Voltage Drop Calculator

Calculate voltage drop for any wire gauge, current, and run length, and instantly check each run against NEC 210.19(A) Informational Note 4's 3% branch-circuit and 5% feeder+branch drop targets. Long runs need thicker wire to stay within the 3% target, and this tool shows exactly where that line sits.

Wire Gauge (AWG, copper)

Current (Amps)

One-Way Distance (Feet)

Source Voltage

Circuit Type

= 3.57V (2.98%)

See full breakdown for 12 AWG at 15A, 75ft

Resistance values are copper conductors at the NEC Chapter 9 Table 8 75°C reference temperature. Aluminum has about 1.3 to 1.4 times the resistance of copper at the same temperature, so aluminum drop will be roughly that much higher for the same gauge; to compare aluminum explicitly, open any Wire Size Calculator page which picks copper and aluminum independently against the same drop target.

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What Is Voltage Drop?

Voltage drop is the loss of voltage as electricity travels through a wire. Every conductor has resistance, and when current flows through that resistance, some voltage is consumed as heat. The longer the wire and the higher the current, the more voltage is lost before it reaches the load.

The Formulas

DC and Single-Phase AC

Voltage Drop = (2 × Length × Current × Resistance) ÷ 1000

The "2" accounts for the round trip: current travels out to the load on one conductor and returns on another. This formula applies to DC circuits and single-phase AC (120V, 240V residential branches).

Three-Phase AC

Voltage Drop = (√3 × Length × Current × Resistance) ÷ 1000

Three-phase systems (208V, 480V commercial / industrial) use the √3 factor rather than a round-trip of 2. The balanced three-conductor geometry means the line-to-line voltage drop is proportional to √3 times the one-way IR product, not twice it. Using the 1-phase formula on a 3-phase circuit overstates the drop by about 15%.

Resistance is the wire's ohms per 1000 feet (see the gauge table below). To get the percentage, divide the drop by the source voltage and multiply by 100.

NEC Drop Targets

NEC 210.19(A) Informational Note 4 cites performance-recommendation targets for branch-circuit and feeder+branch voltage drop:

Circuit Type	Drop Target	At 120V	At 240V
Branch circuit	≤ 3%	≤ 3.6V	≤ 7.2V
Feeder + branch total	≤ 5%	≤ 6.0V	≤ 12.0V

These are performance recommendations, not hard code requirements. Sitting past them typically shows up as motors running hotter and starting slower, incandescent and halogen lighting dimming, and sensitive electronics operating at the low end of their input tolerance, rather than a code violation on its own.

Three Factors That Affect Voltage Drop

Wire gauge: Thicker wire (lower AWG) has less resistance. Doubling the cross-sectional area roughly halves the voltage drop.
Current: More amps means more drop. Voltage drop is directly proportional to current.
Distance: Longer runs mean more resistance. Voltage drop is directly proportional to distance.

You can reduce voltage drop by using thicker wire, shortening the run, or (where possible) increasing the circuit voltage from 120V to 240V.

Wire Gauge Reference

Gauge	Resistance (Ω/1000ft)	Ampacity (60°C)	Ampacity (75°C)	NEC Branch OCP
14 AWG	3.14	15A	20A	15A (240.4D)
12 AWG	1.98	20A	25A	20A (240.4D)
10 AWG	1.24	30A	35A	30A (240.4D)
8 AWG	0.778	40A	50A	50A
6 AWG	0.491	55A	65A	65A
4 AWG	0.308	70A	85A	85A
3 AWG	0.245	85A	100A	100A
2 AWG	0.194	95A	115A	115A
1 AWG	0.154	110A	130A	130A
1/0 AWG	0.122	125A	150A	150A
2/0 AWG	0.0967	145A	175A	175A
3/0 AWG	0.0766	165A	200A	200A
4/0 AWG	0.0608	195A	230A	230A
250 kcmil	0.0515	215A	255A	255A
300 kcmil	0.0429	240A	285A	285A
350 kcmil	0.0367	260A	310A	310A
500 kcmil	0.0258	320A	380A	380A
750 kcmil	0.0171	400A	475A	475A

NEC Branch OCP is the branch-circuit overcurrent cap for that conductor. For 14, 12, and 10 AWG copper, NEC 240.4(D) caps the breaker below the raw 75°C ampacity (the "small conductor rule"). Larger gauges follow the standard ampacity table.

Related Calculators

Frequently Asked Questions

What is voltage drop?expand_more

Voltage drop is the reduction in voltage as electricity travels through a wire. Longer runs and higher currents both increase the drop. NEC 210.19(A) Informational Note 4 cites 3% for branch circuits and 5% for the combined feeder+branch total as planning targets, not hard code requirements.

How do I calculate voltage drop?expand_more

For DC and single-phase AC: Voltage Drop = (2 × Length × Current × Resistance per 1000ft) ÷ 1000. The "2" accounts for the round-trip. For three-phase AC: Voltage Drop = (√3 × Length × Current × Resistance per 1000ft) ÷ 1000, because line-to-line drop across balanced conductors uses the √3 factor rather than round-trip doubling. Percentage = (Drop ÷ Source Voltage) × 100.

What is an acceptable voltage drop?expand_more

NEC 210.19(A) Informational Note 4 recommends branch-circuit drop at or below 3% and feeder+branch total at or below 5% as planning targets. These are performance recommendations, not hard code requirements. Sitting past those targets typically shows up as dimming incandescent lighting, motor starting difficulty, or wasted energy as I²R heat in the conductor, rather than a code violation on its own.

How do I reduce voltage drop?expand_more

Use a larger wire gauge (lower AWG number), shorten the wire run, or increase the source voltage. Doubling the wire gauge area roughly halves the voltage drop.

Does wire material affect voltage drop?expand_more

Yes. Aluminum has about 1.3 to 1.4 times the resistance of copper at the NEC Chapter 9 Table 8 75°C reference temperature, so for the same voltage drop an aluminum conductor is typically one to two gauges larger than copper. The exact gap depends on whether ampacity or voltage drop is binding on the run.

This calculator provides estimates for reference purposes only. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before performing any electrical work.

Standards & References

This page cites the following electrical codes and standards. Always consult the current edition of your local adopted standard for authoritative requirements.

NEC 210.19(A) Informational Note 4. Branch-circuit conductors sized to prevent a voltage drop exceeding 3% at the farthest outlet. Combined with feeders, total voltage drop should not exceed 5%.
National Electrical Code (NFPA 70), Article 210, Branch Circuits. Reference →
NEC 215.2(A)(1) Informational Note 2. Feeder conductors sized to prevent a voltage drop exceeding 3%. Total branch + feeder drop should not exceed 5%.
National Electrical Code (NFPA 70), Article 215, Feeders. Reference →
NEC Table 310.16. Allowable ampacities of insulated conductors rated up to 2000V, 60°C through 90°C, not more than three current-carrying conductors in raceway or cable.
National Electrical Code (NFPA 70), Article 310, Conductors for General Wiring. Reference →
IEC 60364. Low-voltage electrical installations. The international counterpart to the NEC, covering voltages up to 1000V AC / 1500V DC in residential, commercial, and industrial installations.
International Electrotechnical Commission. Reference →

Disclaimer: The information on this page is provided for reference. Always consult a licensed electrician and the current edition of your local adopted electrical code before performing electrical work.