How to use the line power loss calculator
Current flowing through a feeder is partly converted to heat by the resistance of the conductor itself. That loss shows up on every utility bill, and it also pulls down the voltage at the load, which can overheat motors and dim lighting. Enter the load current, the one-way run length and the conductor size to see resistance, power loss, voltage drop and drop percentage at once.
Resistance comes from NEC Chapter 9, Table 8 DC resistance values for stranded uncoated copper at a 75°C operating temperature, expressed in ohms per 1,000 ft. A single-phase, 2-wire circuit loses power in both legs, so P = 2 × I² × R, while a three-phase circuit uses P = 3 × I² × R. Voltage drop is 2IR and √3 × I × R respectively.
If the drop percentage is high, move up one conductor size or relocate the panel closer to the load. NEC 210.19(A) and 215.2(A) informational notes suggest holding branch-circuit drop near 3 percent and the combined feeder plus branch drop near 5 percent for reasonable efficiency.
These numbers treat the run as pure DC resistance, so reactance, conductor heating and harmonic effects are not included. Have a licensed electrical engineer or electrician review the design before it is finalized.
Frequently asked questions
Resistance is inversely proportional to area, so moving from 2 AWG to 1/0 AWG cuts resistance and loss by roughly 37 percent. Weigh that against copper cost and available conduit fill.
No. Enter the one-way distance from the panel to the load. The number of current-carrying conductors is applied automatically based on the system type you select.