Rooftop temperature for sizing PV wires

[alej27]

As you know, for sizing general conductors, one must consider ambient temperature and conductor bundling. When the conductors are installed in a rooftop, and their bottom are 2.3 cm or less from the rooftop, then one must add 33°C to the estimated ambient temperature, as per sec. 310.15(B)(2) of the 2020 NEC. So if the ambient temperature was 40°C, it would become 73°C.

On the other hand, when one is sizing conductors for PV circuits, instead of using the ambient temperature correction factors for general wiring (table 310.15(B)(1)), the NEC allows to use that in the article dedicated to PV systems (table 690.31(A)(a)).

My question is, if I'm sizing the PV wires, which will be installed in a rooftop and less than 2.3 cm, must I still add the 33°C required by sec. 310.15(B)(2) when using table 690.31(A)(a)?

 

[don_resqcapt19]

See 90.3. Rules in Chapters 5 through 7 can modify rule in Chapters 1 through 4.

 

[Carultch]

As you know, for sizing general conductors, one must consider ambient temperature and conductor bundling. When the conductors are installed in a rooftop, and their bottom are 2.3 cm or less from the rooftop, then one must add 33°C to the estimated ambient temperature, as per sec. 310.15(B)(2) of the 2020 NEC. So if the ambient temperature was 40°C, it would become 73°C.

On the other hand, when one is sizing conductors for PV circuits, instead of using the ambient temperature correction factors for general wiring (table 310.15(B)(1)), the NEC allows to use that in the article dedicated to PV systems (table 690.31(A)(a)).

My question is, if I'm sizing the PV wires, which will be installed in a rooftop and less than 2.3 cm, must I still add the 33°C required by sec. 310.15(B)(2) when using table 690.31(A)(a)?

The tables match each other, for your convenience. The table in 690.31(A)(a), includes the 105C column, which the one in article 310 doesn't. I have yet to see 105C rated wire for anything other than medium voltage.

It's all based on the same square root formula, of sqrt((Tc - Ta)/(Tc - T0)), where:
Tc is the temperature rating of the conductor
Ta is the ambient temperature
T0 is the reference ambient temperature at which the conductor has its standard rating, usually 30C.
Either F or C works for this calculation, as long as you are consistent, since it is just temperature differences, that ultimately divide each other.

The tables bin the results of this formula as representative numbers rounded to the nearest percent, for the 5 degree Celsius range in each row, so derate calculations are practical to do on simple 4-function calculators. Since I use Excel for this, I prefer the square root formula, so there is a lot less to look up. You are permitted to use either the tables or the square root formula, whichever is convenient for you.