Q. How is the conductor ampacity for photovoltaic (PV) circuits calculated?
 

A. PV circuit conductors must be sized to the larger of 690.8(B)(2)(a) or 690.8(B)(2)(b).

Sizing according to 690.8(B)(2)(a):
PV circuit conductors must be sized to carry not less than 125% of 690.8(A) current before the application of conductor ampacity correction [310.15(B)(2)(a) and 310.15(B)(3)(c)] and adjustment [310.15(B)(3)(a)].

Example: What's the minimum PV source circuit conductor ampacity before the application of conductor correction or adjustment for a string having a short circuit current rating of 11.80A; assuming all terminals are rated 75°C (see Figure)?
Conductor ampacity = (module Isc × 1.25)* × 1.25
= (11.80A × 1.25)* × 1.25
= (14.75A)* × 1.25
= 18.44A
Conductor ampacity = 14 AWG rated 20A at 75ºC [Table 310.15(B)(16)]

*As per 690.8(A)(1), "the maximum current shall be the sum of parallel module rated short-circuit currents multiplied by 125%."

Conductors terminating on terminals rated 75°C are sized in accordance with the ampacities listed in the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(3)].

Example: What's the minimum PV output circuit conductor ampacity, before the application of conductor correction or adjustment, supplied by two PV source circuits, each having a short-circuit current rating of 11.80A; assuming all terminals are rated 75°C?
Conductor ampacity = (module Isc × 1.25 × number of source circuits)** × 1.25
= (11.80A × 1.25 × 2 strings)** × 1.25
= 29.5A** × 1.25
= 36.88A
Conductor ampacity = 8 AWG rated 50A at 75ºC [Table 310.15(B)(16)]

**As per 690.8(A)(2), "the maximum current shall be the sum of parallel source circuit maximum currents as calculated in 690.8(A)(1)."

Sizing according to 690.8(B)(2)(b):
Circuit conductors must be sized to carry 100% of 690.8(A) current after the application of conductor ampacity correction [310.15(B)(2)(a) and 310.15(B)(3)(c)] and adjustment [310.15(B)(3)(a)]. When performing conductor ampacity correction and adjustment calculations, use the conductor ampacity listed in the 90°C column of Table 310.15(B)(16) for THWN-2 insulated conductors [310.15(B)].

Example: What's the PV source circuit conductor ampacity after temperature correction for two current-carrying size 12 THWN-2 conductors in a circular raceway on the roof, ambient temperature is 90°F with 60°F temperature added in accordance with Table 310.15(B)(3)(c), supplying modules having a nameplate Isc rating of 11.80A?
Conductor ampacity = Table 310.15(B)(16) Ampacity at 90°C Column x Temperature Correction Temperature correction = 0.58, Table 310.15(B)(2)(a) based on 150°F (ambient plus 60°F roof temperature adder)
Therefore, conductor ampacity = 30A x 0.58 = 17.40A, which has sufficient ampacity after correction and adjustment to supply 690.8(A)(1) calculated current of 14.75A (11.80A x 1.25).

Example: What's the PV output current ampacity after temperature correction for two current-carrying 8 size THWN-2 conductors in a circular raceway on the roof where the ambient temperature is 90°F supplied by two parallel PV source circuits, each having a Isc rating 11.80A?
Conductor ampacity = Table 310.15(B)(16) Ampacity at 90°C Column x Temperature Correction
Temperature correction = 0.58, Table 310.15(B)(2)(a) based on 150°F (ambient plus 60°F roof temperature adder)
Therefore, conductor ampacity = 55A x 0.58 [Tables 310.15(B)(2)(a) and 310.15(B)(3)(c)] = 31.90A, which has sufficient ampacity after correction and adjustment to supply 690.8(A)(2) calculated current of 29.5A (11.80A x 1.25 x 2).