Code & permits

NEC 690 for DIY solar builders

Article 690 of the National Electrical Code (NEC) governs solar PV systems in the United States. Most jurisdictions adopt some version of the NEC by reference, so if your install needs a permit, NEC 690 is what your inspector will check it against. Here's the honest breakdown of which rules matter for DIY off-grid and grid-tied builders.

Why DIY builders need to know this

Three reasons. First, code-compliant systems pass inspection — and that's often required for occupancy permits, insurance, and resale. Second, NEC rules exist because real installations have caught fire when they were skipped. Third, even if your system is unpermitted (RV / boat / outbuilding with no permit requirement), following NEC 690 makes the system genuinely safer.

690.7 — Cold-weather Voc derating

The rule: Panel open-circuit voltage (Voc) goes UP in cold weather. A panel rated 22V Voc at 25°C (the spec sheet number) can produce 27V or more at 0°F. If you wire panels in series, the string voltage spike scales with the count. A 4-panel series string at 22V nameplate is 88V nameplate but could hit ~110V on a cold morning.

NEC 690.7 requires you to multiply nameplate Voc by a temperature correction factor before checking against your controller's max PV voltage. A standard safety factor used by most DIY builders is ×1.25 — the same factor the builder on this site applies — which covers temperatures down to about -10°F for typical silicon panels.

Why it matters: most MPPT controllers max out at 100V or 150V PV input. If your derated string Voc exceeds the controller's spec, you'll blow it at sunrise on the coldest day of the year — exactly when you need it most.

690.8 — Ampacity calculations

Two compounding ×1.25 factors apply to PV current sizing:

• ×1.25 for continuous operation — required by NEC 210.19 for any circuit operating ≥3 hours continuously. Solar arrays always qualify during the day.
• ×1.25 specific to solar — NEC 690.8(A)(1) acknowledges that panels can briefly exceed nameplate Isc in real-world conditions (irradiance enhancement, cold/clear days).

The combined factor of 1.25 × 1.25 = 1.56 applies when sizing wire and overcurrent protection between panels and controller. The builder on this site uses the simpler ×1.25 for the controller-amp recommendation since that's the headroom math you care about most; the ×1.56 applies to wire/fuse sizing for the PV-side conductors.

690.9 — Overcurrent protection

Every PV source circuit (panel string), output circuit (controller to battery), and battery circuit needs overcurrent protection — fuses or breakers sized at or above the conductor ampacity.

Practical takeaway: each parallel panel string needs its own combiner-box fuse, the controller-to-battery cable needs a breaker, and the battery-to-inverter cable needs a Class-T (LFP) or ANL/MEGA (lead-acid) fuse within 18 inches of the battery positive terminal.

690.12 — Rapid shutdown

Required for grid-tied systems on buildings (most commonly residential roof installs). A rapid shutdown system de-energizes the array within 30 seconds to a safe touch voltage when a single switch is thrown — typically at the utility meter or a labeled disconnect at building entry.

Most string inverters meet this with module-level shutoffs (Tigo TS4-F, SolarEdge optimizers, Enphase microinverters do it natively). DIY string setups without module-level electronics need to add them — typically the cheapest path is Tigo TS4-F-RS units, ~$50 per panel.

Off-grid systems on non-building structures (RVs, boats, ground mounts, sheds) generally don't need 690.12 compliance. Check with your AHJ.

690.13–690.17 — Disconnects

Every DC source needs a disconnecting means accessible to firefighters and service personnel. Practically:

• · A PV disconnect between the array and the controller
• · A battery disconnect within sight of the inverter
• · An AC disconnect between the inverter and the load center

Listed solar DC disconnects (like Midnite Solar's MNDC-series) are the clean way to satisfy this. Don't substitute regular AC breakers — DC arcs don't self-extinguish like AC arcs, and a normal switch will weld closed under DC fault current.

690.43 — Equipment grounding

Every metallic frame in the system (panel frames, racking, enclosures) must be bonded to a single equipment grounding conductor that ties back to the AC service ground (grid-tied) or a dedicated ground rod (off-grid). Use proper lay-in lugs on aluminum panel frames — standard ring terminals corrode and lose continuity within a few years.

For panel arrays on the same rail, WEEB (Washer Electrical Equipment Bond) washers between panel frames and rails are NEC-approved and dramatically cut the wiring labor versus individual lugs on every frame.

Which rules apply to MY install?

• Permitted residential install (grid-tie or hybrid): all of the above apply. Plan accordingly.
• Permitted residential off-grid install (cabin, shop): all except 690.12 rapid shutdown typically apply. Confirm with AHJ.
• RV, marine, ground-mount in unpermitted areas: code doesn't legally apply, but the safety logic still does. Follow 690.7, 690.8, and 690.9 (overcurrent) at minimum.
• Insurance question: if the system caused a fire and your install bypassed NEC, insurers may deny the claim. Following code even where unenforced protects your coverage.

This is a practical primer, not legal advice. Buy a copy of NFPA 70 (the full NEC) if you're permitting any non-trivial install — the official text is dense but unambiguous, and your inspector is reading from it. The 2023 edition is current in most states; 2017 and 2020 are still in use in some.