Code-Minimum Wire Is a Loan You Repay in Watts

Code-minimum conductor sizing answers one question: will this wire avoid overheating? It says nothing about the second question: how much will this wire cost to own? For a few circuit types, the answer is "more than the bigger wire would have."

The loss formula

Power burned in the conductor is P = Vd × I = I² × R_circuit. It scales with the square of current and linearly with resistance — so heavily loaded circuits waste disproportionately, and each AWG upsize claws back ~21% of the loss.

Three worked cases

• 48 A EV charger, 80 ft, 6 AWG: ~180 W lost while charging. At 4 h/day, ~263 kWh/yr ≈ $42. Upsizing to 4 AWG saves ~$15/yr for roughly $90 extra copper — a 6-year payback that beats most home-efficiency upgrades, and the wire outlives three cars.
• Well pump, 10 A at 240 V, 350 ft, 10 AWG: ~85 W lost while pumping. Irrigation duty (3 h/day) wastes ~93 kWh/yr ≈ $15 — and the pump motor runs hotter for it. 8 AWG halves both numbers.
• Workshop heater, 20 A at 240 V, 100 ft, 12 AWG, 8 h/day all winter: ~154 W lost, ~$30/season. 10 AWG saves ~$11/season forever.

The decision rule

Upsizing pays when the circuit is long, heavily loaded, and high-duty — two of three is the threshold worth checking. Lightly used receptacle circuits never qualify; chargers, pumps, heaters and feeders often do. The energy loss calculator does the full math — duty hours, your kWh rate, annual dollars — so the copper decision becomes a payback number instead of a guess.

Labor and trenching dominate installed cost. The marginal copper is the cheap part of doing it right once.