Solar Wire Sizing Calculator
A solar wire sizing calculator determines the minimum wire gauge required for photovoltaic (PV) system DC wiring, considering both the NEC 690.8 ampacity requirement and voltage drop limits. Undersized wiring reduces system output and creates fire and safety hazards. NEC 690.8 requires solar source circuit conductors to be sized at 156% of the module short-circuit current (Isc). The voltage drop check ensures wire resistance does not cause excessive power loss. This tool calculates both the ampacity-based and voltage drop-based minimum wire sizes and recommends the more conservative (larger) of the two.
Solar wire sizing formulas
NEC design current = Isc x 1.25 x 1.25 = Isc x 1.5625
Ampacity AWG: select smallest AWG with ampacity >= design current (90C PV wire)
Max resistance = (Voc x VD%) / (2 x design current)
Required cross-section (CM) = (21.2 x 2 x run x design current) / (Voc x VD%)
VD AWG: select smallest AWG with circular mils >= required CM
Frequently asked questions
How do I size wire for a solar panel system?
Solar PV wire sizing must satisfy two criteria: (1) ampacity - the wire must handle the short-circuit current (Isc) x 1.25 x 1.25 = 156% of Isc per NEC 690.8, and (2) voltage drop - total voltage drop from array to inverter should be less than 2-3% to minimize power loss. Choose the larger of the two required wire sizes.
What is the NEC 690.8 rule for solar wire sizing?
NEC 690.8 requires that conductors in PV source circuits be sized at 125% of the maximum current (typically the short-circuit current Isc under STC conditions). For overcurrent protection, an additional 125% factor is applied to the breaker size. The wire is therefore sized for 156.25% of Isc.
What wire type is used for solar panels?
PV wire (USE-2 or PV-rated wire, UL listed to 600V or 1000V) is required for direct burial and rooftop applications. THHN/THWN-2 is acceptable in conduit. PV wire has higher temperature ratings (90C wet) and UV resistance. Most residential systems use 10 AWG for source circuits (homerun from each string).
What voltage drop is acceptable for solar wiring?
NABCEP and NEC best practices recommend keeping voltage drop in PV source circuits to less than 2% and in PV output circuits (combiner to inverter) to less than 1%. Higher voltage drop reduces system energy production proportionally, as power lost = current^2 x resistance. Use larger wire or shorter runs to reduce drop.
What is the difference between PV source circuit and output circuit wire?
The PV source circuit connects individual panels or strings to the combiner box. The output circuit connects the combiner box (or string inverter output) to the AC disconnect or main panel. Source circuits carry DC at PV voltage; output circuits carry AC power from the inverter. Each segment has its own sizing requirements.
Official sources
- NFPA: NFPA 70 NEC - Article 690: Solar Photovoltaic (PV) Systems.
- NABCEP: NABCEP PV Installation Professional Study Guide - Wiring and Safety.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.