Rooftop Conduit Ampacity with Six Current-Carrying Conductors
Three strings of 10 AWG PV wire leave a roof-mounted array in a single 1-inch EMT conduit. The conduit is supported 6 in above the roof and contains three positive and three negative conductors, all current-carrying. Each string has Isc 11.2 A. The design ambient temperature is 42°C; terminals are rated 75°C. The installer selected 10 AWG because its 90°C ampacity is 40 A, far above the 11.2 A string current. The AHJ asks for an ampacity calculation that includes the rooftop adder and conductor bundling. The roof has high solar gain, and no part of the conduit is shaded. The conduit must remain 1-inch EMT because of a penetration constraint, but the conductor size can change. All conductors are copper.
1. Calculate the adjusted ampacity of the 10 AWG conductors and determine whether they comply with the continuous PV current requirement.
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First apply the rooftop temperature adder. Because the conduit is 6 in above the roof and exposed to high solar gain, add 33°C to the 42°C design ambient:
42°C + 33°C = 75°C
For 90°C-rated conductors, the correction factor at 75°C is 0.41. The 10 AWG 90°C ampacity is 40 A.
Next apply the bundle adjustment for six current-carrying conductors in one raceway: 0.80.
Adjusted ampacity = 40 A × 0.41 × 0.80 = 13.1 A
The continuous PV current requirement is 1.25 × Isc = 1.25 × 11.2 A = 14.0 A.
13.1 A is less than 14.0 A, so 10 AWG does not comply. The installer's mistake was comparing the raw 40 A ampacity to the string current without the rooftop and bundle adjustments.
2. Select the minimum compliant conductor size and show the calculation. Include the termination check.
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Try 8 AWG copper, 90°C rating = 55 A.
Adjusted ampacity = 55 A × 0.41 × 0.80 = 18.0 A
Required ampacity = 14.0 A. 18.0 A > 14.0 A, so 8 AWG passes the adjustment calculation.
Termination check: the terminals are rated 75°C. The 8 AWG 75°C ampacity is 50 A, and the adjusted conductor ampacity of 18.0 A is below that, so the termination is not overloaded. The conductor is protected against the continuous current and the adjusted ampacity is adequate.
Therefore, 8 AWG is the minimum size for this conduit fill and rooftop condition among the usual choices. 10 AWG fails; 6 AWG would also work but is larger than necessary unless voltage drop or future expansion requires it.
3. Why can't the installer use the 75°C termination rating or the 40 A raw ampacity to justify the 10 AWG?
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The termination rating is an upper limit on the conductor's permitted operating temperature at the terminal, not an exemption from ambient correction or bundle adjustment. The conductor must satisfy all applicable ampacity rules simultaneously: the adjusted ampacity after ambient and bundle corrections must be at least the required continuous current, and the resulting ampacity must not exceed the termination temperature limit.
For 10 AWG, the raw 90°C ampacity of 40 A is irrelevant once the rooftop adder and six-conductor bundle factor are applied. The adjusted value is 13.1 A, below the 14.0 A requirement. Using the 75°C column would be even more restrictive: 10 AWG at 75°C is 35 A before adjustments, and after 0.41 × 0.80 it is 11.5 A.
The correct approach is to start with the conductor's 90°C base ampacity, apply all adjustment factors, compare to the required current, then verify the termination rating. 10 AWG fails that sequence.