Conductor sizing

Wire Size Calculator

Enter the circuit and the drop limit, and this finds the smallest conductor that stays inside it. It also shows, separately, the smallest size whose published ampacity carries the load, because those are two different questions and the answer is whichever size satisfies both.

Your conductor size appears here

Enter a supply voltage, a load current and the run length.

Smallest size that satisfies both checks

4 AWG

Copper, 1.93% drop

Voltage drop is the binding constraint here.

Voltage drop check

4 AWG

1.93% drop, inside your 3% limit

An engineering calculation. It says nothing about whether the conductor may legally carry this current.

Ampacity check

8 AWG

50 A allowable at the 75 °C column, before any correction or adjustment

A table look-up from NEC Table 310.16 at 30 °C ambient with no more than three current-carrying conductors. Real installations need correction for ambient temperature, adjustment for conductor count, and a check against the overcurrent device and the terminal ratings.

How the size was chosen

Voltage drop budget
V_allowed = V_source × (limit ÷ 100)V_allowed = 240 V × (3 ÷ 100)
Result: 7.20 V
This is the largest drop the chosen limit permits over the whole run.
Maximum impedance the conductor may have
Z_max = V_allowed ÷ (2 × I × L)Z_max = 7.20 V ÷ (2 × 50 A × 0.1500 (1000 ft))
Result: 0.48000 Ω per 1000 ft
The smallest listed conductor whose impedance is at or below this figure is the answer.
Smallest conductor that satisfies both checks
Search the size list from smallest upward4 AWG copper
Result: 1.93% drop, 85 A allowable

Every size, compared

Conductor sizes compared by voltage drop and ampacity
Size	Drop	Volts	Ampacity	Verdict
18 AWG	49.69%	119.25 V	—	Over 3%
16 AWG	31.19%	74.85 V	—	Over 3%
14 AWG	19.63%	47.10 V	20 A	Over 3%
12 AWG	12.38%	29.70 V	25 A	Over 3%
10 AWG	7.75%	18.60 V	35 A	Over 3%
8 AWG	4.86%	11.67 V	50 A	Over 3%
6 AWG	3.07%	7.37 V	65 A	Over 3%
4 AWGrecommended	1.93%	4.62 V	85 A	Both OK
3 AWG	1.53%	3.68 V	100 A	Both OK
2 AWG	1.21%	2.91 V	115 A	Both OK
1 AWG	0.96%	2.31 V	130 A	Both OK
1/0 AWG	0.76%	1.83 V	150 A	Both OK
2/0 AWG	0.60%	1.45 V	175 A	Both OK
3/0 AWG	0.48%	1.15 V	200 A	Both OK
4/0 AWG	0.38%	0.91 V	230 A	Both OK
250 kcmil	0.32%	0.77 V	255 A	Both OK
300 kcmil	0.27%	0.64 V	285 A	Both OK
350 kcmil	0.23%	0.55 V	310 A	Both OK
400 kcmil	0.20%	0.48 V	335 A	Both OK
500 kcmil	0.16%	0.39 V	380 A	Both OK
600 kcmil	0.13%	0.32 V	420 A	Both OK
700 kcmil	0.11%	0.28 V	—	Drop OK
750 kcmil	0.11%	0.26 V	475 A	Both OK
800 kcmil	0.10%	0.24 V	—	Drop OK
900 kcmil	0.09%	0.21 V	—	Drop OK
1000 kcmil	0.08%	0.19 V	545 A	Both OK

Drop from NEC Chapter 9 Table 8 resistance at 75 °C. Ampacity from NEC Table 310.16, 30 °C ambient, no more than three current-carrying conductors, before correction or adjustment factors.

The distinction

Two checks, one conductor.

Voltage drop sizing

An engineering calculation. Rearrange the voltage-drop formula for impedance, work out the largest impedance per 1000 ft that keeps the drop inside your limit, and pick the smallest listed conductor at or below it. The answer depends on voltage, current, length, material and power factor. It says nothing about heat, insulation or overcurrent protection, and it carries no code authority of its own. The 3% and 5% figures people size to appear in the NEC only as Informational Notes.

Ampacity sizing

A table look-up. NEC Table 310.16 gives allowable ampacity by size, material and insulation temperature rating, based on 30 °C ambient with no more than three current-carrying conductors in the raceway. Real installations then need correction for ambient temperature, adjustment for conductor count, a check that the terminals are rated for the column you read, and coordination with the overcurrent device. Length does not enter into it at all.

Questions

Common questions.

What is the difference between sizing for voltage drop and sizing for ampacity?

They answer different questions and a conductor has to satisfy both. Ampacity asks how much current the conductor can carry without overheating its insulation: a table look-up from NEC Table 310.16, then modified for ambient temperature, for how many current-carrying conductors are bundled together, and for the temperature rating of the terminals it lands on. Voltage drop asks how much voltage the run loses on the way to the load, which depends on length and has nothing to do with heat. A 14 AWG copper conductor is rated 20 A, but over 100 ft at 20 A on a 120 V circuit it drops 6.6%. It carries the current and still is not the right conductor.

Which constraint usually decides the size?

Short runs are almost always decided by ampacity, long runs by voltage drop. The crossover depends on the voltage: at 120 V it comes surprisingly early, often within 50 ft on a 20 A circuit, because the percentage is measured against a small supply. At 480 V three-phase you can run a long way before drop overtakes ampacity. This calculator shows both answers so you can see which one is binding rather than guessing.

Does this calculator tell me if a conductor is code compliant?

No, and no calculator can. The voltage-drop side is pure engineering with no code content. The ampacity side is the base table value before any of the correction and adjustment factors a real installation needs, and before checking the overcurrent device, the terminal temperature ratings, the small-conductor rules in 240.4(D), the wiring method, and everything else the code requires. Use it to narrow the choice, then verify against the code that applies where the work is being done.

How do parallel conductors change the answer?

Running n identical conductors per phase divides both the voltage drop and the effective impedance by n, and multiplies the available ampacity by n. Two 250 kcmil conductors per phase drop half what one does and carry twice the current. The code has its own conditions on paralleling (generally 1/0 and larger, with all conductors in a set the same length, material, size and termination method), which this calculator does not check.

Why do I need a much larger aluminium conductor than copper?

Aluminium at 61% IACS conductivity has about 1.64 times the resistance of copper at the same cross-sectional area, so it drops about 1.64 times the voltage. Matching a copper conductor on drop generally means going up two AWG sizes. Aluminium also has lower ampacity size for size, so both constraints push the same way.

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Wire Size Calculator