TradesHow this works
A PV DC conductor has to pass two tests, and the bigger answer wins. The first is ampacity: NEC 690.8 makes you size to the short-circuit current times 156 percent, so the wire never overheats even on a bright, cold, edge-of-cloud day. The second is voltage drop: a long DC run bleeds off energy as heat in the copper, and unlike the code minimum, that loss is a design target you pick.
This sizes the conductor both ways and tells you which one governs. On short high-voltage strings the code ampacity is usually enough. On long low-voltage runs, like a battery circuit, voltage drop takes over and pushes the size up. It stops at planning grade: it does not apply rooftop heat or conduit-fill derating, so on a hot roof step up from what it shows.
Common questions
How do you size PV DC wire per NEC 690.8?
The conductor has to carry the module or array short-circuit current multiplied by 156 percent. That is a 125 percent factor for the edge-of-cloud irradiance boost and another 125 percent for continuous operation, and the two compound to 1.5625. The smallest conductor whose table ampacity meets that current is the code floor, before any temperature or conduit-fill derating.
Does the NEC set a DC voltage-drop limit?
No. The code has no voltage-drop requirement for PV DC circuits, so the target is a design choice. Around two percent is a common goal because DC losses run for the whole day, and three percent is a looser ceiling. The tool sizes to whichever you set and shows the drop the recommended conductor actually lands at.
Why does the recommendation sometimes jump above the code minimum?
On a long run or a low DC voltage, the voltage-drop target needs more copper than the ampacity floor does, so voltage drop governs and the size steps up. On a short run or a high-voltage string, ampacity governs and the code minimum is enough. The tool shows both sizes so you can see which one is driving it and where the crossover is.
Sources: NEC (NFPA 70) Article 690.8 conductor sizing (2023) · NEC Table 310.16 ampacity + Chapter 9 Table 8 (via necData) (2023) · Voltage-drop target: industry design recommendation (no NEC DC limit). Engine version 1.0.0. Risk tier 2.