What size wire do I need for 30, 40, 50 or 60 amps?

For copper on a short run, using the 75°C column of NEC Table 310.16: 30 A needs 10 AWG, 40 A needs 8 AWG, 50 A needs 8 AWG and 60 A needs 6 AWG. Where the terminals are only rated 60°C, use the 60°C column: 50 A then needs 6 AWG. Aluminum is one or two sizes larger: 6 AWG for 50 A at 75°C. On a long run, voltage drop can push any of these up a size or more, which is what the calculator checks.

How do I size wire for distance?

Find the size the current needs, then check the voltage drop over the run and go up until the drop is at or under your limit, usually 3% for a branch circuit. A 40 A load at 240 V needs 8 AWG copper for ampacity, but 150 feet away 8 AWG drops 9.34 V (3.89%), so 6 AWG is the size: 5.89 V, or 2.46%.

What do the 60°C, 75°C and 90°C columns mean?

They are the insulation temperature ratings in Table 310.16. A wire can only be loaded to the lowest rating of anything it connects to (NEC 110.14(C)). Terminals on circuits of 100 A or less, or for 14 through 1 AWG, are treated as 60°C unless the equipment is listed for 75°C; larger circuits use 75°C. The 90°C column of THHN or XHHW-2 is mostly a starting point for derating in hot spaces or full raceways.

Does the breaker size change the wire size?

Yes. The breaker or fuse protects the wire, so the wire's ampacity has to suit it (NEC 240.4). For small copper, 240.4(D) limits the breaker to 15 A on 14 AWG, 20 A on 12 AWG and 30 A on 10 AWG (aluminum: 15 A on 12 AWG, 25 A on 10 AWG), whatever the table says. A continuous load, one running three hours or more, is sized at 125%. Motors, air conditioners and some other equipment follow their own articles and nameplate ratings.

Wire Size Calculator

Find the AWG wire size for a load by ampacity and voltage drop, from amps, run length, voltage and phase, for copper or aluminum. Enter the load, the voltage and the one-way run to get the minimum size for the current, the minimum for your allowed drop, and the size to use, with its voltage drop. An estimate: a licensed electrician and the local code decide.

Your numbers

Conductor

Temperature column

Load current

A

Voltage

V

120, 208, 240, 277, 480; 12, 24 or 48 for DC.

One-way length

Source to load, along the route.

Length in

Allowed voltage drop

%

3% is the NEC's recommendation for a branch circuit, 5% for feeder and branch together.

Continuous load

Runs at full current for 3 hours or more; sized at 125%.

Recommended wire size

–

Set by: –
Minimum by ampacity: –
Minimum for 3% drop: –
Voltage drop at the recommended size: –
Voltage at the load: –
Current the wire is sized for: –
Ampacity of the recommended size: –
Smallest standard breaker for the load: –

Ampacity from NEC Table 310.16 at 30°C ambient with no more than three current-carrying conductors together. Hotter spaces, more conductors in one raceway and the terminals' temperature rating can call for a larger wire.

An estimate: a licensed electrician and the local code decide.

The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.

How the wire size is worked out

Two checks, and the larger answer wins. First, ampacity: the smallest wire whose rating in NEC Table 310.16, in the temperature column you choose, covers the current. A continuous load is counted at 125%, and 14, 12 and 10 AWG copper are held to their 240.4(D) breaker limits of 15, 20 and 30 A. Second, voltage drop: the smallest wire whose drop over the one-way length stays at or under the percent you allow, using the NEC Chapter 9 Table 8 resistance at 75°C.

Worked example: a 40 A, 240 V single-phase load 150 feet away, copper, 75°C column, 3% allowed. By ampacity, 10 AWG is capped at 30 A, so 8 AWG (50 A) is the minimum. By voltage drop, 3% of 240 V is 7.2 V; 8 AWG drops 2 × 40 × 0.778 × 150 ÷ 1,000 = 9.34 V, too much, and 6 AWG drops 5.89 V, or 2.46%. The recommended size is 6 AWG copper, set by voltage drop. The smallest standard breaker for the load is 40 A.

Which temperature column to use

The default is the 75°C column, which applies when the wire and everything it lands on (breaker, panel, disconnect, equipment) are rated at least 75°C. NEC 110.14(C) says circuits of 100 A or less, or wire from 14 to 1 AWG, use the 60°C column unless the equipment is listed and marked for 75°C. If you are not sure what the terminals are marked, choose 60°C.

The 90°C column is for conductors like THHN, THWN-2 and XHHW-2 and is where adjustment starts when a wire runs through a hot attic or shares a raceway with more than three current-carrying conductors. After derating, the result still cannot exceed the terminal rating. Those corrections are not in this calculator.

Breaker and fuse sizing

The calculator shows the smallest standard breaker or fuse rating, from NEC 240.6(A), at or above the design current. The wire's ampacity has to be at least that rating, except that 240.4(B) lets you use the next standard size up when the wire's ampacity falls between standard sizes, on circuits up to 800 A that do not feed several receptacles for cord-and-plug equipment. The 240.4(D) limits on 14, 12 and 10 AWG copper always apply.

Dwelling services and the main feeder of a house follow 310.12, which allows a smaller size than Table 310.16. Motors (Article 430), air conditioners and heat pumps (Article 440, using the nameplate's minimum circuit ampacity and maximum breaker), EV chargers and solar circuits each add their own multipliers. Use the nameplate and the article for those.

AWG ampacity and resistance chart

The values the calculator uses. Ampacity in amps from NEC Table 310.16 (not more than three current-carrying conductors, 30°C ambient); DC resistance in ohms per 1,000 feet at 75°C from NEC Chapter 9, Table 8 (stranded). A dash means the table lists no value. Breakers on 14, 12 and 10 AWG copper and 12 and 10 AWG aluminum are limited by 240.4(D).

Copper and aluminum ampacity by temperature column and DC resistance, 18 AWG to 500 kcmil
Size	copper, amps			aluminum, amps			Ω per 1,000 ft
Size	60°C	75°C	90°C	60°C	75°C	90°C	Cu	Al
18 AWG	–	–	14	–	–	–	7.95	13.1
16 AWG	–	–	18	–	–	–	4.99	8.21
14 AWG	15	20	25	–	–	–	3.14	5.17
12 AWG	20	25	30	15	20	25	1.98	3.25
10 AWG	30	35	40	25	30	35	1.24	2.04
8 AWG	40	50	55	35	40	45	0.778	1.28
6 AWG	55	65	75	40	50	55	0.491	0.808
4 AWG	70	85	95	55	65	75	0.308	0.508
3 AWG	85	100	115	65	75	85	0.245	0.403
2 AWG	95	115	130	75	90	100	0.194	0.319
1 AWG	110	130	145	85	100	115	0.154	0.253
1/0 AWG	125	150	170	100	120	135	0.122	0.201
2/0 AWG	145	175	195	115	135	150	0.0967	0.159
3/0 AWG	165	200	225	130	155	175	0.0766	0.126
4/0 AWG	195	230	260	150	180	205	0.0608	0.1
250 kcmil	215	255	290	170	205	230	0.0515	0.0847
300 kcmil	240	285	320	195	230	260	0.0429	0.0707
350 kcmil	260	310	350	210	250	280	0.0367	0.0605
400 kcmil	280	335	380	225	270	305	0.0321	0.0529
500 kcmil	320	380	430	260	310	350	0.0258	0.0424

For electricians and solar installers on a job walk

The inputs that decide the size come up as you walk the job: the load, where the panel is, how the run gets there, whether it goes through the attic. On a long run the wire is often the largest material cost on the quote, so a size up or down changes the price.

Run the circuit here while you are still on site, then write the size and the reason into the quote, so the customer sees why the run to the shop needs bigger wire than the one to the kitchen.

An estimate: a licensed electrician and the local code decide. Ambient temperature, conductors bundled in one raceway, terminal ratings and the equipment's nameplate all affect the final size.

Sources, read on 2026-10-23:

Already picked the wire? The voltage drop calculator checks one size, with parallel sets. Sizing the load first? Try the BTU calculator for heating and cooling, or the solar calculator for an array.

The load and the run get said on the walk-through.

Electricians and solar installers hear the numbers out loud on site: a 50 A range circuit, 150 feet to the shop, a subpanel in the garage. Record the walk-through in ParrotNotes and get a structured note, a summary and the action items, so the quote has every run and load in it. Free for 100 minutes of recording a month.

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