Conductor Ampacity

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

Derating Sequence

1. Base ampacity (NEC Table 310.16, 90°C column).
2. × temperature factor (NEC Table 310.15(B)(2)(a)).
3. × conduit-fill factor (NEC 310.15(B)(3)(a)).
4. Not to exceed terminal rating (60°C or 75°C).
5. 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.

Related Reading

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.