The second calculation and comparison is outlined in 690.8(B)(2)—690.8(B)(2)(b) in the 2011 version. Here, after conditions of use have been applied, we compare the conductor’s ampacity to the maximum current as calculated in 690.8(A). This section does not require multiplying by 125% a second time.
Let’s examine these calculations with an example three-string batteryless grid-tied PV system in Sacramento, California. The source circuits run in EMT from a rooftop junction box and down to the inverter. To begin, we consider the type of conductor used: THWN-2, which is rated at 90°C and for use in wet or dry environments. Use the ampacity values in the 90°C column of Table 310.15(B)(16) when adjusting the conductor’s ampacity.
The temperature limitation of the conductor’s terminals must also be considered. Although the direct reference to 110.14(C), which states that the conductor’s ampacity rating needs to correlate with the terminal’s rating, has been removed in the 2014 NEC, that Code section is still necessary. If the terminal is rated for 75°C (common for terminals in PV junction boxes), the ampacity of the conductor at the terminal is considered to be that of a conductor with the 75°C rating, as long as the actual conductor rating is 75°C or more.
To start sizing the conductor, first consider 690.8(B)(1) from the 2014 NEC. The source circuit’s maximum current is 8.8 A × 1.25 = 11 A. The conductor must have an ampacity of 11 A × 1.25 = 13.75 A. Referring to Table 310.15(B)(16), in the 75°C column (because of the terminal limitations), 14 AWG is the smallest conductor listed with an ampacity greater than 13.75 A.
For 690.8(B)(2), the effects of temperature and the number of conductors in a conduit are considered. The 90°C rating for the conductors can be used. We need to know the local ambient temperature, the number of current-carrying conductors in the conduit or raceway, and whether any additional conditions apply. In this system, the EMT is installed 4 inches above the roof and runs 12 feet before going down to the inverter. Therefore, the correction factors in Table 310.15(B)(3)(c) need to be applied to determine the temperature imposed on the conductors. Looking at our correction factors:
• ASHRAE 2% ambient temperature: 38°C. The ASHRAE data is referenced in an Informational Note to Table 310.15(B)(3)(c). (See bit.ly/SolarABCmap.)
• For a raceway that lies 4 inches above the rooftop and is exposed to sunlight, add 17°C to the ambient temperature for correction-factor calculations:
38°C + 17°C = 55°C
Temperature correction factor for conductors rated at 90°C in 55°C environment:
0.76 – 310.15(B)(2)(a)
• With three source circuits, there are six current-carrying conductors in the raceway.
Table 310.15(B)(3)(a) correction factor is 80%
• To compare the 14 AWG conductor found in 690.8(B)(1), use the ampacity of 14 AWG from the 90°C column and apply conditions of use:
25 A × 0.76 × 0.8 = 15.2 A
This ampacity must be greater than the maximum current calculated in 690.8(A): 15.2 A > 11 A, so the 14 AWG fits for this calculation.
To complete the conductor sizing, verify that the conductors are properly protected by any OCPD in the circuit (refer to 690.9 in the 2014 NEC or 690.8(B)(1) in the 2011 version for the OCPD requirements).
Ryan Mayfield is the principal at Renewable Energy Associates, a design, consulting, and educational firm in Corvallis, Oregon, with a focus on PV systems.