NEC 690 solar cable sizing (AWG) — how it works

What NEC 690 actually covers

NEC Article 690 (NFPA 70) is the section of the National Electrical Code specific to photovoltaic systems — conductor sizing and ampacity, grounding and bonding (Part V), and ground-fault protection all fall under it, alongside the general wiring rules elsewhere in the Code. An off-grid, non-utility-interactive system is still covered by 690, though some provisions written primarily for grid-tied systems apply differently or not at all — this is exactly the kind of distinction worth confirming with a licensed electrician rather than assuming from a generic guide.

AWG conductor sizing and ampacity

Conductor ampacity in the US is read from NEC Table 310.16 (the 75°C column is the common reference for most off-grid equipment terminations), then adjusted for the real installation: ambient temperature, how many current-carrying conductors are bundled together, and the run length. A cable that's the right gauge on paper at 25°C can be undersized once it's actually bundled with three other cables in a hot enclosure — the derating factors aren't optional extras, they're what makes the number correct for your install.

String cables are typically sized with a 25% safety margin on the panel's short-circuit current — standard practice independent of which code you're working under. What changes country to country is which ampacity table and which conductor markings (AWG vs mm²) you're reading the number from.

Grounding and bonding — NEC 690 Part V

For an off-grid inverter, the neutral-to-ground bonding configuration matters and depends on the inverter's topology — this isn't a detail to guess at from the datasheet alone. NEC 690 Part V (690.41, 690.43, 690.47) governs grounding and bonding for PV systems, and the equipment grounding conductor itself is sized per NEC 250.66, a separate table from the ampacity one above. Grid-interactive and off-grid systems are treated differently here, so a wiring diagram copied from a grid-tied guide won't necessarily apply as-is.

DC ground-fault protection

NEC 690.41(B) requires DC ground-fault protection (GFPD) for PV source circuits. On the AC side, dwelling circuits fall under the AFCI/GFCI requirements in NEC 210.12 and 210.8 — a different mechanism from the RCD Type B requirement used in the UK and EU, not a direct equivalent despite serving a similar purpose. If you're used to European wiring conventions, this is the point where the two codes diverge the most.

Common mistakes