CSLB C-46 Solar ContractorElectrical and Plumbing CodesHard
A solar contractor is installing a PV array on a commercial building rooftop. The DC PV source circuit conductors are run in metallic conduit along the roof surface. The conduit is exposed to direct sunlight. According to NEC Article 310.15(B)(2)(a), what is the minimum temperature adder that must be applied to the ambient temperature when calculating conductor ampacity for conduits exposed to direct sunlight on or above rooftops?
- A33°C (60°F)
- B22°C (40°F)
- C17°C (30°F)
- D28°C (50°F)
Show answer & explanationAnswer & explanation
Correct answer: A. 33°C (60°F)
NEC Table 310.15(B)(2)(a) (now 310.15(B)(1)) specifies temperature adders for conductors in raceways exposed to direct sunlight on or above rooftops. For heights from 0.5 inches to 13 inches above the roof, a 33°C (60°F) adder is required. This accounts for the significant heat buildup in such installations.
Why the other options are wrong
- B. 22°C (40°F) is an adder for raceways 0.5-1.5 inches above the roof, but a specific case.
- C. 17°C (30°F) is a lower adder for raceways less than 0.5 inches from the roof, not typically for 'on the roof surface'.
- D. 28°C (50°F) is an adder for raceways 1.5-3.5 inches above the roof.
Rooftop Conduit Temp Adder
NEC Table 310.15(B)(2)(a) (now 310.15(B)(1)) requires temperature adders for conductors in raceways on or above rooftops exposed to sunlight, with 33°C (60°F) being common for surface-mounted conduits.
- Accounts for solar heat gain.
- Increases conductor operating temperature.
- Reduces conductor ampacity, requiring larger wire sizes.
Memory trick: Rooftop Wires: Add 60 Degrees Fahrenheit for Safety's Light.