Wattplate
Tools

Wire Size Calculator

The smallest wire for your load and your run, copper or aluminum.

Size from
Units
Metal

NM-B is the sheathed cable used in most houses; wires in conduit means single THHN/THWN-2 wires.

System

Rated output

Sized to fit

Use at least

4AWG

Use at least: 4 AWG copper (21.15 mm²).

MetalCopper
Area21.15 mm²
Drop4.93 V
Drop %2.05%
Run at 3%233 ft

Because: distance sets this size; current alone needs 6 AWG (60 °C rating 55 A typical).

  • Rating column: 60 °C, because NM-B cable is limited to its 60 °C rating.
  • 4 or more current-carrying wires in one conduit, hot attics (above 86 °F) and bundled cables lower the rating.
  • Some installs may use a smaller size under code exceptions; we size so the wire's typical rating is at least the load.

Working

  1. Current: 50 A
  2. Current alone: 6 AWG copper, 60 °C rating 55 A typical (the first size rated at least 50 A)
  3. Drop with 6 AWG = 2 × 160 ft × 50 A × 0.4901 Ω ÷ 1,000 = 7.84 V (3.27%), over 3%
  4. Drop with 4 AWG = 2 × 160 ft × 50 A × 0.3082 Ω ÷ 1,000 = 4.93 V (2.05%), within 3%
  5. Answer: the larger of the two, 4 AWG copper

Model WV-01

Typical figures from physics and maker data, one rating per input. The lamp lights only when no size can be given. Check your code edition and your local inspector (AHJ).

Quick answer

For a 50 A load at 240 V over 160 ft of copper NM-B cable, use at least 4 AWG copper (21.15 mm²): current alone needs 6 AWG (60 °C rating 55 A typical), and the distance needs the larger wire to keep the drop at 2.05%.

Rule of thumbTypical: 50 A on copper takes 6 AWG at 60 °C (8 AWG only with 75 °C terminals); long runs need bigger wire.

NoteTypical figures. Check your code edition and your local inspector (AHJ). This is an estimate from physics and maker data, not engineering or code advice. Hire a licensed electrician for the work.

How far can you run it? 240 V, copper, 3% drop
Wire size, AWG
Load10 AWG8 AWG6 AWG4 AWG3 AWG2 AWG
30 A96153244389490619
40 A-115183292368464
50 A--146233294371
60 A---194245309

The longest one-way run, in feet, before the drop on a 240 V single phase circuit reaches 3% (stranded copper). A dash means that size is too small for the current.

How the calculator picks a size

A wire has to pass two tests. It must carry the current without getting too hot, and over the length of the run it must not lose too much voltage. Each test gives a smallest size, and the answer is the larger of the two.

  1. Current. The calculator takes the load in amps, or the breaker size if you pick one, and finds the first wire whose typical current rating from the makers’ charts is at least that much. Small copper wire has a fixed limit whatever the chart says: 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG.
  2. Distance. It then finds the smallest wire that keeps the voltage drop within your limit at the real load, with the wire at 75 °C. The method is the same as on the voltage drop calculator.

On a short run the current decides. On a long run the distance usually does, and the result tells you which one set the size.

60 °C or 75 °C: which rating applies

Wire carries more current when it is allowed to run hotter, so the makers’ charts list a rating for each temperature. The rating you can use is limited by the coolest part of the circuit, which is usually where the wire connects. Breakers and equipment for circuits of 100 A or less are treated as 60 °C parts unless they are marked 75 °C (NEC 110.14(C)(1)). NM-B cable, the plastic-sheathed cable used in most houses, is limited to its 60 °C rating in any case (NEC 334.80).

So the calculator uses the 60 °C rating unless you choose wires in conduit and tick that both ends are marked 75 °C. That is why the usual answer for 50 A is 6 AWG copper, and 8 AWG copper only with 75 °C terminals at both ends.

Loads that run for hours

A load that runs 3 hours or more, such as an EV charger, is counted at 125% when choosing the wire (NEC 210.19(A)(1)). Tick “runs 3 hours or more” and the calculator multiplies the load by 1.25 for the current test. The voltage drop still uses the real current, because that is what flows in the wire. If you pick a breaker size instead, it is used as it is.

When the typical rating is too high

The charts assume surroundings at 86 °F and no more than three current-carrying wires together. Hot attics, four or more current-carrying wires in one conduit, and cables bundled tightly together all lower what a wire can carry. The calculator does not apply those corrections, so in those places choose a larger wire or ask an electrician.

What this calculator does not do

It stops at 100 A: feeders and services need more checks than one current rating, so it asks you to call an electrician. It does not pick a breaker, size the ground wire, or offer aluminum smaller than 8 AWG. Some installations may use a smaller wire under exceptions in the code; this tool always sizes so the typical rating is at least the load, which errs on the large side.

Worked example

  1. Inputs: 50 A load, 160 ft one way, copper NM-B cable, single phase 240 V, 3% drop limit.
  2. Current: 50 A
  3. Current alone: 6 AWG copper, 60 °C rating 55 A typical (the first size rated at least 50 A)
  4. Drop with 6 AWG = 2 × 160 ft × 50 A × 0.4901 Ω ÷ 1,000 = 7.84 V (3.27%), over 3%
  5. Drop with 4 AWG = 2 × 160 ft × 50 A × 0.3082 Ω ÷ 1,000 = 4.93 V (2.05%), within 3%
  6. Answer: the larger of the two, 4 AWG copper

Questions people ask

What size wire for a 50 amp circuit?

6 AWG copper for NM-B cable or unmarked terminals; 8 AWG copper only where the wires run in conduit and both ends are marked 75 °C. Those are the typical ratings at 60 °C (55 A) and at 75 °C (50 A). Distance can push it larger: at 240 V, 6 AWG copper reaches a 3% drop at about 146 ft, so a 160 ft run needs 4 AWG.

Does distance change the wire size?

Yes. The current sets the smallest wire that stays cool enough, and the distance sets the smallest wire that keeps the voltage drop in check. The calculator works out both and gives the larger. A long run often needs a wire one or two sizes bigger than the current alone calls for.

What is a continuous load?

A load that runs 3 hours or more, such as an EV charger. It is counted at 125% when choosing the wire (NEC 210.19(A)(1)), so a 16 A continuous load is sized as 20 A. Tick "runs 3 hours or more" and the calculator does this for you; the voltage drop still uses the real 16 A.

Copper or aluminum?

Aluminum carries less current for its size and drops about 1.64 times as much voltage as copper of the same size, so it needs a bigger wire: at 50 A on the 60 °C rating, 4 AWG aluminum against 6 AWG copper. The calculator offers aluminum from 8 AWG up. Check that the breaker and the equipment terminals accept aluminum wire.

Can I use a bigger wire than the minimum?

Yes. A bigger wire runs cooler and loses less voltage, and long runs are the usual reason to go up a size. Check that it fits the terminals on the breaker and the equipment. The breaker size still depends on the load and the equipment, not on the bigger wire.

Does it work for 12 V RV and van wiring?

Pick DC and 12 V and it sizes the wire the same way, with the voltage drop over your run. The current rating it uses is for building wire, and boat or RV cable can be rated differently, so check the label on your cable. For the 3% or 10% marine limits, the voltage drop calculator has a chip for each.

Related guides

Sources

Data last verified: Page updated

Written and checked by Ali, who is not a licensed electrician.

Before you wire anything

These calculators give estimates worked out from physics and published maker data. They are not engineering, design or code advice, and they do not replace an inspection.

Electrical codes differ by edition and by town. Your local inspector, the authority having jurisdiction (AHJ), has the final say on wire size, conduit, boxes and breakers.

Mains wiring can kill and can start fires. Unless you are trained for it, hire a licensed electrician, and get a permit where your town requires one.

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