ContractorHandbook

Wire Size Chart by Amps and Distance

The smallest conductor that holds voltage drop under 3 percent, for ten loads across seven distances, copper and aluminum.

Two separate things decide a wire size, and mixing them up is the most common mistake on a long run. The first is ampacity: the conductor has to carry the current without overheating, and that is NEC Table 310.16. The second is voltage drop: the conductor has to deliver usable voltage at the far end, and that is what the tables below are about.

On a short run ampacity governs and voltage drop is irrelevant. Past roughly a hundred feet the two swap places, and voltage drop starts demanding a conductor two or three sizes above the one ampacity would accept. Whichever of the two asks for more copper is the one you install.

Three percent is the figure every one of these tables is built on. It is the recommendation in the informational notes to NEC 210.19 and 215.2 for branch circuits and feeders, and it is a note rather than a rule, so no inspector fails a job on it. It is still the right target, because what is on the other end of the wire does not care what the code enforces.

Copper at 240 V, single phase

Load25 ft50 ft75 ft100 ft150 ft200 ft300 ft
15 A14 AWG14 AWG14 AWG12 AWG10 AWG8 AWG8 AWG
20 A14 AWG14 AWG12 AWG10 AWG8 AWG8 AWG6 AWG
30 A14 AWG12 AWG10 AWG8 AWG8 AWG6 AWG4 AWG
40 A14 AWG10 AWG8 AWG8 AWG6 AWG4 AWG3 AWG
50 A12 AWG10 AWG8 AWG6 AWG4 AWG4 AWG2 AWG
60 A12 AWG8 AWG8 AWG6 AWG4 AWG3 AWG2 AWG
100 A10 AWG6 AWG4 AWG4 AWG2 AWG1 AWG2/0
125 A8 AWG6 AWG4 AWG3 AWG1 AWG1/03/0
150 A8 AWG4 AWG4 AWG2 AWG1 AWG2/03/0
200 A6 AWG4 AWG2 AWG1 AWG2/03/0250 kcmil

Voltage drop only. This column is the smallest conductor that stays under 3 percent over that one way distance, and it is NOT an ampacity rating. A 20 amp circuit still needs 12 AWG minimum for ampacity no matter what this table says at 25 feet. Install the larger of the two.

Copper at 120 V, single phase

Load25 ft50 ft75 ft100 ft150 ft200 ft300 ft
15 A14 AWG12 AWG10 AWG8 AWG8 AWG6 AWG4 AWG
20 A14 AWG10 AWG8 AWG8 AWG6 AWG4 AWG3 AWG
30 A12 AWG8 AWG8 AWG6 AWG4 AWG3 AWG2 AWG
40 A10 AWG8 AWG6 AWG4 AWG3 AWG2 AWG1/0
50 A10 AWG6 AWG4 AWG4 AWG2 AWG1 AWG2/0
60 A8 AWG6 AWG4 AWG3 AWG2 AWG1/02/0
100 A6 AWG4 AWG2 AWG1 AWG2/03/0250 kcmil
125 A6 AWG3 AWG1 AWG1/03/04/0350 kcmil
150 A4 AWG2 AWG1 AWG2/03/0250 kcmil350 kcmil
200 A4 AWG1 AWG2/03/0250 kcmil350 kcmil500 kcmil

The same loads at half the voltage need a great deal more copper, because drop is a percentage of the supply and the supply is half as large. This is the whole argument for running a long circuit at 240 V wherever the load allows it.

Aluminum at 240 V, single phase

Load25 ft50 ft75 ft100 ft150 ft200 ft300 ft
15 A14 AWG12 AWG10 AWG10 AWG8 AWG6 AWG4 AWG
20 A14 AWG12 AWG10 AWG8 AWG6 AWG6 AWG4 AWG
30 A12 AWG10 AWG8 AWG6 AWG4 AWG4 AWG2 AWG
40 A12 AWG8 AWG6 AWG6 AWG4 AWG3 AWG1 AWG
50 A10 AWG8 AWG6 AWG4 AWG3 AWG2 AWG1/0
60 A10 AWG6 AWG4 AWG4 AWG2 AWG1 AWG2/0
100 A8 AWG4 AWG3 AWG2 AWG1/02/04/0
125 A6 AWG4 AWG2 AWG1 AWG2/03/0250 kcmil
150 A6 AWG3 AWG2 AWG1/02/04/0350 kcmil
200 A4 AWG2 AWG1/02/04/0250 kcmil500 kcmil

Aluminum carries about 61 percent of copper's conductivity, which is roughly two sizes up for the same drop. It is normal and code compliant for services, feeders and sub panel runs, and it needs terminations rated for it and an antioxidant compound on the connections.

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Ampacity first, then this table

Look the circuit up in NEC Table 310.16 for the conductor that carries the current at the temperature rating of the terminations, which on most residential equipment is 60 or 75 degrees Celsius. That gives you a floor: 14 AWG for 15 amps, 12 AWG for 20, 10 AWG for 30 and so on for copper.

Then look the run up in the table above. If the voltage drop answer is larger, the voltage drop answer wins and you pull the bigger wire. If it is smaller, ignore it and pull what ampacity requires. The tables here will happily return 14 AWG for a 20 amp circuit at 25 feet, and installing that would be a code violation. The table is answering one question, not both.

Length here means one way

Every distance in these tables is the one way run from the panel to the load. The formula doubles it internally, because the current goes out on one conductor and comes back on another and both of them drop voltage.

Measure the route the wire takes, not the straight line. A run to a detached garage goes up the wall, across the attic, down to the trench, through the trench, and up the far wall, and that path is routinely half again as long as the distance across the yard.

What voltage drop actually does

Motors are the worst affected. A low voltage motor draws more current to make the same power, runs hotter, starts harder and dies earlier, which is why well pumps and shop compressors on long undersized runs fail in ways that look like equipment faults. Resistive loads simply produce less: a water heater at 5 percent drop takes noticeably longer to recover.

Electronics mostly cope, and LED lighting mostly copes, which is why a long lighting circuit can be well out of spec for years without anyone noticing. That is an argument for getting it right at install time rather than an argument that it does not matter.

Where the 3 percent figure comes from

The informational notes to NEC 210.19 and 215.2 suggest 3 percent on a branch circuit or a feeder, and no more than 5 percent total from the service to the furthest outlet. Informational notes are not enforceable text, so this is guidance rather than a rule an inspector cites.

Where it does become enforceable is in specific equipment standards and in some local amendments, and in the manufacturer instructions for a particular piece of equipment, which NEC 110.3(B) does make binding. Check the nameplate and the manual on anything expensive at the end of a long run.

Questions people ask

What size wire do I need for a 100 amp sub panel 150 feet away?

For voltage drop at 240 V, 2 AWG copper or 1/0 aluminum, from the tables above. Check ampacity separately for your termination temperature rating and install whichever of the two is larger.

What size wire for a 50 amp circuit 100 feet away?

6 AWG copper at 240 V keeps the drop under 3 percent, which is also what ampacity wants at 50 amps and a 75 degree termination, so the two answers agree at that distance. At 200 feet voltage drop moves it to 4 AWG and ampacity does not.

Can I use aluminum wire for a long run?

Yes, and for services and feeders it is usually the cheaper answer at the larger sizes. Go up about two sizes from the copper column, use terminations listed for aluminum, and apply the antioxidant compound. Aluminum's bad reputation comes from small branch circuit wire in the 1960s and 70s, not from modern feeder conductors.

Is voltage drop a code violation?

Not by itself in the NEC, because 210.19 and 215.2 put the 3 percent figure in informational notes rather than in enforceable text. Local amendments, specific equipment standards and manufacturer instructions under 110.3(B) can each make it binding, and exceeding it is a real performance problem whether or not anyone can cite it.

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See all 30 reference charts.

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