Voltage Drop Calculator

Voltage drop is the loss of electrical potential as current travels through a conductor's resistance. The longer the run and the higher the current, the more voltage is dropped along the wire, and the less voltage reaches the load.

Voltage drop = current × total resistance. Total resistance equals the resistance per 1000 ft (from a published table or the spec sheet) divided by 1000, times the total conductor length (the round trip for DC and single-phase circuits; one-way times √3 for three-phase line-to-line drop).

Current flows out to the load and back to the source, so the total conductor length the current travels is twice the one-way distance. Three-phase circuits have a more efficient geometry; the line-to-line drop uses one-way length times √3.

The NEC recommends (in informational notes) keeping branch-circuit voltage drop below 3% and total feeder plus branch drop below 5%. These are guidelines, not code requirements in most installations. Sensitive electronics, motors, and long runs benefit from staying well below the limit.

Approximate stranded-wire DC resistance values at 75°C, sourced from common electrical reference tables similar to NEC Chapter 9 Table 8. Real-world resistance depends on temperature, conductor temperature rating, and whether the wire is solid or stranded. For exact values, use the manufacturer's spec sheet or the current edition of the NEC.

No. This is an educational estimation tool. NEC compliance requires the official tables, derating for ambient temperature and number of current-carrying conductors, AC reactance for larger conductors, and approval by a qualified electrical inspector. Always consult a licensed electrician for permitted work.