The three ampacity checks every PV circuit must pass

Ask three installers how to size a PV conductor and you may get three answers — because there isn't one calculation, there are three, and the conductor has to pass all of them. Run only your favorite and you'll eventually undersize a wire. Here's the full checklist in the order that keeps you out of trouble.

Check 1: minimum current (690.8)

Start with how much current the conductor must carry. Under 690.8, the maximum PV circuit current is Isc x 1.25 (690.8(A)), and the conductor's continuous-duty requirement adds another 1.25 (690.8(B)(1)) — the two stack to 156% of Isc. The conductor's ampacity must beat this number before any environmental derate. See how to size a residential solar system for how this fits the whole design flow.

Check 2: temperature and conduit-fill derating

Now punish the wire for its environment. Against the 90 C column of Table 310.16, apply:

• Ambient temperature correction, including the rooftop temperature adder for conduit above the roof surface, and
• Conduit-fill adjustment when more than three current-carrying conductors share a raceway.

Multiply both factors against the 90 C ampacity. The derated result must exceed the circuit's continuous current.

Check 3: termination temperature (110.14(C))

The sting in the tail. Even a 90 C conductor is capped at the temperature rating of the terminals it lands on — usually 75 C on breakers and lugs. So the conductor's 75 C column ampacity (no derates) must independently meet the load. The 90 C rating buys derating headroom; the 75 C terminal sets the ceiling.

Take the worst case

The three checks compete, and which one governs changes with the job:

• A hot rooftop with a low conduit standoff makes Check 2 dominate.
• A high-current circuit into 75 C lugs makes Check 3 dominate.
• A modest run in mild climate may be set by Check 1.

Run all three, and size to the largest conductor any of them demands. Don't stop at the first pass.

A worked example

Take a string with 10 A Isc. Check 1: 10 x 1.56 = 15.6 A minimum ampacity. Check 2: on a hot roof your 90 C conductor's rating might derate by 0.82 (temp) and 0.80 (fill) to well under its table value — suppose that lands at 19 A, still above 15.6 A, so it passes. Check 3: the conductor's 75 C column rating must cover the load at its termination — confirm the wire you picked has enough 75 C ampacity where it lands on the lug. If any single check fails, step up one wire size and re-run all three, because bumping the conductor changes every number.

That "re-run all three" loop is the discipline. A change made to satisfy one check can nudge another, and the only conductor you can trust is the one that clears every test at once.

Then size the OCPD

Conductor settled, the overcurrent device follows under 690.9 and rounds up to a standard rating in 240.6 — and it must protect the conductor's final ampacity, not its table value. Size it at 125% of the 690.8(A) current, confirm it doesn't exceed the module series-fuse rating, and you have a circuit that survives both the calculation and the inspection.

Correction factors, rooftop adders, and continuous-current wording vary across 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. For the conductor-and-OCPD relationship, see our 690.8 reference.

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.