Sizing solar conductors and breakers, worked

Sizing a PV circuit runs in one direction: start with the module current, inflate it with the NEC 690.8 factors, correct the conductor's ampacity for heat and conduit fill, then pick a breaker that protects the wire. Miss a step and the plan gets corrected. Here is the full worked chain.

Step 1 — Maximum circuit current (690.8)

NEC 690.8(A) sets the maximum current as Isc x 1.25 to cover irradiance above the standard test condition. Then 690.8(B) applies the continuous-load 1.25 when sizing conductors and OCPD. Combined, that is:

• Design current = Isc x 1.25 x 1.25 = Isc x 1.5625.

Example: a module string with Isc = 11.4 A:

• 11.4 x 1.25 = 14.25 A (maximum current).
• 14.25 x 1.25 = 17.8 A (continuous-adjusted design current).

Our PV conductor and OCPD 690.8 reference walks the code text.

Step 2 — Pick a base conductor

Choose a conductor whose 90 °C ampacity (from NEC 310.16) comfortably exceeds the design current before corrections. For 10 AWG copper THWN-2, the 90 °C column is 40 A. That looks generous — but corrections come next.

Step 3 — Apply temperature correction

Rooftop conduit runs hot. NEC requires a rooftop temperature adder plus the ambient correction factor from 310.15(B). Suppose the corrected ambient is 45 °C, giving a temperature correction factor of about 0.87 for a 90 °C conductor:

• 40 A x 0.87 = 34.8 A.

Step 4 — Apply conduit-fill adjustment

If more than three current-carrying conductors share a raceway, apply the fill factor from 310.15(C). With, say, 4 to 6 conductors, the factor is 0.80:

• 34.8 A x 0.80 = 27.8 A corrected ampacity.

Compare to the design current: 27.8 A corrected vs 17.8 A design. The 10 AWG conductor still clears the load, so it holds. See the wire ampacity 310.16 reference for the tables and factors.

Step 5 — Size the overcurrent device

The OCPD must:

• Be at least the 690.8(B) continuous design current (17.8 A here), rounded up to a standard size — 20 A.
• Not exceed the corrected conductor ampacity (27.8 A). A 20 A breaker fits.

The corrected ampacity (27.8 A) sits above the OCPD (20 A), which sits above the design current (17.8 A) — the ordering a reviewer confirms.

Don't forget the AC side

The chain above sizes a DC PV circuit, but the inverter's AC output conductors get the same treatment with their own numbers. Start from the inverter's rated continuous AC output current, multiply by 1.25 for continuous duty, then correct the conductor ampacity for its ambient and fill just as you did on the DC side, and pick an OCPD that protects the wire. For microinverter systems, remember the branch circuit carries the combined output of every microinverter on it, so the current adds up quickly and can drive a larger conductor than you'd expect. Terminal temperature ratings (the 75 °C column at most breakers and lugs) can also cap the ampacity you're allowed to use, even when the 90 °C conductor rating is higher.

Your exact correction factors depend on the module Isc, roof temperature, and raceway fill, so run them for your own site and confirm the enforced NEC edition with your AHJ. A licensed electrician or PE must review and stamp the final design. To see where these labels appear in the package, read what is in a solar permit set.

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.