Voltage Drop Calculator

Nominal voltage at the source: line-to-line for three-phase, line-to-line or line-to-neutral for single-phase, whichever the load uses.
Distance from the source to the load. The formula already accounts for the return conductor.
NEC informational notes recommend 3 % on a branch circuit and 5 % feeder plus branch circuit combined.
Leave blank to use 12.9 for copper or 21.2 for aluminium at 75 degC. Supply your own value for a different conductor temperature.
Voltage drop
3.292 %
Voltage Drop Calculator
Voltage drop
7.902 V
Voltage at the load
232.098 V
Conductor area
6,530 cmil
Circuit resistance
0.39510 ohm
Power lost in the conductors
158.04 W
Inside the allowable drop
no
Minimum area for the limit
7,166.666667 cmil
Smallest size meeting the limit
10 AWG
Longest run at this size
91.1 ft
Worked out from NFPA 70, National Electrical Code — Chapter 9, Table 8 (Conductor Properties) and 2 other sources.
The K method uses direct-current resistance at 75 degC and ignores conductor reactance, so it understates drop on large AC feeders and long runs in steel raceway.
The 3 % and 5 % figures come from NEC informational notes; they are recommendations, not code requirements, and your authority having jurisdiction may impose different limits.
The drop of 3.29 % exceeds the 3 % allowance. Increase the conductor size, shorten the run, raise the voltage or parallel conductors.
Advisory only. Voltage drop is one of several constraints on conductor size; ampacity, termination temperature, fault duty and the edition of NFPA 70 adopted by your authority having jurisdiction all govern the final design. Have a licensed electrical professional confirm any installed circuit.

How this is worked out

6 steps
  1. Multiplier for a single-phase circuit: 2 (out and back).
  2. K for copper at 75 degC = 12.9 ohm-cmil/ft; 12 AWG = 6,530 cmil.
  3. VD = 2 x 12.9 x 20 A x 100 ft / 6,530 cmil = 7.902 V.
  4. Percent drop = 7.902 / 240 = 3.292 %, against an allowance of 3 %.
  5. Voltage at the load = 240 - 7.902 = 232.098 V.
  6. Area needed for 3 % (7.200 V): 7,167 cmil, met by 10 AWG.

Results are general information, not financial, tax, legal, medical or engineering advice.