Code Q&A: Figuring Conductor Ampacity for Photovoltaic Circuits

Code Q&A: Figuring Conductor Ampacity for Photovoltaic Circuits

Your most pressing National Electrical Code (NEC) questions answered

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).

Hide comments

Comments

  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
Publish