How voltage drop is calculated
Every foot of wire has a little resistance, so the current flowing to the load loses some voltage along the way. The trade K-factor method turns that into one line of arithmetic using the wire's area in circular mils from NEC Chapter 9 Table 8.
voltage drop (V) = 2 × K × amps × one-way feet ÷ circular mils
three-phase: use √3 (1.732) in place of 2
percent drop = voltage drop ÷ system voltage × 100
voltage at the load = system voltage − voltage drop
K is the resistance of one foot of a one-circular-mil conductor: 12.9 Ω·cmil/ft for copper and 21.2 for aluminum at 75°C. DC and single-phase circuits use 2 because the current goes out on one conductor and back on the other. Turn the formula around and you get the longest run for a 3% drop:
longest run (ft) = 0.03 × system voltage × circular mils ÷ (2 × K × amps)
The calculator then tries each size of the same metal, smallest first (aluminum starts at 12 AWG), and reports the first one that keeps the drop at 3% or less.
Worked example: 20 A over 75 ft of 12 AWG copper
The calculator opens on a 20 A load at the end of a 75 ft run of 12 AWG copper on a 120 V single-phase circuit.
- Area of 12 AWG: 6,530 circular mils (Table 8).
- Voltage drop: 2 × 12.9 × 20 × 75 ÷ 6,530 = 5.926 V.
- Percent drop: 5.926 ÷ 120 × 100 = 4.94%, so the load sees 114.07 V.
- Longest run at 3%: 0.03 × 120 × 6,530 ÷ (2 × 12.9 × 20) = 45.6 ft.
- 10 AWG would drop 3.11%, still over 3%. 8 AWG drops 1.95%, so 8 AWG is the smallest size that meets 3%.
The 4.94% drop is under the 5% total but over the 3% branch recommendation, so the calculator flags it. A 20 A circuit still needs a conductor rated for the breaker. Upsizing for voltage drop never lets you use a smaller wire than the ampacity tables require.
Voltage drop by wire size and distance
| Wire size | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|
| 12 AWG | 3.3% | 6.6% | 9.9% | 13.2% |
| 10 AWG | 2.1% | 4.1% | 6.2% | 8.3% |
| 8 AWG | 1.3% | 2.6% | 3.9% | 5.2% |
| 6 AWG | 0.8% | 1.6% | 2.5% | 3.3% |
For the longest run each size can carry at 3%, at other currents and at 240 V, see the voltage drop chart.
The 3% and 5% recommendations
The NEC does not set a maximum voltage drop for ordinary branch circuits and feeders. The informational note to 210.19 and Informational Note No. 2 to 215.2(A)(2) recommend sizing conductors so that the drop is no more than 3% on a branch circuit or feeder and no more than 5% from the service to the farthest outlet. Informational notes are explanatory material, not enforceable rules.
Excess drop wastes energy as heat in the wire. Motors run hotter and draw more current, lights dim when loads switch on, and electronics may reset or shut down. Long runs to detached garages, wells, outbuildings, gates and EV chargers are where the drop usually becomes the deciding factor in wire size.
Measuring the run and choosing a size
- Measure the actual path of the wire, not the straight-line distance: up and over walls, along the ceiling, down to the box. Add the slack you leave in panels and boxes.
- Use the current the load actually draws for long periods. For a branch circuit with several outlets, the load at the far end is the worst case.
- For a feeder plus a branch circuit, work out each run separately and add the percentages to compare against the 5% total.
- Size the wire for ampacity first (NEC Table 310.16 and 240.4(D)), then step it up until the drop is acceptable.
- Larger conductors may not fit the device or breaker terminals. Check the lug ranges, and use a splice or pigtail where the code allows.
- The K-factor method uses DC resistance. For large conductors on AC circuits, NEC Chapter 9 Table 9 (AC impedance) gives a more exact result.
Sources
- K-factor method: VD = 2 × K × I × L ÷ CM (single-phase and DC) and √3 × K × I × L ÷ CM (three-phase), K = 12.9 Ω·cmil/ft copper and 21.2 Ω·cmil/ft aluminum at 75°C
- NFPA 70, National Electrical Code (2023), Chapter 9 Table 8, conductor properties (circular mil areas)
- NFPA 70, National Electrical Code (2023), 210.19 Informational Note and 215.2(A)(2) Informational Note No. 2 (3% branch or feeder, 5% total)