ConduitCount

Voltage drop calculator

Wire size, load and one-way length in. Out comes the drop in volts and percent, and the smallest size that keeps it under 3 %.

6.34 % dropOver 3 %
Volts lost
7.60 V
At the load
112.4 V
Smallest for 3 %
#8
Smallest for 5 %
#10

2 × 16 A × 1.98 Ω per 1,000 ft × 120 ft ÷ 1,000 = 7.60 V

About the numbers

Resistance is the DC value at 75 °C from Chapter 9, Table 8 (uncoated copper, or aluminum). For runs up to about 2 AWG that is close to the AC value; for big conductors on AC, Table 9 gives a slightly higher effective impedance, so treat large-size results as a floor.

The 3 % for a branch circuit and 5 % for feeder plus branch are the NEC's informational notes (210.19(A) and 215.2(A)), not a requirement, except where a rule makes them one (for example 647.4(D) for sensitive electronic equipment).