wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
There's an error in the second part of the example, as the text does the ampacity correction for ambient temperature starting at the 75C termination rating, rather than the 90C insulation temperature rating.
Cheers, Wayne
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 of38°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.
Cheers, Wayne
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
145.25/
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.