Wire size calculator

How wire size is calculated

The calculator sizes the wire for heat first, then for voltage drop, and takes whichever is bigger.

Next it walks up NEC Table 310.16, in the column for your termination rating, to the first size whose ampacity covers the required ampacity and that the breaker is allowed to protect. For the small sizes, 240.4(D) sets a hard breaker limit: copper 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A; aluminum 12 AWG at 15 A, 10 AWG at 25 A. For larger sizes, 240.4(B) lets you use the next standard breaker above the wire's ampacity when the ampacity is not itself a standard size. Aluminum is not listed below 12 AWG.

K is 12.9 for copper and 21.2 for aluminum (ohm-circular mils per foot), and circular mils come from NEC Chapter 9 Table 8. Voltage drop uses the actual load current, not the 125% figure. If the ampacity size drops more than your limit, the calculator steps up one size at a time until it does not.

Worked example: 50 A at 240 V over 100 ft

The calculator opens on a 50 A noncontinuous load at 240 V single-phase, copper, 75°C terminations, 100 ft one way, with a 3% drop limit.

1. Required ampacity: 50 A × 1 = 50.00 A. Breaker: 50 A, a standard size.
2. Table 310.16, copper, 75°C: 8 AWG is rated 50 A, enough for 50 A, and 240.4(D) does not apply to it. Minimum size by ampacity: 8 AWG.
3. Drop on 8 AWG: 2 × 12.9 × 50 × 100 ÷ 16,510 = 7.81 V, or 3.26% of 240 V. That is over 3%.
4. Drop on 6 AWG: 2 × 12.9 × 50 × 100 ÷ 26,240 = 4.92 V, or 2.05%.
5. Result: 6 AWG copper (65 A at 75°C) on a 50 A breaker.

Common sizes by load

Minimum size by Table 310.16 ampacity and the 240.4(D) limits for a noncontinuous load, before voltage drop. Long runs and continuous loads may need a bigger wire.

Choosing the termination rating

The wire is only as good as the terminal it lands on. NEC 110.14(C) limits terminations on circuits of 100 A or less, or marked for 14 through 1 AWG conductors, to the 60°C column unless the equipment is listed and identified for 75°C. Most modern breakers, panels and lugs are listed 60/75°C, so 75°C is the usual choice; check the label on the breaker and the equipment. Circuits over 100 A, and terminals marked for conductors larger than 1 AWG, use the 75°C column.

• NM cable (Romex) is sized from the 60°C column (334.80) even though its conductors are rated 90°C.
• The 90°C column is for derating only: start there when you correct for heat or bundling, then make sure the result does not exceed the termination's column.
• Aluminum needs terminals marked AL or CU/AL and the right torque.

What this calculator leaves out

It assumes not more than three current-carrying conductors in a raceway or cable and a 30°C (86°F) ambient, the conditions of Table 310.16. It does not apply ambient temperature correction (310.15(B)), adjustment for more than three current-carrying conductors (310.15(C)), the 83% dwelling service and feeder rule of 310.12, or motor circuits sized under Article 430. Hot attics, rooftop conduit, bundled cables and motor or HVAC loads all need those rules. Have an electrician or engineer size services, feeders and anything over these simple cases, and check what your local inspector adopts.