Understanding wire gauge sizing
Why ampacity, not just current, decides the gauge
Every conductor has a maximum current it can carry continuously without its insulation overheating, called its ampacity, and NEC Table 310.16 sets that ceiling for each wire gauge and insulation type. A circuit's load current isn't the number you size against directly. Continuous loads, meaning anything running 3 or more hours at a stretch, which describes most daytime PV output, get a 125% factor applied first, the same continuous-load logic used in the disconnect switch sizing tool. A 25A continuous circuit needs wire rated for at least 31.25A before you even look at temperature or bundling. The full ampacity table behind these calculations is NEC Table 310.16.
Why the same wire performs differently on different roofs
A wire's table ampacity assumes a 30C ambient baseline. Run that same conductor through a conduit baking on a dark asphalt roof in Phoenix, and the surface temperature can hit 60C or higher, which cuts the wire's safe carrying capacity substantially under NEC 310.15(B)(1). This is one of the more commonly skipped steps in DIY wire sizing, and it's a real gap: a 10 AWG conductor that comfortably handles 40A at 30C ambient may only be good for about 23A once a 55C correction factor of roughly 0.76 is applied.
Why conductor count in the same raceway matters
Bundling more current-carrying conductors into one conduit or cable traps heat, since each wire has less opportunity to dissipate what it generates. NEC 310.15(C)(1) accounts for this with an adjustment factor that starts cutting in once you exceed three current-carrying conductors, dropping to 80% at four to six, and continuing to shrink from there. Combined string circuits, worked out with the parallel strings current calculator or a combiner box sizing calculator, often push a conduit run past the three-conductor threshold faster than expected once you count both current-carrying legs of every string.
Ampacity sizing is the floor, not the whole answer
This calculator finds the smallest gauge that won't overheat under your stated conditions. That's a real constraint, but it's independent from voltage drop, which is a separate concern about how much voltage the wire itself eats up over distance. A long run from a rooftop array to a ground-mounted inverter can pass the ampacity check on 10 AWG and still lose enough voltage over 150 feet to justify jumping to 8 AWG or larger. Always run both checks; ampacity keeps the wire from overheating, voltage drop keeps the system efficient. NREL's PVWatts calculator can help confirm what either gauge choice ultimately means for your site's annual production.
What this tool doesn't cover
This calculator uses copper conductors with 90C-rated insulation (THWN-2 and XHHW-2 are common choices for solar), the standard assumption for most rooftop and array wiring in current solar design guides. Aluminum conductors have different ampacity tables entirely, and if your local jurisdiction terminates equipment at a 75C rating, the equipment's terminal rating can become the limiting factor even when the wire itself is rated for more. Choosing the gauge is one step; keeping the connections sound afterward is another, covered by the connector and wiring inspection cost estimator, and if your run feeds a detached structure, the system size for detached garage calculator covers the array-planning side of that same layout. Always confirm the final gauge and insulation type against your local electrical code and a licensed electrician before purchasing.