🪢 Cable Sizing (Voltage Drop Method)

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Related standards: NEC

Choosing a conductor size large enough to keep voltage drop within an allowed percentage over a given run.

Every conductor has some resistance, so current flowing through it produces a voltage drop proportional to current, resistance, and run length. Undersized wire for a long run can drop enough voltage that equipment at the far end receives noticeably less than nominal voltage — motors run hot and inefficient, and electronics may malfunction. The NEC recommends keeping voltage drop within about 3% for branch circuits (5% combined feeder+branch).

This calculator uses the circular-mil method: it computes the minimum conductor area (in circular mils) that keeps voltage drop under the allowed percentage, then matches that up to the next standard wire size.

Key formula

Required area (circular mils) = (2 × L × I × ρ) / V_allowed
V_allowed = (%Drop / 100) × System Voltage

Variables

ρ

conductor resistivity (≈10.4 Ω·cmil/ft for copper at 20°C)

L

one-way conductor run length

I

load current

How to use the Cable Sizing calculator

Use this when a long run is the problem: it finds the smallest copper conductor that keeps voltage drop within the percentage you allow, then rounds up to the next standard size. Over a few hundred feet, voltage drop rather than heating is what decides the wire size, and it is the check people forget.

It does one job only. It does not check ampacity, the current a conductor can carry without overheating, and on short runs ampacity usually asks for a larger wire than voltage drop does. Size for both and use the larger of the two results.

Current (A)

The load current in amperes. For a motor use the full-load current from its nameplate rather than an estimate from horsepower.

Allowed V Drop (%)

The maximum voltage drop you will accept as a percentage of the supply voltage. Three percent is the usual target for a branch circuit.

Cable Length (ft)

The one-way distance from the source to the load in feet, along the actual cable route. The out-and-back factor is included in the calculation.

System Voltage (V)

The supply voltage of the circuit, which sets how many volts three percent actually is: 3.6 V on a 120 V circuit, 14.4 V at 480 V.

Worked example: a pump 200 feet from the panel

A 120 V, 20 A well pump sits 200 ft from the panel. The most drop you will accept is 3%. A 20 A circuit is normally wired in #12 copper. Is that enough for this distance?

Worked by hand:

1. Voltage you can afford to lose. 3% of 120 V = 3.6 V.
2. Required area. Circular mils = 2 × L × I × ρ ÷ allowed volts, using ρ = 10.371 Ω·cmil/ft for copper: 2 × 200 × 20 × 10.371 ÷ 3.6 = 23,047 cmil, or 23.05 kcmil.
3. Round up to a standard size. The next standard conductor at or above 23.05 kcmil is 26.24 kcmil, which is #6 AWG.
4. Actual drop with that wire. 2 × 200 × 20 × 10.371 ÷ (26.24 × 1,000 × 120) = 2.63%, inside the 3% target.
5. What #12 would do. #12 is 6.53 kcmil, and over this distance it would drop about 12.7 V, or 10.6%, which is far too much.

This run needs #6 AWG for voltage drop, three sizes larger than the #12 that ampacity alone would suggest for a 20 A circuit. That jump is typical of long runs and is the reason the check matters. Confirm that #6 also satisfies the ampacity and terminal requirements for the breaker you use, and treat the larger of the two answers as the wire size.

Reading the result: two limits, one wire

A conductor has to satisfy two independent limits. Ampacity is a thermal limit, set by how much current the insulation can stand in the installed conditions; voltage drop is a performance limit, set by how much voltage the load can afford to lose. Short, heavily loaded runs are governed by ampacity, and long, lightly loaded ones by voltage drop. The wire you install is the larger of the two.

• The result is for copper. Aluminum needs roughly 1.6 times the area for the same drop, and its terminations must be rated for it.
• The formula here is single-phase (two conductors carrying the current). For a balanced three-phase circuit the required area is about 87 percent of the single-phase figure, which would bring this example down to about 20 kcmil, still #6.
• Bigger wire brings its own limits: terminals and conduit have maximum sizes, and very large conductors are harder to pull and bend.
• The calculator uses copper resistivity at about 20°C. In a hot location the conductor's resistance is higher and the real drop a little more than shown, which is another reason to leave margin.

Notes & limitations

• Voltage-drop sizing and ampacity (heat/current-carrying capacity) sizing are two separate NEC checks — a conductor must satisfy both, and for long runs, voltage drop is very often the governing constraint, not ampacity.
• This assumes a single-phase circuit (the factor of 2 accounts for the round-trip conductor length); three-phase circuits use a √3 factor instead.

Common mistakes

• Stopping at the voltage-drop size and skipping the ampacity check. The calculator does not do it, and the size it returns can be too small to carry the current safely on a short run.
• Entering the round-trip length. The factor of two for the return conductor is already in the formula.
• Mixing up supply voltage and load voltage. The percentage is measured against the circuit voltage, so a 240 V circuit has twice the volts to lose that a 120 V circuit has at the same percentage.
• Taking the "next standard size" as a target. It is the smallest size that meets the drop limit, so a larger one is always allowed and often wise.
• Sizing from the load's rated power rather than its actual current. Use amperes, and use the full-load current for motors, not a figure derived from horsepower.

Frequently asked questions

Papers worth reading

The calculation of the temperature rise and load capability of cable systems Neher, J. H. & McGrath, M. H. (1957), AIEE Transactions. The paper behind the standard method for working out how much current a cable can carry before it overheats, which is where published ampacity tables come from.