Wire Size Calculator

What size wire do I need for this load?

A
Continuous load (3+ hours)
Conductor
Wiring

Use at least

#12copper

Table 310.16 for 60 °C wiring with no more than 3 current-carrying wires at 86 °F. Long runs, hot attics and bundling can need larger wire. Planning figure only. Confirm against the electrical code your area has adopted and have a licensed electrician check the work.

#12 copper: load against ampacity20 A limitSizing load: 16 ASpare: 4 A
Load used for sizing16 A
Table ampacity20 A
Limit under 240.4(D)20 A

Find the smallest copper or aluminum wire that meets the published ampacity for your load and wiring method.

How it works

Multiplies continuous loads by 1.25, then looks up the electrical code Table 310.16 column for your wiring and applies the small-conductor limits in 240.4(D).

Before you order

  • Nonmetallic cable uses the 60 °C column in most homes, even with 90 °C insulation inside.
  • Long runs may need a larger size for voltage drop, so check that separately.
  • Hot attics and bundled cables need ampacity corrections this table does not include.

Common questions

Why is 12 AWG limited to 20 amps?

Section 240.4(D) limits the breaker on small conductors even though the table lists a higher ampacity.

Can I use aluminum wire at home?

Aluminum feeders are common with the right terminations and antioxidant. Small branch circuits generally use copper, so check with an electrician.