Wire Size Calculator

How the wire size calculator works

Design current. Your load in amps, multiplied by 1.25 if it runs for three hours or more.
Base ampacity. The calculator walks up NEC Table 310.16 in the column you choose (60°C, 75°C or 90°C) for copper or aluminum.
Correction and adjustment. Each candidate is multiplied by the ambient temperature factor from Table 310.15(B)(1) and the bundling factor from Table 310.15(C)(1).
Small-conductor limits. 14, 12 and 10 AWG are capped at the overcurrent limits of 240.4(D) — 15, 20 and 30 A for copper.
Voltage drop (optional). If you enter a length, the result is upsized until the drop is within your target.

Wire sizing formula

Required ampacity = Load × (1.25 if continuous)
Adjusted ampacity = Table 310.16 value × ambient factor × bundling factor
Choose the smallest size where adjusted ampacity ≥ required ampacity

Worked example

A 40 A non-continuous load on copper THWN conductors with 75°C terminals, three conductors in a conduit, 30°C ambient. Table 310.16 gives 8 AWG copper 50 A at 75°C, and no correction applies, so 8 AWG is the answer. Put the same circuit in a 40°C attic with six current-carrying conductors and the 8 AWG drops to 50 × 0.88 × 0.80 = 35.2 A — too small, so the calculator moves up to 6 AWG (65 × 0.88 × 0.80 = 45.8 A).

Wire ampacity chart (NEC Table 310.16)

Allowable ampacity with no more than three current-carrying conductors at 30°C (86°F).

Size	Cu 60°C	Cu 75°C	Cu 90°C	Al 60°C	Al 75°C	Al 90°C
14 AWG	15	20	25	—	—	—
12 AWG	20	25	30	15	20	25
10 AWG	30	35	40	25	30	35
8 AWG	40	50	55	35	40	45
6 AWG	55	65	75	40	50	55
4 AWG	70	85	95	55	65	75
3 AWG	85	100	115	65	75	85
2 AWG	95	115	130	75	90	100
1 AWG	110	130	145	85	100	115
1/0 AWG	125	150	170	100	120	135
2/0 AWG	145	175	195	115	135	150
3/0 AWG	165	200	225	130	155	175
4/0 AWG	195	230	260	150	180	205
250 kcmil	215	255	290	170	205	230
300 kcmil	240	285	320	195	230	260
350 kcmil	260	310	350	210	250	280
400 kcmil	280	335	380	225	270	305
500 kcmil	320	380	430	260	310	350

14–10 AWG are further limited by 240.4(D): copper 15/20/30 A and aluminum 15 A (12 AWG) and 25 A (10 AWG).

Copper vs aluminum

Aluminum conducts about 61% as well as copper, so it needs to be roughly two sizes larger for the same current. It is lighter and cheaper on big feeders, but terminations must be listed for aluminum (marked AL/CU) and torqued correctly.

After you size the wire

Check how many of those conductors fit in your raceway with the conduit fill calculator, confirm the drop on long runs with the voltage drop calculator, and see the full derating breakdown in the ampacity calculator.

Frequently asked questions

What size wire do I need for 30 amps?

For copper in the 60°C or 75°C column, 10 AWG is rated 30 A or 35 A and is the usual choice for a 30 A circuit. With aluminum you need 8 AWG. Long runs may need a bigger size for voltage drop.

What size wire for a 50 amp circuit?

Copper 8 AWG is rated 50 A at 75°C, so it works where both ends are rated 75°C. If the equipment terminals are only rated 60°C, use 6 AWG copper (55 A). Aluminum needs 6 AWG at 75°C (50 A).

What size wire for 100 amps?

At 75°C, 3 AWG copper (100 A) or 1 AWG aluminum (100 A). For a dwelling service or main feeder, NEC 310.12 allows a smaller size (83% rule), which this calculator does not apply.

Why does the calculator use 125% for continuous loads?

NEC 210.19(A) and 215.2(A) require conductors for continuous loads (three hours or more) to be sized at 125% of that load. A 16 A continuous load therefore needs a conductor good for 20 A.

Should I use the 90°C column for THHN?

You can use the 90°C ampacity as the starting point for derating, but the final ampacity is usually limited by the 60°C or 75°C rating of the breaker and equipment terminals (110.14(C)).

Does wire size depend on distance?

Ampacity does not, but voltage drop does. On long runs, a wire that is large enough for the current can still lose too much voltage. Enter the one-way length to have the calculator upsize for a 3% limit.

Last reviewed 2026-11-15. Values from NFPA 70 (NEC) 2017–2023. For estimating only — confirm with the code edition adopted by your authority having jurisdiction.