Resistance is the root of every drop number on this site. Pick a conductor and length; get the ohms one-way and round-trip, the per-kft and per-km figures, and — with a current entered — the volts lost and watts of heat.
DC resistance at 75 °C from NEC Chapter 9, Table 8 — the same dataset every calculator here runs on. Copper runs about 61% the resistance of aluminum at equal gauge, which is why aluminum installs at roughly two sizes larger. Above 4/0 on AC circuits, skin effect and reactance make the effective impedance slightly higher than DC resistance (Chapter 9 Table 9 territory); below that, DC values are accurate within a percent or two for ordinary power work.
| r | Ω per 1000 ft for the gauge — NEC Chapter 9, Table 8 (75 °C) |
| L | conductor length, feet |
| R | resistance of that length, ohms |
Round trip doubles it. Volts lost = I × R(round trip); watts of heat = I² × R(round trip).
0.193 Ω one-way, 0.386 Ω for the round trip — which is exactly why 20 A over that run drops 7.7 V.
Temperature (values here are at 75 °C operating; room-temp wire reads ~17% lower), terminations, and meter lead resistance all show up in real measurements.