NEC 690.8 vs 690.9 — currents versus overcurrent protection

Two adjacent sections in Article 690 trip up more solar designs than almost any others: 690.8 and 690.9. They sound similar, but one sizes the wire and the other sizes the fuse or breaker. Confusing them produces conductors that are too small or overcurrent devices that don't protect what they should.

690.8: how much current, and how big the wire

690.8 answers two questions in sequence.

690.8(A) — maximum circuit current. For a PV source or output circuit, the maximum current is the sum of module Isc multiplied by 1.25. That 1.25 accounts for irradiance above the standard test rating; it is a property of the current itself, not a safety pad.

690.8(B) — conductor ampacity. The conductor must have an ampacity at least equal to the larger of two calculations:

• 690.8(B)(1): the 690.8(A) maximum current times 1.25 (the continuous-duty factor), without any temperature or conduit-fill adjustment; or
• 690.8(B)(2): the 690.8(A) maximum current after applying temperature and conduit-fill correction factors.

Because 690.8(A) already applied 1.25 and 690.8(B)(1) applies another 1.25, the two factors stack: 1.25 x 1.25 = 1.56. That is why the familiar rule of thumb is "size the ungrounded PV conductor to 156% of Isc" before derating.

You take the worse of the continuous-duty result and the derated result — see conductor ampacity derating explained for how the two compete.

690.9: the overcurrent device

690.9 governs the fuse or breaker. Here the factors do not stack the same way. The overcurrent device rating must be at least 1.25 times the 690.8(A) maximum current — one 1.25, applied to the already-1.25'd current. It must also protect the conductor and any module series-fuse rating.

• Overcurrent protection is required where the available fault current from parallel sources or a battery could exceed a conductor or module rating.
• Modules carry a maximum series fuse rating on their datasheet; the OCPD must not exceed it.
• OCPDs are rounded up to the next standard size in NEC 240.6 — see OCPD standard sizes.

Where designers go wrong

Two mistakes show up over and over on submitted plans:

• Applying 156% to the OCPD. The stacked factor belongs to the conductor, not the device. The breaker or fuse gets one 1.25 on the 690.8(A) current. Double-stacking oversizes the device and can leave the conductor unprotected.
• Sizing the OCPD to the raw conductor table value. The device must protect the conductor's final, derated ampacity — after temperature and conduit-fill corrections — not its Table 310.16 starting number.

A third subtle trap: a battery or a utility feed can push fault current backward into a circuit that the PV output alone wouldn't overload. That backfeed is exactly the condition 690.9 asks you to evaluate for overcurrent protection.

Putting them side by side

The clean mental model: 690.8 protects the conductor by making it big enough; 690.9 protects everything by making the device trip in time. Size the wire with the stacked 156% (or the derated value, whichever is larger), then size the device at 125% of the 690.8(A) current, confirm it protects the conductor's final ampacity, and round to a standard size. On a small two-source circuit where no fuse or breaker is even required, 690.9 lets you omit the OCPD entirely — but you still size the conductor under 690.8.

For the raw conductor-and-OCPD relationship, our 690.8 conductor and OCPD reference walks a worked example.

Adoption of the NEC varies, and continuous-current handling saw wording changes across editions. Confirm the enforced code and local amendments with your AHJ, and remember a licensed electrician or PE must 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.