How to use the voltage drop calculator
Choose the system, enter the source voltage, the load current and the one-way length of the run, then pick the conductor material and size. The calculator gives the drop in volts and as a percentage of the source voltage, the voltage left at the load, the longest run that stays inside your limit, and the smallest wire size that would meet it.
Voltage drop is only half of sizing a wire. The conductor must first be big enough to carry the current safely under NEC Table 310.16; the wire size calculator checks both at once.
The formulas
single-phase AC or DC: VD = 2 × L × I × R ÷ 1,000
three-phase AC: VD = 1.732 × L × I × R ÷ 1,000
percent drop = VD ÷ source voltage × 100
L is the one-way length in feet, I the current in amps and R the conductor's resistance in ohms per 1,000 ft from NEC Chapter 9 Table 8. Single-phase and DC use 2 because current flows out and back; three-phase uses √3.
Worked example: a 40 A, 240 V circuit, 100 ft of 8 AWG copper
- Table 8 resistance for stranded 8 AWG copper: 0.778 Ω per 1,000 ft.
- Drop: 2 × 100 × 40 × 0.778 ÷ 1,000 = 6.22 V.
- Percent: 6.22 ÷ 240 = 2.59%, inside the 3% guideline.
- The same run at 120 V would be 5.2%, which is why the circuit voltage matters as much as the wire.
Resistance by size (NEC Table 8, stranded, 75 °C)
| AWG | Copper Ω / 1,000 ft | Aluminum Ω / 1,000 ft |
|---|---|---|
| 14 | 3.14 | — |
| 12 | 1.98 | 3.25 |
| 10 | 1.24 | 2.04 |
| 8 | 0.778 | 1.28 |
| 6 | 0.491 | 0.808 |
| 4 | 0.308 | 0.508 |
| 2 | 0.194 | 0.319 |
| 1/0 | 0.122 | 0.201 |
| 4/0 | 0.0608 | 0.100 |
Reducing voltage drop
- Upsize the conductor. Each step of three AWG sizes roughly halves the resistance.
- Raise the voltage. A 240 V circuit has half the current of the same load at 120 V, and a quarter of the percentage drop.
- Shorten the run. Put a subpanel near distant loads.
- Remember the equipment ground. When you upsize phase conductors for voltage drop, NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally.
Common mistakes
- Using the round-trip length. Enter the one-way distance; the formula already doubles it.
- Sizing for drop only. A big enough wire for voltage drop can still be too small for ampacity at short distances. Check both.
- Ignoring start-up current. Motors draw several times their running current when starting, so drop is worse for a moment. Size motor circuits with that in mind.
Questions people ask
How do you calculate voltage drop?
For single-phase AC and DC: VD = 2 × length × current × resistance per foot. For three-phase: VD = 1.732 × length × current × resistance per foot. Length is one way, in feet; resistance comes from NEC Chapter 9 Table 8 (ohms per 1,000 ft). A 100 ft run of 8 AWG copper carrying 40 A drops 2 × 100 × 40 × 0.778 ÷ 1,000 = 6.2 V.
What is the maximum allowable voltage drop?
Informational notes in NEC 210.19 and 215.2 recommend no more than 3% on a branch circuit or feeder and 5% overall from the service to the farthest outlet. They're recommendations, not requirements, but most designers follow them, and some equipment and local codes are stricter.
Does voltage drop depend on the voltage?
The drop in volts doesn't; it depends on current, length and wire. But the percentage does: 6 V is 5% of 120 V and only 2.5% of 240 V. That's why long runs are often wired at 240 V.
Is aluminum wire worse for voltage drop?
Aluminum has about 1.6 times the resistance of copper of the same gauge, so it drops more. An aluminum conductor is usually upsized by about two AWG sizes to match copper.
How long can a wire run be before voltage drop is a problem?
It depends on the load. At 120 V, 15 A on 14 AWG copper hits 3% at about 38 ft one way; 20 A on 12 AWG at about 45 ft. Above that, upsize the wire.
Sources and assumptions
- NFPA 70 (NEC) Chapter 9, Table 8: DC resistance at 75 °C, stranded conductors (2011 edition table pages; values are unchanged in later editions)
- NEC informational notes to 210.19 (branch circuits) and 215.2 (feeders): 3% / 5% voltage drop guidance
- Uses DC resistance at 75 °C and unity power factor. For large AC conductors with motor loads, NEC Chapter 9 Table 9 (impedance) gives a more exact answer.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.