Wire size calculator

What gauge wire a breaker needs, from the National Electrical Code ampacity tables, with temperature, conductor count, voltage drop and house service sizes.

Start from:

Sizing A circuit (breaker) House service or main feeder

Breaker size A Conductor Insulation rating Breaker and equipment terminals Air temperature around the wire °F Current-carrying wires together wires Run length, one way (optional) ft Voltage V System

Minimum wire size

10 AWG copper

Allowable ampacity: 30.0 A for a 30 A breaker
Terminal column: 60 °C
Temperature and wire-count factor: 1.00
Largest breaker on this size: 30 A (240.4(D))
Voltage drop at this size: %
83% of the rating: 25 A (310.12)

Over 3% voltage drop at this length. Use to stay within it.
No size up to 500 kcmil keeps the drop within 3% at this length; shorten the run or raise the voltage.
Nothing in the table fits these conditions (or the insulation isn't permitted at this temperature). Have a licensed electrician design this circuit.

From NEC 2023 Table 310.16 for up to three current-carrying wires at 86 °F, corrected for your temperature and wire count, limited by the terminal rating (110.14(C)) and the small-wire breaker limits (240.4(D)). Follow the code edition your area has adopted, and have a licensed electrician size and inspect the work.

The wire is only one part of a safe circuit

The breaker must suit the load and the equipment, the cable or raceway must suit the location, and equipment such as air conditioners, motors and EV chargers have their own sizing rules on the nameplate and in the code. This calculator doesn't check those. Have a licensed electrician design, install and check the circuit, and get a permit where one is required.

How to choose a wire size

The wire has to carry the breaker's current without overheating. The National Electrical Code gives each size an ampacity, adjusted for heat and crowding, and limits it by the temperature the terminals can take.

Start from the wire's ampacity in Table 310.16 for its insulation rating (60, 75 or 90 °C)
Correct for air hotter or cooler than 86 °F (Table 310.15(B)(1)(1)) and for more than three current-carrying wires together (Table 310.15(C)(1))
Limit it to the terminal column: 60 °C for circuits of 100 A or less unless the equipment is listed for 75 °C, 75 °C above that (110.14(C)(1))
Pick the smallest size whose ampacity is at least the breaker rating, without going over the 240.4(D) breaker limits for 14, 12 and 10 AWG
Check voltage drop on long runs, aiming for 3% or less on a branch circuit

Worked example

A 50 A breaker with copper wire rated 90 °C, terminals not marked for 75 °C: the circuit is 100 A or less, so the 60 °C column applies. 8 AWG copper is 40 A at 60 °C, too small; 6 AWG is 55 A, so the answer is 6 AWG copper. If the breaker and the equipment are both listed for 75 °C, 8 AWG's 75 °C ampacity of 50 A is enough, and the answer is 8 AWG.

Wire size chart by breaker amps

Minimum size for 90 °C wire at 86 °F with up to three current-carrying wires, from the same rules as the calculator. "Unmarked" uses 60 °C terminals up to 100 A; "75 °C" assumes equipment listed and marked for 75 °C.

Breaker	Copper, unmarked	Copper, 75 °C	Aluminum, unmarked	Aluminum, 75 °C
15 A	14 AWG	14 AWG	12 AWG	12 AWG
20 A	12 AWG	12 AWG	10 AWG	10 AWG
25 A	10 AWG	10 AWG	10 AWG	10 AWG
30 A	10 AWG	10 AWG	8 AWG	8 AWG
40 A	8 AWG	8 AWG	6 AWG	8 AWG
50 A	6 AWG	8 AWG	4 AWG	6 AWG
60 A	4 AWG	6 AWG	3 AWG	4 AWG
70 A	4 AWG	4 AWG	2 AWG	3 AWG
80 A	3 AWG	4 AWG	1 AWG	2 AWG
90 A	2 AWG	3 AWG	1/0 AWG	2 AWG
100 A	1 AWG	3 AWG	1/0 AWG	1 AWG
125 A	1 AWG	1 AWG	2/0 AWG	2/0 AWG
150 A	1/0 AWG	1/0 AWG	3/0 AWG	3/0 AWG
175 A	2/0 AWG	2/0 AWG	4/0 AWG	4/0 AWG
200 A	3/0 AWG	3/0 AWG	250 kcmil	250 kcmil

House service and main feeder wire sizes

NEC 2023 Table 310.12(A), for single-phase 120/240 V services and main feeders of a one-family dwelling or an individual unit, where no temperature or wire-count corrections apply. These conductors only need 83% of the service rating.

Service rating	Copper	Aluminum or copper-clad aluminum
100 A	4 AWG	2 AWG
110 A	3 AWG	1 AWG
125 A	2 AWG	1/0 AWG
150 A	1 AWG	2/0 AWG
175 A	1/0 AWG	3/0 AWG
200 A	2/0 AWG	4/0 AWG
225 A	3/0 AWG	250 kcmil
250 A	4/0 AWG	300 kcmil
300 A	250 kcmil	350 kcmil
350 A	350 kcmil	500 kcmil
400 A	400 kcmil	600 kcmil

Wire ampacity chart (NEC Table 310.16)

Allowable ampacity in amps, up to three current-carrying wires at 86 °F (30 °C), from NFPA 70, National Electrical Code, 2023 edition. 14, 12 and 10 AWG are also limited by 240.4(D).

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

Wire size questions

What size wire for a 20, 30, 40, 50 or 60 amp breaker?

With copper wire and terminals that aren't marked for 75 °C: 20 A takes 12 AWG, 30 A takes 10 AWG, 40 A takes 8 AWG, 50 A takes 6 AWG and 60 A takes 4 AWG. Where the breaker and equipment are listed for 75 °C, 50 A drops to 8 AWG and 60 A to 6 AWG. The chart above lists aluminum and more breaker sizes, and the calculator handles hot attics, crowded conduit and long runs.

Why can't 12 AWG go on a 25 or 30 amp breaker?

Section 240.4(D) caps the breaker for small wires no matter what the ampacity table says: 15 A for 14 AWG copper, 20 A for 12 AWG copper and 30 A for 10 AWG copper; 15 A for 12 AWG aluminum and 25 A for 10 AWG aluminum. Some equipment, like air conditioners and motors, has its own rules that can allow more; follow the nameplate and the code for those.

What size wire for a 100 or 200 amp service?

For a house's main service or main feeder at 120/240 V, Table 310.12(A) allows 4 AWG copper or 2 AWG aluminum for 100 A, and 2/0 AWG copper or 4/0 AWG aluminum for 200 A, because the code lets these conductors carry 83% of the rating. That table only applies to the dwelling's main service or a feeder carrying its whole load. A 100 A subpanel feeder that doesn't carry the whole house is sized as a circuit: choose "A circuit" above.

How does heat or a full conduit change the wire size?

The ampacity table assumes 86 °F and no more than three current-carrying wires together. In a hot attic or with more wires in one conduit, the ampacity is multiplied down: at 104 °F a 90 °C wire keeps 91% of its rating, and four to six current-carrying wires keep 80%. Enter the temperature and wire count above to see the effect.

Does a long run need bigger wire?

Often. Long runs lose voltage, and the code's informational notes suggest keeping a branch circuit within 3%. Enter the one-way length and voltage above and the calculator checks the drop with the voltage drop calculator's method, suggesting a larger size if needed. Low-voltage DC runs, like 12 V lighting or solar, are usually sized by voltage drop rather than ampacity.

What size wire for an air conditioner, motor or ground wire?

Air conditioners list a minimum circuit ampacity and a maximum breaker size on the nameplate: size the wire for the minimum circuit ampacity and the breaker no larger than the maximum shown. Motors marked with design letter B, C or D may use the 75 °C column under 110.14(C)(1)(a)(4), and motor circuits have their own sizing rules. The equipment grounding wire has its own sizing rules too, which this calculator doesn't cover. An electrician should size all of these.