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Wire size calculator

A 16 A load at the end of a 100 ft, 120 V single-phase copper run needs 10 AWG to stay inside a 3% allowance - 12 AWG would carry the current but drops too much over that distance. At 10 AWG the drop is 3.2 V, or 2.7% of 120 V.

Your circuit

Distance to the load, not there and back - the calculator doubles it.

Result

Size to use
10 AWG
Cross-section the drop limit demands
4.67 mm²
Voltage drop
3.2 V
Drop as a percentage
2.7 %
Allowance left unused
0.3 %
Ampacity of that size
30 A

NEC Table 310.16, copper, 60 °C (140 °F) column - Copper conductors, not more than three current-carrying conductors in a raceway or cable, 30 °C (86 °F) ambient, 60 °C termination rating. Higher ambient temperatures, more conductors in the same raceway, or conductors in sunlight on a rooftop all reduce these figures. The 60 °C column is used here because it is the column 240.4(D) leaves you with on 14, 12 and 10 AWG anyway, and because most 15 A and 20 A devices are only listed for 60 °C terminations.

One size either way, for this run

  • 12 AWG4.2 % · 5.1 VOver the allowance
  • 10 AWG2.7 % · 3.2 VWithin the allowanceyour cable
  • 8 AWG1.7 % · 2.0 VWithin the allowance

Permitted drop: 3.0 % · ASTM B258

How the calculation works

Sizing a conductor is two separate questions, and the calculator above answers the one that tables do not. Ampacity asks whether the wire can carry the current without overheating; that comes from NEC Table 310.16 with its correction and adjustment factors. Voltage drop asks whether enough voltage survives the trip to the load, and that is pure arithmetic:

A ≥ k · ρ(T) · L · I / ΔU

A is the cross-section in mm², k is 2 for DC and single phase (current goes out and comes back) or √3 for three phase, ρ(T) is the resistivity of the metal at its working temperature, L is the one-way length, I the load current, and ΔU the volts you are willing to lose. Copper at 20 °C is 0.017241 Ω·mm²/m; the calculator corrects it with α = 0.00393 per K when you raise the temperature.

The result is then rounded up to the next size that actually exists - AWG sizes come from the ASTM B258 definition rather than a typed-in table, so they cannot drift. Rounding down would hand you a wire that misses the allowance you just typed in, which is why the figure you buy is always at least the figure the formula asks for.

What the calculator will not do is invent an ampacity for a size. Current-carrying capacity depends on the table, the termination rating, the ambient temperature and how many conductors share the raceway - so the US edition consults the 60 °C copper column of Table 310.16 and says so, and nothing is implied about installation methods it cannot see.

Worked example

Your circuit

System
Single phase AC
Supply voltage (V)
120.0 V
Load current (A)
16 A
Run length, one way
100 ft (30.5 m)
Conductor material
Copper
Size series
AWG
Voltage drop allowed (%)
3.0 %

Result

Cross-section the drop limit demands
4.67 mm²
Size to use
10 AWG
Voltage drop
3.2 V
Drop as a percentage
2.7 %
Allowance left unused
0.3 %
Ampacity of that size
30 A
Formula
A ≥ k · ρ(T) · L · I / ΔU
Standard
ASTM B258 (AWG), IEC 60028
Limits of the model
The resistive model is used: conductor reactance is not included, which is negligible below roughly 95 mm² and on the short runs this tool is for.

Computed by Voltacalc with the same engine as the tool above.

What applies in the United States

The 60 °C column of NEC Table 310.16 and the small-conductor rule in 240.4(D) set what the breaker may be - 15 A on 14 AWG, 20 A on 12 AWG, 30 A on 10 AWG - and no amount of upsizing for voltage drop changes those limits. Upsizing the wire for a long run is normal practice and does not entitle you to a bigger breaker, because the breaker still protects the smallest conductor in the circuit.

Who decides
NFPA / the authority having jurisdiction (AHJ) in your state or city
Standard
NEC (NFPA 70), National Electrical Code
Ampacity table
NEC Table 310.16, with the correction and adjustment factors of 310.15(B) and (C), and the small-conductor rule in 240.4(D)
Permitted drop
The NEC does not make voltage drop a rule for most branch circuits: 210.19(A) and 215.2(A)(1) carry Informational Notes recommending no more than 3% on the branch circuit and 5% for feeder plus branch combined. Your AHJ, or a specific article such as 647.4(D), can make a lower figure mandatory. (Checked: September 12, 2026)

Questions electricians ask

Does the calculator use one-way or round-trip length?

One way. Enter the distance from the panel to the load and the calculator doubles it for you, because on a 120 V or 240 V circuit the current travels out on one conductor and back on another.

Why does it give me a bigger wire than the ampacity table?

Because it sizes for voltage drop, not heat. Ampacity answers "will this wire survive the current"; voltage drop answers "will the load still see enough volts". On long 120 V runs the second question almost always asks for more copper than the first.

Is 3% a code requirement?

No. The NEC carries 3% on the branch circuit and 5% total as Informational Notes - recommendations, not rules - although your AHJ or a specific article such as 647.4(D) can make a tighter figure mandatory.

Can I use aluminum instead?

Yes, and the calculator has it as an option, but aluminum's resistivity is about 64% higher than copper's, so the same drop allowance needs roughly two gauge sizes more. Terminations have to be listed for aluminum too.

Standards and sources

  • NEC (NFPA 70), National Electrical Code
  • NEC Table 310.16, with the correction and adjustment factors of 310.15(B) and (C), and the small-conductor rule in 240.4(D)

Last updated: September 12, 2026

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