U.S. Electrical and HVAC Calculators

This calculator estimates the voltage drop on a copper or aluminum run in the United States. It uses a DC-resistance method and published National Bureau of Standards wire data. It is for learning and field estimates only. It is not a substitute for the local codebook, manufacturer data, or design by a licensed electrician.

What you enter

The default target is a 3% drop. You can change that to 5% or to a custom percentage.

What you get

How to measure length

Enter the one-way length: the path of the cable from the source to the load, along the route the wire actually takes, not a straight-line shortcut.

Do not multiply that length by 2. The single-phase formula already includes the outgoing and return conductors. The three-phase formula already uses the √3 factor for a three-wire circuit.

The formulas

For a single-phase circuit:

VD = 2 × I × L × R / 1000

For a three-phase circuit:

VD = √3 × I × L × R / 1000

where:

Resistance starts from the 20 °C volume resistivity and the 20 °C temperature coefficient of resistance in the handbooks, then is adjusted to 75 °C:

ρ75 = ρ20 × (1 + α20 × (75 − 20))

R = ρ75 / CM × 1000

where CM is the conductor’s nominal area in circular mils.

Copper values come from NBS Handbook 100:

Aluminum values come from NBS Handbook 109, Table 1, EC-H19:

Nominal AWG areas, for both metals, are the American Wire Gage areas in Handbook 100. For example, 12 AWG is 6,530 circular mils. This first version offers these commonly stocked sizes: 14, 12, 10, 8, 6, 4, 3, 2, 1, and 1/0 through 4/0 AWG. It does not include odd sizes such as 13, 11, 9, 7, or 5 AWG, which are rarely available in the field.

Some field shortcuts use a single “K” constant, often 12.9 for copper or 21.2 for aluminum. This calculator does not use those constants. It uses the handbook resistivity adjusted to 75 °C, so a result may differ slightly from a 12.9 / 21.2 shortcut.

A worked example

Take 12 AWG copper, 20 A, 100 ft one-way, 120 V, single-phase, at 75 °C.

  1. Area of 12 AWG = 6,530 circular mils (Handbook 100).
  2. ρ75 = 10.371 × (1 + 0.00393 × 55) ≈ 12.613 ohm-circular mil/ft.
  3. VD = 2 × 20 × 100 × 12.613 / 6,530 ≈ 7.73 V.
  4. Drop as a percentage: 7.73 / 120 ≈ 6.4%.
  5. Voltage at the load: 120 − 7.73 = 112.3 V.

(The corresponding resistance is about 1.931 Ω per 1,000 ft at 75 °C.)

In this example the drop is above a 3% or 5% target, so a larger conductor, a shorter run, or a lower current would be needed to meet those targets. Before you rely on this example, check the resistivity and the 12 AWG area against the original Handbook 100 tables.

Copper and aluminum

Aluminum has a higher resistivity than copper in the handbooks, so the same AWG size and the same run will show a larger voltage drop with aluminum. If you switch metals in the calculator, the suggested size and the drop will change for that reason alone. This tool does not address other copper-versus-aluminum issues, such as terminations or ampacity.

Why larger or longer runs need extra care

This is a DC-resistance method. It does not include reactance or power factor. On 2/0 AWG and larger conductors, on very long runs, and for motor starting current, the real AC voltage drop can be higher than the number you see here. In those cases, treat the result as a first estimate only and have a licensed electrician apply a method that includes those effects.

The calculator also assumes solid or equivalent DC resistance for the listed AWG size. Stranded conductors of the same size are typically a little higher in resistance. The first version does not model that difference separately.

Disclaimer

Results are for learning and estimation only. They are not a substitute for a licensed electrician, the local electrical code, or manufacturer instructions. The method is written for U.S. practice and is not directly applicable outside the United States. If you use these results, you do so at your own risk.

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