Wire Size Calculator

Wire Size formula

Circular mils = 2 x K x Amps x Distance (ft) / (Volts x Drop %)   (K = 12.9 copper, 21.2 aluminum; use 1.732 instead of 2 for three-phase)
Wire size = smallest AWG with circular mils >= required and ampacity >= amps
Voltage drop (V) = 2 x K x Amps x Distance / circular mils of the chosen wire
Continuous load: Amps x 1.25

Worked example

A 30 A, 240 V circuit to a workshop 150 ft away in copper with a 3% limit: 2 x 12.9 x 30 x 150 = 116,100. Divide by 240 x 0.03 = 7.2 to get 16,125 circular mils. 10 AWG (10,380 CM) is too small even though it carries 30 A; 8 AWG at 16,510 CM is the answer. Actual drop: 116,100 / 16,510 = 7.03 V, or 2.9%.

A 50 A, 240 V feeder 100 ft long in aluminum: 2 x 21.2 x 50 x 100 = 212,000, divided by 7.2 = 29,444 circular mils. 4 AWG aluminum (41,740 CM, 65 A) covers both requirements, with a drop of 212,000 / 41,740 = 5.08 V, or 2.1%. 6 AWG aluminum would carry 50 A but drops 8.1 V, or 3.4%.

Quick reference

How wire size is chosen

Two rules decide the gauge. The first is ampacity: the wire must carry the load without overheating, and the NEC tables give the allowed current for each size. In a house the practical limits are 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A, 8 AWG for 40 A and 6 AWG for 50 to 55 A in copper.

The second is voltage drop. Long runs lose voltage in the wire itself, which makes motors run hot and lights dim. The NEC recommends no more than 3 percent on a branch circuit. The calculator finds the circular mils that hold the drop to your limit and then picks the smallest gauge that satisfies both rules. On short runs ampacity wins; past about 50 ft at 120 V, voltage drop usually forces a larger wire.

What this calculator does not cover

This is a planning tool. Ampacities here are the 75°C column with no derating, but bundling more than three conductors in a conduit, ambient temperatures above 86°F, and 60°C terminations on older devices all reduce the allowed current. Aluminum needs compatible lugs and anti-oxidant compound, and is not permitted for most small branch circuits.

Continuous loads such as water heaters, EV chargers and heat pumps must be sized at 125 percent of the nameplate current. Motors have their own rules. Before buying wire or pulling a permit, have a licensed electrician confirm the size against the current NEC and your local amendments.

Key facts

• The NEC recommends limiting voltage drop to 3% on a branch circuit and 5% on the feeder and branch circuit combined (informational notes to 210.19 and 215.2).
• K is the resistance of a 1 ft conductor with a cross section of one circular mil: about 12.9 ohms for copper and 21.2 for aluminum at 75°C.
• NEC 240.4(D) limits 14 AWG copper to a 15 A breaker, 12 AWG to 20 A and 10 AWG to 30 A regardless of table ampacity.
• Every three AWG steps roughly doubles the conductor area, so 7 AWG has about twice the circular mils of 10 AWG.
• A load that runs for 3 hours or more is continuous, and the NEC requires conductors and breakers rated for 125% of it.

Frequently asked questions

Sources and references

• •NFPA 70, National Electrical Code (Table 310.16 ampacities, 240.4(D), 210.19 and 215.2 informational notes) (NFPA)
• •Voltage Drop Formulas (International Association of Electrical Inspectors)
• •Re3 Voltage Drop Calculator (Southwire)