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Voltage drop calculator

Twenty amps down 150 ft of 10 AWG copper on a 120 V circuit loses 6.0 V - 5.0% of the supply, so the load sees only 114.0 V and the cable wastes 120 W as heat. That is past the 3% the NEC recommends for a branch circuit.

Your circuit

Distance to the load, not there and back - the calculator doubles it.

Result

Voltage drop
6.0 V
Drop as a percentage
5.0 %
Voltage at the load
114.0 V
Resistance of one conductor
0.150 Ω
Power lost as heat in the cable
120 W

Over the allowance

The resistive model is used: conductor reactance is not included, which is negligible below roughly 95 mm² and on the short runs this tool is for.

One size either way, for this run

  • 12 AWG7.9 % · 9.5 VOver the allowance
  • 10 AWG5.0 % · 6.0 VOver the allowanceyour cable
  • 8 AWG3.1 % · 3.8 VOver the allowance

Permitted drop: 3.0 % · ASTM B258

How the calculation works

The tool works from the resistance of the conductor you already have:

R₁ = ρ(T) · L / (A · n)      ΔU = k · R₁ · I

R₁ is the resistance of one conductor over the one-way length, n the number of conductors in parallel per phase, and k is 2 on DC and single phase or √3 on three phase - the factor that accounts for how many conductor-lengths the current actually crosses. The percentage is ΔU divided by the nominal supply voltage.

Two outputs matter beyond the percentage. Voltage at the load is what the equipment actually sees, and it is the number to quote when a motor trips on undervoltage or an LED driver hums. Power lost as heat is I²R in every current-carrying conductor - two of them on a single-phase run, three on three phase – and on a long circuit it is the running cost of having undersized the cable: watts that you pay for and that end up warming a wall.

The verdict line compares the result against the allowance set for this edition, which the US edition starts at 3% because that is the figure the NEC’s Informational Notes recommend for a branch circuit. It is a recommendation rather than a rule, so the calculator shows you the number and lets you decide - and leaves the final judgement to the authority having jurisdiction, which is where it belongs.

Worked example

Your circuit

System
Single phase AC
Supply voltage (V)
120.0 V
Load current (A)
20 A
Run length, one way
150 ft (45.7 m)
Conductor size (mm²)
5.26 mm²
Conductor material
Copper
Voltage drop allowed (%)
3.0 %

Result

Resistance of one conductor
0.150 Ω
Voltage drop
6.0 V
Drop as a percentage
5.0 %
Voltage at the load
114.0 V
Power lost as heat in the cable
120 W

Over the allowance

Formula
ΔU = k · ρ(T) · L · I / A
Standard
IEC 60228, IEC 60028
Limits of the model
The resistive model is used: conductor reactance is not included, which is negligible below roughly 95 mm² and on the short runs this tool is for.

Computed by Voltacalc with the same engine as the tool above.

What applies in the United States

At 120 V a 3% allowance is 3.6 V, which is why American branch circuits reach their drop limit at roughly half the distance a 230 V circuit does. Running the same load at 240 V across both legs halves the current and quarters the percentage drop, and for detached garages, well pumps and EV chargers that is usually the cheaper fix - the alternative is paying for copper on every foot of the run.

Who decides
NFPA / the authority having jurisdiction (AHJ) in your state or city
Standard
NEC (NFPA 70), National Electrical Code
Ampacity table
NEC Table 310.16, with the correction and adjustment factors of 310.15(B) and (C), and the small-conductor rule in 240.4(D)
Permitted drop
The NEC does not make voltage drop a rule for most branch circuits: 210.19(A) and 215.2(A)(1) carry Informational Notes recommending no more than 3% on the branch circuit and 5% for feeder plus branch combined. Your AHJ, or a specific article such as 647.4(D), can make a lower figure mandatory. (Checked: September 12, 2026)

Questions electricians ask

Why is the drop a percentage of the supply voltage?

Because what matters to the load is the fraction of its nameplate voltage that reaches it. Six volts lost is a nuisance on a 240 V circuit and a real problem on a 120 V one, and the percentage is what the NEC's Informational Notes and most equipment specs are written against.

Does the calculator include reactance?

No, it uses the resistive model. Reactance is negligible on the sizes and lengths a branch circuit uses and starts to matter above roughly 95 mm² (about 3/0 AWG) or on long inductive feeders, where a designer works from the cable manufacturer's impedance data instead.

The result is over the limit. What are my options?

Four, in rough order of cost: move the load to a higher voltage, go up a wire size, shorten the run, or split the load across two circuits. The calculator makes the comparison quick - change one input at a time and watch the percentage.

What temperature does it assume?

20 °C by default, which is the conductor's resistance at rest. A conductor running warm has higher resistance - at 70 °C copper is about 20% more resistive - so a drop figure computed at 20 °C is the optimistic end of the range.

Standards and sources

  • NEC (NFPA 70), National Electrical Code
  • NEC Table 310.16, with the correction and adjustment factors of 310.15(B) and (C), and the small-conductor rule in 240.4(D)

Last updated: September 12, 2026

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