Wire Size Calculator

Wire size calculator: the smallest copper gauge for your run

A wire size calculator works backwards from the voltage drop you will accept. It computes how many circular mils of copper the circuit needs, then picks the smallest standard American Wire Gauge (AWG) that has at least that much. For a 120 V, 20 A load 100 feet away with a 3 percent drop limit, the answer is 8 AWG.

Conductor sizing is safety-critical and governed by code. Confirm the final choice with a licensed electrician and your local code before wiring.

The formula in plain words

Wire pushes back against current, and the longer and thinner it is, the harder it pushes. That push eats voltage before it reaches the load. You decide how much you can lose, and the formula tells you how fat the wire must be.

• Allowed drop in volts = voltage × max drop %
• Required circular mils = (2 × 12.9 × one-way feet × amps) ÷ allowed drop
• Pick the smallest gauge whose circular mils are at least that number

The 12.9 is a resistivity constant for copper. The 2 covers the round trip.

Worked examples

1. 120 V, 20 A load, 100 ft one-way, 3% max drop

• Allowed drop: 120 × 0.03 = 3.6 V
• Required: (2 × 12.9 × 100 × 20) ÷ 3.6 = 14,333 CM
• 10 AWG has 10,380 CM, which is too small. 8 AWG has 16,510 CM, which works.
• Actual drop at 8 AWG: 2.60 percent

2. 240 V, 50 A load, 150 ft one-way, 3% max drop

• Allowed drop: 240 × 0.03 = 7.2 V
• Required: (2 × 12.9 × 150 × 50) ÷ 7.2 = 26,875 CM
• 6 AWG has 26,240 CM, just 635 short. The pick is 4 AWG (41,740 CM).
• Actual drop at 4 AWG: 1.93 percent

In example 2, 6 AWG misses by only about 2 percent, and the calculator still bumps you up to 4 AWG.

Results for 120 V, 20 A, 3% limit

One-way length	Required CM	Calculator pick	Drop at that gauge
25 ft	3,583	12 AWG	1.65%
50 ft	7,167	10 AWG	2.07%
100 ft	14,333	8 AWG	2.60%
150 ft	21,500	6 AWG	2.46%
200 ft	28,667	4 AWG	2.06%

In the 25 ft row, voltage drop alone would allow 14 AWG, but 14 AWG copper is only good for 15 A, so the calculator returns 12 AWG and shows “Size set by: ampacity”. On the longer rows, voltage drop sets the size.

How to gather the inputs

• Voltage: Use the supply voltage at the panel, such as 120 or 240.
• Current: Use the load’s running amps. For a continuous load, ask your electrician about derating.
• Length: Measure the cable path, not the straight line. Include vertical runs and detours.
• Max drop: Start at 3 percent. Lower it for long runs to sensitive equipment.

Common mistakes

• Ignoring the breaker size. The calculator sizes the wire for the load current you enter. If the breaker is larger than the load, size the wire for the breaker instead.
• Entering the round-trip length. The formula already doubles it.
• Sizing aluminum with copper numbers. Aluminum needs a larger gauge.
• Ignoring temperature and bundling. Wire in hot attics or crowded conduit carries less current than the nameplate number.
• Rounding down. If the required circular mils land between two gauges, you must take the larger conductor.

Where this calculator stops

The tool checks one thing: voltage drop for steady current. It does not account for conductor temperature, bundled cables in one conduit, or the brief surge a motor draws when it starts, which can be several times its running current.

A common planning guide splits the allowance, with about 3 percent on a branch circuit and 5 percent on the whole path from service to load. If a feeder already eats part of that budget, set a lower limit here. Then have the final gauge checked against code.

Related calculators

After choosing a gauge, check it with the voltage drop calculator, confirm the breaker with the breaker size calculator, and see whether the cables fit your raceway using the conduit fill calculator.