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Voltage Drop Calculator
Aim for about 3% voltage drop or less on a branch circuit and 5% or less in total, as the NEC notes suggest. For example, 10 AWG copper carrying 30 amps for 100 feet at 240 volts drops about 7.4 volts, or 3.1%. Enter your wire and run below for an estimate.
About 7.44 V lost. About 232.6 V reaches the load.
Over 3%, under 5%. Above the 3% branch guideline, but within the 5% total guideline (feeder plus branch). These notes are advice, not code rules.
Estimate: VD = 2 x length x R x amps / 1000, R = 1.24 ohms per 1,000 ft (NEC Chapter 9 Table 8, 75 C).
This is an estimate for planning. Electrical work needs an expert electrician and, in most places, a permit.
How the calculator works
Every wire has a little resistance, so some voltage is used up on the way to the load. The tool uses the resistance of each wire size from NEC Chapter 9 Table 8, in ohms per 1,000 feet at 75 C, for copper and aluminum.
For a single-phase circuit it uses: voltage drop = 2 x one-way length x resistance x amps / 1,000. The 2 counts the trip out on one wire and back on the other. The percent drop is the volts lost divided by the circuit voltage. The voltage at the load is the circuit voltage minus the drop.
The tool also checks the amps against the wire's rating in NEC Table 310.16 (75 C column) and the small-wire limits in NEC 240.4(D). If your load is higher than the wire can carry, it shows a warning. To pick a wire size from your amps instead, use the wire size calculator.
What changes the answer
Length. Double the distance and you double the drop. Use the one-way length from the panel to the load, along the path the wire really takes, not a straight line.
Amps. Double the amps and you double the drop. Use the real load. A breaker size gives a worst case.
Voltage. The same volts lost is a smaller percent on a 240 V circuit than on a 120 V circuit. That is one reason long runs to a shed or EV charger often use 240 V.
Metal and size. Aluminum has about 1.6 times the resistance of copper of the same size. A bigger wire cuts the drop. From 14 AWG to 4 AWG, each common step (14, 12, 10, 8, 6, 4) cuts it by a bit more than a third.
Heat. Resistance rises as wire gets hot. The table values are at 75 C, a common working limit, so the estimate leans to the high side for lightly loaded wire.
Why voltage drop matters
Too much drop can make lights dim when a motor starts, make motors and compressors run hot, and make chargers and electronics work poorly. The wire also turns the lost voltage into heat, which is wasted energy you pay for.
The NEC sets no hard limit for most home circuits. It gives advice in informational notes: about 3% for a branch circuit, and about 5% for the feeder and branch circuit together. Some equipment makers and local codes ask for less.
When to call an expert electrician
Call an expert electrician for long runs to a garage, shed, well pump, pool or EV charger, and for any sub-panel. They check what this tool does not: the cable type, heat, conduit fill, terminal ratings and the ground wire. When the hot wires are made bigger for voltage drop, NEC 250.122(B) says the ground wire must be made bigger in proportion. Electrical work needs a permit in most places.
The code basis
Resistance values come from NEC Chapter 9 Table 8. Amp ratings come from NEC Table 310.16 (75 C column) and 240.4(D). The 3% and 5% guidelines come from informational notes to NEC 210.19(A) and 215.2(A). Informational notes are advice, not rules, unless a local code adopts them. You can read the NEC for free on the NFPA website.
| Copper wire | Load | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 14 AWG | 15 A | 4.71 V (3.9%) | 9.42 V (7.9%) | 14.13 V (11.8%) |
| 12 AWG | 20 A | 3.96 V (3.3%) | 7.92 V (6.6%) | 11.88 V (9.9%) |
| 10 AWG | 20 A | 2.48 V (2.1%) | 4.96 V (4.1%) | 7.44 V (6.2%) |
| 8 AWG | 20 A | 1.56 V (1.3%) | 3.11 V (2.6%) | 4.67 V (3.9%) |
| 6 AWG | 20 A | 0.98 V (0.8%) | 1.96 V (1.6%) | 2.95 V (2.5%) |
Worked example: a 30 A, 240 V circuit 100 feet away
- The circuit is 240 V, single phase, 30 A, on 10 AWG copper, 100 feet one way.
- Resistance of 10 AWG copper: 1.24 ohms per 1,000 feet (NEC Chapter 9 Table 8, 75 C).
- Voltage drop = 2 x 100 x 1.24 x 30 / 1,000 = 7.44 V.
- Percent drop = 7.44 / 240 = 3.10%. That is just over the 3% branch guideline, but under the 5% total guideline.
- Voltage at the load = 240 - 7.44 = 232.56 V.
- With 8 AWG copper (0.778 ohms per 1,000 feet), the drop is 2 x 100 x 0.778 x 30 / 1,000 = 4.67 V, or about 1.9%. Estimate only.
Voltage Drop Calculator questions
Short answers to common questions.
What is an acceptable voltage drop?
The NEC suggests about 3% or less for a branch circuit and about 5% or less for the feeder and branch circuit together. These are informational notes, so they are advice for most homes, not hard rules.
How do I calculate voltage drop?
For a single-phase circuit: multiply 2 x the one-way length in feet x the wire's resistance in ohms per 1,000 feet x the amps, then divide by 1,000. Divide that by the circuit voltage and multiply by 100 for the percent.
How far can I run 12 gauge wire on a 20 amp circuit?
At 120 V and a full 20 A load, 12 AWG copper reaches about 3% drop at roughly 45 feet one way. With a lighter load it can go farther. Most outlets do not carry the full 20 A all the time.
Does aluminum wire have more voltage drop than copper?
Yes. Aluminum has about 1.6 times the resistance of copper of the same size, so it drops more voltage. That is why aluminum is usually one or two sizes larger for the same job.
Does voltage drop matter on a 240 volt circuit?
Yes, but less in percent. The same number of volts lost is half the percent on 240 V that it is on 120 V. Long runs still need a check, such as a feed to a detached garage or an EV charger.
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