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E-01 · Electrical

Voltage drop calculator

Voltage drop in volts and percent for single-phase, three-phase and DC circuits, checked against the wire's ampacity. Conductor data from NEC 2023 Chapter 9.

System
V

Source to load, not round trip.

Conductor

Result

2.67%

3.20 V drop

012463% branch5% total

Within the 3% branch target.

Voltage at load
116.80 V
Effective impedance
2.000 Ω/1000 ft
Ampacity, 12 AWG Cu
20 A@60 °C
Smallest size for 3% branch
12 AWG Cu

3% / 5% are NEC informational-note recommendations, not requirements. Ampacity uses the column 110.14(C) requires for this circuit, with the load current standing in for the breaker rating.

How the calculation works

Voltage drop is the load current times the impedance of the wire, out to the load and back. For AC circuits the calculator takes the resistance and reactance per 1,000 ft for your wire size, metal and conduit from NEC Chapter 9 Table 9, and combines them with the load's power factor into an effective impedance. DC circuits use the DC resistance from Table 8.

Formula

Single-phase
VD = 2 × L × I × Ze ÷ 1000
Three-phase
VD = 1.732 × L × I × Ze ÷ 1000
DC
VD = 2 × L × I × R ÷ 1000
where
Ze = R × PF + X × sin(arccos PF)

L is one-way length in feet, I is load current in amps, R and X are ohms per 1,000 ft. With parallel sets, divide Ze by the number of sets.

Worked example

A 240 V, 40 A single-phase load sits 150 ft from the panel. Copper in PVC conduit, power factor 1.0.

SizeDrop%3% target
8 AWG9.36 V3.90Over
6 AWG5.88 V2.45Meets

8 AWG carries the 40 A load (40 A at 60 °C), but loses 3.90% over that distance. Going up one size to 6 AWG brings the drop to 2.45%. Long runs are where voltage drop, not ampacity, decides the wire size.

Assumptions

  • Conductors at 75 °C, the temperature Table 8 and Table 9 values are based on.
  • Uncoated, stranded conductors; for AC, three single conductors in one conduit at 60 Hz.
  • The ampacity check uses Table 310.16 conditions: up to three current-carrying conductors at 30 °C ambient. Apply your own adjustment for more conductors or hotter spaces.
  • By default the ampacity column follows 110.14(C): 60 °C for circuits of 100 A or less or conductors 14 through 1 AWG, 75 °C otherwise. The load current stands in for the breaker rating.
  • Length is one way. The formula accounts for the return path.

References

  • NFPA 70, National Electrical Code, 2023 edition: Chapter 9, Tables 8 and 9.
  • NFPA 70, 2023: Table 310.16, Section 110.14(C) termination temperature limits, Section 240.4(D).
  • NFPA 70, 2023: informational notes to 210.19 and 215.2 on voltage drop.

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