Voltage drop per foot of feeder is the current multiplied by the conductor resistance for that length, and doubled for a single-phase loop. Start with the load current, the conductor size and material, and the one-way run length. You get volts lost per foot and a percentage of system voltage. Check that figure before the wire is bought.
Voltage drop per length formulas
The resistance comes from the conductor tables of the NEC edition your jurisdiction has adopted. That means Chapter 9, Table 8 for DC resistance or Table 9 for AC resistance and reactance, published by NFPA 70. Free copies of those tables on third-party sites disagree with each other in the second decimal place. For example, two secondary tables list different values for 12 AWG copper, 1.98 and 1.93 Ω/kft. Read the value from your own edition, the conductor manufacturer's data sheet, or the project specification. This page does not publish resistance values, because the NEC text was not available to verify them.
| System | Voltage drop per foot (V/ft) | Percent drop |
|---|---|---|
| Single-phase, 2-wire | 2 × I × R ÷ 1000 | (V/ft × L ÷ V) × 100 |
| Three-phase, 3-wire | 1.732 × I × R ÷ 1000 | (V/ft × L ÷ V) × 100 |
- I: load current in amperes. Use the generator's rated output current or the actual feeder load, not the breaker rating, unless the breaker rating is your design current.
- R: conductor resistance in ohms per 1,000 ft (Ω/kft) at the stated temperature.
- L: one-way feeder length in feet.
- V: system voltage. Use line-to-line voltage for three-phase and for 240 V single-phase loops.
For metric work, use R in Ω/km. The drop in volts per metre is then I × R ÷ 1000, times 2 for single-phase or 1.732 for three-phase.
Worked example
This is an illustration, not a design. The inputs are:
- 240 V single-phase feeder
- 40 A load
- 100 ft one way
- 6 AWG copper
- an assumed R of 0.491 Ω/kft (confirm against your NEC edition before relying on it)
- Drop per foot = 2 × 40 × 0.491 ÷ 1000 = 0.0393 V/ft.
- Drop over the run = 0.0393 × 100 = 3.93 V.
- Percent drop = 3.93 ÷ 240 × 100 = 1.64%.
The same run at 200 ft gives 7.86 V, or 3.28%. Drop scales linearly with length and current for a fixed conductor.
Limits of this table
- Not a code limit. The NEC's 3% figure for branch circuits and 5% combined feeder and branch circuit drop appear in informational notes, such as 210.19(A) and 215.2(A). These are recommendations, not mandatory rules. Project specifications, local amendments, or specific articles can set enforceable limits.
- Ampacity is separate. A conductor that passes the drop check can still be too small for the load. Size it against the wire ampacity table and the equipment's terminal temperature rating.
- Inputs it does not cover. Motor starting current causes a larger, brief dip that this steady-state figure does not predict. Power factor, conductor temperature, parallel sets, and long runs with significant reactance also fall outside it. Table 9 values are the better starting point for larger AC feeders.
- Installation checks still apply. Generator nameplate data, the manufacturer's installation manual, the transfer switch requirements, and a qualified electrician or engineer determine the final conductor, grounding, and neutral arrangement. The generator manufacturer's literature, such as Cummins generator guidance, covers unit-specific application limits.
Related tools
- Run the whole feeder through the generator feeder wire calculator.
- Add raceway costs with the conduit material cost calculator.
- See the generator installation guide for the wider installation context.