Electrical Wire Gauge Calculator

How Electrical Wire Size Is Calculated

Choosing the correct wire gauge prevents overheating, voltage drop, and fire hazards. In the US, wire size follows the AWG (American Wire Gauge) system, where lower numbers mean thicker wire and higher current capacity.

This page is aimed at residential branch circuits: 120V or 240V, copper, a 3% drop limit, and it takes the run in feet or metres. It stops at 1/0. For aluminium conductors, a 12V or 24V direct-current run, a drop limit other than 3%, an outdoor derate, or anything up to 4/0, use the wire gauge size calculator, which shows every gauge with a pass or fail beside it. Both pages use the same NEC 310.16 60°C copper ampacities and the same Chapter 9 Table 8 resistances, so where they overlap they return the same gauge.

Voltage Drop Formula: V_drop = (2 × L × I × R_per_1000ft) / 1000

Where:

• L = one-way cable length in feet
• I = current in amps
• R_per_1000ft = conductor resistance in ohms per 1,000 feet, from NEC Chapter 9, Table 8
• 2 = accounts for the outbound and return conductors, since the current goes down one and back the other

The units matter and are easy to get wrong. The resistances in every table below are ohms per 1,000 feet, which is how the NEC publishes them, so the length has to be in feet and the ÷ 1,000 is what cancels the “per 1,000 ft”. If you work in metres, convert the length first: the calculator does that for you when you pick metric.

Maximum Allowable Voltage Drop:

• Branch circuits (NEC): 3% for branch, 5% total including feeder
• At 120V: max drop = 3.6V; at 240V: max drop = 7.2V

AWG Current Capacity (Ampacity) Reference:

This is the full table the calculator works from, resistances included, so you can check any answer it gives by hand.

Worked Example: A 20A circuit runs 25 meters (about 82 ft) to a workshop at 120V. The calculator checks each gauge against the 3% drop limit and picks the first one that passes:

• 12 AWG: about 5.4% drop, too much
• 10 AWG: about 3.4% drop, still over the line
• 8 AWG: about 2.1% drop, this is the pick

Notice that raising the amps or the distance pushes you to thicker (lower-number) wire fast. The calculator uses the conductor resistances from NEC Chapter 9, Table 8, which sit a little above ideal-copper textbook values, so it leans conservative on long runs. Ampacity is only half the job: a 12 AWG wire is rated for 20A, but over 82 feet it still drops too much voltage, which is why this run lands on 8 AWG even though 12 AWG would carry the current.