Solar PV Wire Sizing Guide (NEC 690) | PES Supply

Solar PV Wire Sizing Guide (NEC 690)

Complete wire sizing reference for solar installations per National Electrical Code

NEC 690.8 — Current Calculations

Solar PV circuit current must be calculated at 125% of short-circuit current (Isc):

NEC 690.8(A)(1): Maximum Current = Isc × 1.25

NEC 690.8(B)(1): Conductor Ampacity = Max Current × 1.25

Total multiplier: Isc × 1.5625

Common Solar Panel Specs

Panel Model Wattage Voc Isc Imp Min Wire (series)
VSUN 490W 490W 49.8V 13.2A 12.5A 14 AWG
Silfab 460W 460W 50.1V 11.7A 11.1A 14 AWG
EG4 450W 450W 48.5V 11.5A 10.9A 14 AWG
Mission 400W 400W 47.2V 10.3A 9.7A 14 AWG

Wire Sizing by Array Configuration

String Wiring (Series)

In series, current stays the same as one panel. Wire is sized based on Isc × 1.5625:

Panel Isc NEC Ampacity Min Wire (Copper) Breaker/Fuse
10A 15.6A 14 AWG 15A
12A 18.8A 12 AWG 20A
15A 23.4A 10 AWG 25A

Parallel Wiring

In parallel, currents add up. Wire is sized based on (total Isc) × 1.5625:

Panels in Parallel Isc Each Total Isc NEC Ampacity Wire Size Breaker
2 12A 24A 37.5A 8 AWG 40A
3 12A 36A 56.3A 6 AWG 60A
4 12A 48A 75.0A 4 AWG 80A
6 12A 72A 112.5A 2 AWG 125A

Home Run Wire Sizing (Array to Inverter)

The home run wire must handle the combined array current and account for voltage drop over distance:

Array Size Distance Wire Size (3% VD) Wire Type
3 kW (120V string) 50 ft 10 AWG PV Wire / THWN-2
5 kW (400V string) 50 ft 12 AWG PV Wire
5 kW (400V string) 100 ft 10 AWG PV Wire
10 kW (400V string) 50 ft 10 AWG PV Wire
10 kW (400V string) 150 ft 8 AWG PV Wire
15 kW (600V string) 100 ft 10 AWG PV Wire

Wire Types for Solar

Type Rating Use Temperature
PV Wire 600V / 2000V Module interconnects, exposed 90°C wet/dry
USE-2 600V Module interconnects (older) 90°C wet/dry
THWN-2 600V Conduit runs 90°C dry, 75°C wet
XHHW-2 600V Conduit, direct burial 90°C wet/dry
RHW-2 600V Direct burial 90°C wet/dry