Conductor ampacity derating — temperature and conduit fill

A PV conductor has to pass more than one ampacity test, and beginners often stop after the first. Between the wire's table rating and the current it actually carries sit two derates and one termination cap. Skip any of them and you get either an overheated conductor or a plan-review correction.

Start with the table rating

Every conductor has a base ampacity from NEC Table 310.16 at a given insulation temperature rating — 60 C, 75 C, or 90 C. PV wire and THWN-2 are commonly 90 C rated, which is useful because derating is applied against the 90 C column, giving you headroom before the final termination check. See the wire ampacity table.

Derate 1: ambient temperature

Ampacity tables assume a 30 C ambient. Where it's hotter, multiply by the temperature-correction factor from Table 310.15(B). Two twists matter for solar:

• Use the 90 C column factor if your conductor is 90 C rated — that's the whole point of using 90 C wire.
• Rooftop adder. Conduit in sunlight above a roof runs hotter than air temperature. Add the rooftop temperature adder for the conduit's height above the surface before picking the correction factor. This is one of the most-missed steps on residential DC runs.

Derate 2: conduit fill

When more than three current-carrying conductors share a raceway, apply the conduit-fill adjustment factor (for example, 80% for 4-6 conductors). Count carefully — on the DC side, series strings and the way you count conductors affect this.

The two derates multiply together against the 90 C ampacity:

The termination check: 110.14(C)

Here's the catch that trips everyone. Even after you derate from the 90 C column, 110.14(C) caps the usable ampacity at the temperature rating of the terminals the conductor lands on — commonly 75 C for breakers and lugs. So you run two parallel calculations:

1. Derated ampacity from the 90 C column (both derates applied) must exceed the circuit's continuous current. 2. Terminal check: the conductor's 75 C column ampacity (no derates) must meet the required load per the terminal rating.

The conductor must satisfy both. The 90 C rating buys derating headroom; the 75 C terminal rating sets the connection ceiling.

A quick worked feel

Say a THWN-2 conductor lists 40 A at 90 C. On a hot roof the ambient-plus-rooftop correction might be 0.82, and sharing a raceway with five other current-carrying conductors adds a 0.80 fill factor. Multiply: 40 x 0.82 x 0.80 = about 26 A of usable derated ampacity. That single conductor went from a comfortable 40 A on paper to 26 A in the real installation — and if the circuit's continuous current is 24 A, you're close enough that the next hotter day or an added conductor pushes you over. This is why derating is never optional.

How it fits the bigger picture

This derating is one of three ampacity checks a PV conductor must clear: the 690.8 minimum current, this environmental derate, and the 110.14 termination. See how it fits the full design in how to size a residential solar system. The checks compete — sometimes the derate governs, sometimes the terminal cap does — so you run all of them and take the largest conductor. Miss the rooftop adder and a summer inspection can find undersized DC conductors that were "fine" on the spreadsheet.

Correction factors and rooftop adders shifted between NEC editions; confirm the enforced code and local amendments with your AHJ, and have a licensed electrician or PE review, sign and stamp the design before filing or energizing.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.