Conductor Ampacity and Voltage Drop Calculator

Sizing a conductor means satisfying two separate checks at the same time: the conductor has to carry the required current safely (ampacity), and it can’t drop so much voltage over its length that it hurts system performance (voltage drop). This tool runs both checks together and steps through wire sizes to find the smallest one that passes both.

It uses:

• NEC Table 310.16 ampacity values (copper and aluminum, 60C/75C/90C columns)
• NEC Chapter 9 Table 8 DC resistance values for voltage drop
• The ambient temperature correction formula behind NEC 310.15(B)(1), so temperature correction is continuous rather than snapped to a table bin
• The rooftop conduit temperature adder from NEC 310.15(B)(3)(c)
• The Isc x 1.25 x 1.25 sizing convention from NEC 690.8 for PV source and output circuits

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A few notes on how it works

PV source/output circuits use the module or string Isc rating as the starting current. NEC 690.8(A)(1) requires multiplying Isc by 1.25 to account for irradiance above standard test conditions, and 690.8(B)(1) requires an additional 1.25 continuous-duty multiplier on top of that, for a combined 1.5625x. Other continuous duty circuits get a single 1.25x, and non-continuous circuits get no multiplier.

Rooftop conduit runs hotter than free air because of reflected and re-radiated heat from the roofing material. NEC 310.15(B)(3)(c) adds a temperature adder based on how far the conduit sits above the roof surface, from 33C for conduit resting directly on the roof, down to no adder above 3.5 inches.

This is a design aid for estimating and learning; always verify final conductor sizing against the current NEC edition adopted in your jurisdiction and, where required, a licensed professional’s stamped design.