conductors are installed in an ambient temperature of 38°C (100°F), the conductor ampacity must be multiplied by the appropriate correction factor in Table 310.15(B)(1)(1). In this case, we will use an XHHW-2 conductor, so we use a correction factor of 0.91 to find the minimum conductor ampacity and size:
145.25/.91 = 159.6 amperes
In accordance with Table 310.16, a 2/0 AWG Cu or a 4/0 AWG Al would be required.
If no temperature correction or ampacity adjustment factors are required, the following table includes conductor sizes calculated using the requirements in 310.12. This table is based on 75°C terminations and without any adjustment or correction factors.

 

[wwhitney]

If a 175-ampere service rating is selected, a service conductor is then
175 amperes × 0.83 = 145.25 amperes per 310.12.
If the conductors are installed in an ambient temperature of 38°C (100°F), the conductor ampacity must be multiplied by the appropriate correction factor in Table 310.15(B)(1)(1). In this case, we will use an XHHW-2 conductor, so we use a correction factor of 0.91 to find the minimum conductor ampacity and size:
145.25/.91 = 159.6 amperes
In accordance with Table 310.16, a 2/0 AWG Cu or a 4/0 AWG Al would be required.

This is not correct. As is always the case, temperature correction factors may be applied to the 90C ampacity of XHHW-2; this is even explicitly stated in 310.12(D).

So in consulting Table 310.16 for a conductor with a base ampacity of at least 159.6A, we can use he 90C column. That means the conductor needs to be at least a 1/0 Cu or 3/0 Al, same as in your previous example.

Cheers, Wayne

 

[wwhitney]

We can flip the question in the OP around and ask: for a standard 200A residential service, with no ampacity adjustment, what's the highest ambient temperature at which the usual 2/0 Cu conductors with 90C insulation are still good for 200A?

The base 90C ampacity of 2/0 Cu is 195A. So the minimum temperature correction factor allowed would be (200 * .83) / 195 = 0.851. Table 310.15(B)(1) gives a correction factor of 0.87 for 105-113F and 0.82 for 114-122F. Per the table, at 114F ambient we'd need to upsize the 2/0 Cu if the calculated load is exactly 200A. (The formula in 310.15(B)(1) gives a sharper answer of 116F).

If the calculated load is less than 200A, 2/0 Cu would be fine at an even higher temperature. At 176A of calculated load, we only need a 176A service rating but can still protect those conductors at 200A per 240.4(B). That would make the minimum temperature correction factor (176 * .83) / 195= 0.749. In which case per Table 310.15(B)(1) we'd fine at an ambient temperature of up to 131F.

Cheers, Wayne

 

[jar546]

This is not correct

Sharp as always wwhitney. What I posted is copied and pasted directly from Appendix D of the 2023 NEC. Nothing was altered, therefore, if you are correct, then the NFPA should be notified and this section in Appendix D needs to be changed. Are you sure?

 

[wwhitney]

Sharp as always wwhitney. What I posted is copied and pasted directly from Appendix D of the 2023 NEC. Nothing was altered, therefore, if you are correct, then the NFPA should be notified and this section in Appendix D needs to be changed. Are you sure?

Yes. PIs for the 2026 NEC closed this week, but fortunately I have already submitted one, as it came up on another forum earlier this year. My PI (where I missed referencing the last sentence of 310.12(D)) is reproduced below:


With Required Temperature Correction Factor.

If a 175-ampere service rating is selected, a service conductor is then
175 amperes × 0.83 = 145.25 amperes per 310.12.
If the conductors are installed in an ambient temperature of 38°C 49°C (100°F120°F), the conductor ampacity must be multiplied by the appropriate correction factor in Table 310.15(B)(1)(1). In this case, we will use an XHHW-2 conductor, so we use a correction factor of 0.91 0.82 to find the minimum conductor ampacity and size:
145.25/0.91 0.82 = 159.6 177.1 amperes
In accordance with the 90°C column of Table 310.16, a 2/0 AWG Cu or a 4/0 AWG Al would be required.

Statement of Problem and Substantiation for Public Input

The current example errs in applying the temperature correction factor to the 75C (termination temperature) ampacity column of Table 310.16, rather than the 90C (insulation temperature for XHHW-2). 110.14(C), 310.15(A), and Annex D Example D3(a) section "Ungrounded Feeder Conductors" all indicate that when applying temperature correction factors to determine ampacity, the insulation temperature rating may be used as the starting point for the ampacity calculation.

Therefore in keeping with the idea that the example is intended to show the need to upsize the conductors from the previous case, I have increased the ambient temperature to the minimum necessary to get that result. If the 49C / 120F temperature necessary is deemed unrealistic, I would suggest instead changing the conductor insulation type to one with only a 75C rating. Then the current example temperature of 38C / 100F gives a temperature correction factor of 0.88, and the resulting wire size is still the same.

 

[jar546]

We can flip the question in the OP around and ask: for a standard 200A residential service, with no ampacity adjustment, what's the highest ambient temperature at which the usual 2/0 Cu conductors with 90C insulation are still good for 200A?

Great question. As usual, it depends on ambient temperature and what table you are using. For example, most SFR's get their ampacity from Table 310.16, therefore, if that were the case, Table 310.15(B)(1)(1) would apply and ambient temperatures above 30C or 86F would apply and be the maximum temp for the 2/0 in question. That certainly won't fly in south Florida, for example.

 

[wwhitney]

For example, most SFR's get their ampacity from Table 310.16, therefore, if that were the case, Table 310.15(B)(1)(1) would apply and ambient temperatures above 30C or 86F would apply and be the maximum temp for the 2/0 in question. That certainly won't fly in south Florida, for example.

When the ambient temperature exceeds 30C or 86F, that just means to have to do the computation to see if it's an issue, not that it's necessarily a problem. In my post I did the computation, and using just the tables, 2/0 Cu with 90C insulation is fine for a 200A residential service at ambient temperatures up to 113F.

Cheers, Wayne