Conductor Ampacity (NEC 310): Solar PV Wire Capacity

Quick answer

Conductor ampacity is the maximum current, in amperes, a conductor can carry continuously without exceeding its insulation temperature rating. NEC 2023 Table 310.16 lists base values for 60°C, 75°C and 90°C insulation. Solar designers then apply temperature and conduit-fill corrections and size PV circuits using the NEC 690.8 current rules.

Ampacity by AWG (90°C insulation, NEC 310.16)

AWG	Ampacity
14	30 A
12	40 A
10	55 A
8	75 A
6	105 A
4	140 A
2	190 A
1/0	260 A
4/0	405 A

Derating Sequence

Base ampacity (NEC Table 310.16, 90°C column).
× temperature factor (NEC Table 310.15(B)(2)(a)).
× conduit-fill factor (NEC 310.15(B)(3)(a)).
Not to exceed terminal rating (60°C or 75°C).
Must satisfy 1.25 × Isc × 1.25 minimum.

Key Takeaways

Conductor ampacity = max continuous current capacity.
NEC Table 310.16 provides base values.
Apply temperature + conduit-fill derating.
Final ampacity capped by terminal temperature rating.
Required ampacity for solar = 1.5625 × Isc (NEC 690.8).

Worked Example

Take a 10 AWG PV Wire home run rated 55 A at 90°C insulation (see the table above). If the conduit runs across a rooftop at a 50°C design ambient, the temperature derating factor from NEC Table 310.15(B)(2)(a) is 0.82. If that same conduit bundles 4-6 current-carrying conductors, the conduit-fill factor from NEC 310.15(B)(3)(a) is 0.80.

Multiplying the base ampacity by both factors, 55 A × 0.82 × 0.80, gives a corrected ampacity of roughly 36 A. That corrected value is then checked against the terminal temperature rating (60°C or 75°C, per NEC 110.14(C)) and against the minimum ampacity NEC 690.8 requires for the array’s short-circuit current; whichever constraint is more restrictive sets the final wire size.

Conductor ampacity rarely gets checked in isolation, it’s one part of the electrical single-line diagram package that also has to reconcile overcurrent protection, conduit fill, and terminal ratings, which is why our guide on NEC 2023 auto-generation in solar SLD software is worth a read alongside this term. Sizing decisions made here also feed directly into permit-ready electrical CEIG drawings, where inspectors check derated ampacity against the schedule of conductors.

For related string-and-array electrical terms, see wire sizing, conduit fill, and the underlying NEC 690 requirements that set the minimum ampacity threshold in the first place. On the grounding side of the same conductor schedule, QBITS Energy’s solar inverter grounding and earthing guide covers the equipment-grounding conductors that ampacity tables also apply to.

Disclosure: Qbits Energy is a Heaven Designs group company.

Frequently Asked Questions

4 commonly searched questions about Conductor Ampacity.

How do you calculate ampacity for a PV circuit?

Start with the NEC 690.8 maximum circuit current, typically 1.25 x module Isc, then multiply by 1.25 for continuous duty. Select a conductor whose corrected ampacity, after ambient temperature and conduit fill adjustments, still exceeds that value, and check terminal temperature limits at both ends.

Why does rooftop conduit reduce ampacity?

Conduit exposed to sunlight on a roof runs hotter than the surrounding air. NEC 310.15(B)(2) requires adding a temperature adder for raceways within 7/8 inch of the roof surface, and the higher ambient temperature lowers the correction factor applied to the conductor's ampacity.

What is the difference between ampacity and voltage drop?

Ampacity is a safety limit based on heat in the conductor. Voltage drop is a performance issue caused by conductor resistance over distance. A wire can pass the ampacity check yet still need upsizing on long runs to keep voltage drop near the common 2% design target.

Which terminal temperature rating applies?

Even with 90°C insulation such as PV wire or THWN-2, equipment terminals are often rated only 75°C. NEC 110.14(C) requires the final conductor size to satisfy the terminal rating, so designers derate from the 90°C column but verify the 75°C column at terminations.

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