Solar PV Wire Size Calculator

The 690.8 method, step by step

A PV array is a current source that produces its rated short-circuit current into a bolted fault, and irradiance above standard test conditions pushes the current above the nameplate Isc. NEC 690.8(A)(1) accounts for that by defining the maximum circuit current as 125 percent of the rated Isc. Because a PV circuit carries that current for hours at a time, it is a continuous load, so 690.8(B) and 690.9 apply a second 125 percent factor to the conductor and the overcurrent device.

The conductor is sized for the larger of two requirements. The 690.8(B)(1) path takes 125 percent of the maximum current and compares it to the conductor ampacity before any derating. The 690.8(B)(2) path takes the maximum current itself and compares it to the ampacity after ambient and conductor-count derating. On a cool run with only two conductors the first path usually governs; on a hot rooftop with many strings sharing a raceway the derated path governs and forces a larger conductor. This tool evaluates both and reports the larger result.

Worked example: 8 A source string

1. The module rated short-circuit current is 8 A, one series string.
2. Maximum circuit current is 125 percent of Isc: 8 x 1.25 = 10 A (NEC 690.8(A)(1)).
3. 690.8(B)(1): 125 percent of 10 A = 12.5 A. 14 AWG copper is 20 A in the 75 degrees C column, which meets it.
4. 690.8(B)(2): 10 A against the derated ampacity. With two conductors at a normal ambient there is no derating, so 14 AWG still qualifies.
5. The larger of the two paths is 14 AWG copper.
6. 690.9(B)(1): overcurrent device at 125 percent of 10 A = 12.5 A, rounded up to the next standard rating, a 15 A PV-listed fuse.

Isc versus Imp: which current sizes the wire

Two module currents matter for PV, and they do different jobs. The short-circuit current Isc sizes the conductor ampacity and the overcurrent device under 690.8 and 690.9, because those protect against the maximum current the array can push. The maximum power current Imp, which is smaller, is the operating current used for energy yield and voltage-drop calculations. Sizing conductors on Isc is conservative: because Isc is larger than Imp, a conductor sized on Isc is never undersized for the operating current. Use the Solar Voltage Drop calculator, which works from Imp, for the separate voltage-drop check.

Overcurrent protection and the module fuse rating

The overcurrent device this tool reports is the minimum rating required by 690.9(B)(1): at least 125 percent of the maximum circuit current, rounded up to a standard 240.6(A) rating. Devices in PV source and output circuits must be listed for PV use per 690.9. There is a second limit the tool cannot check for you: every module datasheet lists a maximum series fuse rating, and the overcurrent device may not exceed it. When two or more source circuits are paralleled without series fuses, the fault current from the other strings is what makes those fuses necessary in the first place. Always confirm the recommended device against the module datasheet.

Why this tool may size one step larger than others

This calculator holds the 690.8 and 690.9 figures to their exact values and rounds the conductor up, never down. Where another PV wire-size tool rounds a required ampacity down to the nearest standard conductor, it can show a conductor one size smaller. That is a documented, intentional deviation, not a disagreement about the method: it is the never-undersize choice this site is built on. The overcurrent device is always rounded up to the next standard 240.6(A) rating, as 690.9 requires.

Frequently asked questions

How do I size PV wire under NEC 690.8?

Take 125 percent of the module Isc for the maximum circuit current, then size the conductor for the larger of 690.8(B)(1) (125 percent of that current, no derating) and 690.8(B)(2) (that current, with derating). An 8 A Isc string needs 14 AWG 90 degrees C conductor and a 15 A fuse.

Why is there a 125 percent factor twice?

One 125 percent is the 690.8(A) maximum-current factor for irradiance above standard test conditions; the other is the continuous-duty factor in 690.8(B)(1) and 690.9(B). Together they give 156 percent of Isc for the no-derate conductor and the overcurrent device.

Do I use Isc or Imp to size PV conductors?

Isc for conductor ampacity and overcurrent protection (690.8, 690.9). Imp is the operating current for energy and voltage-drop work. Isc is larger, so sizing on it never undersizes.

What size fuse does a solar string need?

At least 125 percent of the maximum circuit current, rounded up to a standard 240.6(A) rating. A 10 A Isc string needs a 20 A PV-listed fuse. Confirm it does not exceed the module maximum series fuse rating.

Does the rooftop temperature adder still apply?

No. The former 310.15(B)(3)(c) rooftop adder was removed in NEC 2020. Enter the actual ambient and the tool applies the Table 310.15(B)(1) correction.

Related calculators

• Solar Voltage Drop Calculator: PV string and array voltage drop against the 2 and 3 percent solar targets, using the operating current.
• Wire Ampacity Calculator: the Table 310.16 ampacity of a chosen conductor, with ambient and fill derating.
• DC Wire Size Calculator: DC conductor sizing by ampacity and voltage drop for solar, battery, and marine circuits.
• Fuse Size Calculator: standard 240.6(A) fuse ratings and the conductor a fuse can protect for general-use loads.