Wire Size Calculator
Estimate using typical 75 °C ampacities. NM cable (Romex) is limited to the 60 °C column, and the NEC and local code govern. A licensed electrician must verify the size, breaker and terminations.
How the wire size calculation works
Find the circular mils the run needs
CM = 2 × K × amps × length ÷ (volts × allowed drop). For 40 A over 150 ft at 240 V and 3%: 2 × 12.9 × 40 × 150 ÷ 7.2 = 21,500 circular mils.
Pick the first size with at least that many
8 AWG copper has 16,510 circular mils, too few. 6 AWG has 26,240, so it passes the voltage-drop test.
Check the wire can carry the current
The size must also meet its ampacity. 6 AWG copper is typically good for 65 A at 75 °C, well over 40 A. Continuous loads are checked at 125% of the current.
Let an electrician confirm
Ampacity depends on insulation, terminal ratings, conduit fill and ambient temperature. The NEC and local code decide the final size.
Worked example. A 240 V single-phase load draws 40 A at the end of a 150 ft copper run, with a 3% drop limit. It needs 21,500 circular mils, so 6 AWG copper (26,240 cmil) is the minimum. Voltage drop is 5.90 V, or 2.46%. On ampacity alone 8 AWG would do, so here voltage drop sets the size.
Wire Size FAQ
What size wire do I need for 40 amps?
On ampacity alone, 8 AWG copper is typically enough: 40 A in the 60 °C column used for NM cable and 50 A at 75 °C. A continuous 40 A load is checked at 50 A, the full 75 °C rating of 8 AWG, so 8 AWG only works there when the breaker, the equipment and every termination are rated 75 °C. With NM cable, 60 °C terminals or a long run, plan on 6 AWG or larger, and have an electrician confirm.
What size wire for 30 amps?
In the typical 75 °C figures this calculator uses, 10 AWG copper is the smallest size for 30 A; small copper sizes are capped by the breaker rule in NEC 240.4(D). Aluminum needs at least 8 AWG. Long runs may need a larger size for voltage drop.
What size wire for 50 amps?
8 AWG copper is rated 50 A in the 75 °C column, so it is the minimum when every termination is rated 75 °C. NM cable uses the lower 60 °C column, which usually means 6 AWG copper. Aluminum needs at least 6 AWG at 75 °C.
What size wire for 60 amps?
6 AWG copper (typically 65 A at 75 °C) or 4 AWG aluminum (65 A). Enter your voltage and run length: past about 120 ft at 240 V with a 3% limit, voltage drop pushes the size up.
What size wire for a 100 amp service?
Commonly 4 AWG copper or 2 AWG aluminum for the service conductors of a dwelling, under special NEC rules for services. Feeders and long runs differ, so an electrician must size it.
How does distance affect wire size?
Voltage drop rises in direct proportion to length. Doubling the run doubles the circular mils you need, which is usually two AWG sizes larger.
How do I size wire for 12 volt DC?
Use the same formula with 12 V. Low voltage loses a large share to resistance, so a 10 A load 20 ft away at 3% needs about 14,300 circular mils, which is 8 AWG copper.
Is aluminum wire OK?
Aluminum is common for feeders and services. It needs a larger size than copper for the same load, and connectors rated for aluminum.
Sources: NFPA 70, National Electrical Code 2023 (free read-only access)