Wire Gauge Calculator

Picking a wire size looks like one question and is actually two, asked of the same piece of copper. Can it carry the current without its own insulation cooking? And can it deliver the voltage at the far end after the resistance of the run has taken its cut? The first is a table lookup. The second is a formula. The answer is whichever of them asks for more copper — and knowing which one asked is usually more useful than the size itself, because it tells you what to change.

Limit one: ampacity

A conductor's ampacity is the current it can carry continuously without exceeding the temperature its insulation is rated for. It is not a property of the metal alone — it is a property of the metal, the insulation around it, and how easily the assembly can shed heat. That last part is why the same 12 AWG copper wire is rated 25 A in a raceway and considerably less if you bundle nine of them together in a hot attic.

The published starting point is NEC Table 310.16, which assumes no more than three current-carrying conductors in a raceway or cable at 30°C (86°F) ambient. In that condition 14 AWG copper is 20 A, 12 AWG is 25 A, and 10 AWG is 35 A in the 75°C column. Three things then move the number:

Ambient temperature. Above 30°C the table value is multiplied by a correction factor. A 50°C attic — 122°F, which a roof space reaches easily — costs a 75°C conductor 25% of its rating, so that 20 A 14 AWG becomes 15 A and no longer carries a 20 A circuit at all.

Bundling. Past three current-carrying conductors in the same raceway, heat has nowhere to go, so an adjustment factor applies: 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20. Note that the equipment grounding conductor is not current-carrying and does not count, and on a normal circuit the neutral does.

Continuous load. A load that runs three hours or more — the classic examples are lighting circuits and EV chargers — must be served by a conductor rated for 125% of it (NEC 210.19(A)(1)). A 20 A continuous load therefore needs a 25 A conductor, which is the rule that quietly pushes a lot of 14 AWG answers up to 12 AWG.

The termination rule that catches everyone

Most wire sold today is 90°C-rated: THHN, THWN-2, XHHW-2. It is tempting to read the 90°C column, which is generous. You usually cannot. NEC 110.14(C) says the whole circuit is limited by the lowest-rated component in it, and breakers, lugs and receptacles are listed for 60°C or 75°C, not 90°C. So a 90°C conductor landing on 75°C lugs is sized from the 75°C column.

The 90°C column is not useless — it is the number you derate from. Start at the 90°C ampacity, apply the ambient and bundling factors, and the result may not exceed the 60°C or 75°C table value. That two-step is how a 90°C conductor buys you headroom in a hot or crowded install without ever letting the terminations run hotter than they are listed for. This calculator takes the simpler and more conservative route of reading the lower column directly, which gives the same answer in every ordinary case and a slightly larger conductor in the derated ones.

Limit two: voltage drop

Copper is a good conductor, not a perfect one. Push current down it and some of your voltage is spent getting there. For DC and single-phase AC the arithmetic is:

V_drop = 2 × I × R × L ÷ 1000, with I in amps, R the conductor's resistance in ohms per 1000 feet, and L the one-way distance in feet.

The 2 is the part people drop, and it is the whole game: current goes out on one conductor and comes back on the other, so a 50-foot run is 100 feet of copper. For three-phase, the factor is √3 (about 1.732) rather than 2, because the return current is shared across the phases rather than travelling a dedicated conductor.

Worked through: 20 A down 100 feet of 12 AWG copper, which is 1.98 Ω per 1000 ft, gives 2 × 20 × 1.98 × 0.1 = 7.92 volts. On a 120 V circuit that is 6.6% — more than double the usual target, and the appliance at the end sees 112 V.

The NEC's guidance here is informational rather than mandatory: Informational Note 4 to 210.19(A) suggests keeping branch-circuit drop to 3%, with 5% total across feeder and branch. It is a recommendation, not a violation, which is exactly why so much residential wiring is at 4 or 5% and works fine. Treat the 3% default here as a design target you can relax knowingly, not a line you must not cross.

What voltage drop actually costs you is real though. Motors are the worst affected: torque falls with the square of voltage, so a 10% drop costs about 19% of starting torque, and a motor that struggles to start draws locked-rotor current for longer, heating both itself and the undersized wire that caused the problem. Resistive loads simply run cooler and slower — a heater at 10% low delivers 19% less heat. LED drivers mostly do not care until they fall off a cliff.

Which limit wins, and what that tells you

Short and heavy: ampacity wins. A 30 A circuit 10 feet away needs 10 AWG because 10 AWG is the first size rated for 30 A — the voltage drop at that length is a third of a percent and irrelevant.

Long and light: voltage drop wins, often by a lot. A 15 A circuit 100 feet out is thermally happy on 14 AWG, which is rated for exactly that. But 14 AWG over 100 feet drops 9.42 V, or 7.85% of 120 V, and even 10 AWG is still at 3.1% — so a 3% target pushes you to 8 AWG, three sizes up, on a run where the wire never gets warm. That gap is the single most useful thing this calculator shows, because it is invisible in an ampacity chart and it is where the money goes.

The default worked example on this page is the crossover: 20 A at 50 feet on 120 V. Ampacity alone says 14 AWG. Voltage drop says 10 AWG, which drops 2.48 V (2.07%) and delivers 117.5 V. Change the distance to 25 feet and both limits land on 14 AWG at once — which is the calculator's way of saying you have nothing left to gain by shortening the run.

Copper or aluminum

Aluminum has roughly 61% of copper's conductivity — at 12 AWG the code's aluminum resistance is 1.64× the copper figure — so it needs about 64% more cross-sectional area to match. Two AWG sizes up buys 59% more area, which is why two sizes larger is the rule of thumb and why it lands very slightly on the tight side. A 50 A subpanel 100 feet away wants 6 AWG in copper and 4 AWG in aluminum.

It is still often the cheaper answer on larger feeders and service entrances, where the metal price difference outweighs the extra size and the conduit that has to hold it. Two cautions. Aluminum needs terminations listed for it (marked AL or CU-AL) and an antioxidant compound at the joints, because aluminum oxide is an insulator where copper oxide is not. And solid aluminum branch-circuit wiring of the 1960s–70s variety has a genuinely bad safety record; modern AA-8000 series alloy is a different material with a different history.

Reading the AWG scale

American Wire Gauge runs backwards — bigger number, smaller wire — and it is geometric rather than arbitrary. The scale is defined so that 36 AWG is exactly 5 mils across and 4/0 is exactly 460 mils, with 39 steps between them, which gives d = 0.005 × 92^{(36 − n)/39} inches.

Two consequences worth carrying around. Three sizes down doubles the area — 10 AWG has about twice the copper of 13 AWG, and 6 AWG about twice that of 9 — so three sizes also roughly halves the resistance and halves the voltage drop. Six sizes down doubles the diameter and quadruples the area. And below 1 AWG the numbering runs out, so the scale continues into the "aughts": 1/0, 2/0, 3/0, 4/0, each larger than the last. Above 4/0 the naming changes system entirely to thousands of circular mils — 250 kcmil, 500 kcmil — because there is no gauge number left to use.

What this calculator does not do

It sizes one conductor. Above 500 kcmil, real installations run parallel sets rather than a single enormous conductor, and paralleling has its own rules (NEC 310.10(G)) about matching length, material and termination — so the table stops there rather than offering a size that would need them.

It uses DC resistance from NEC Chapter 9 Table 8 and assumes unity power factor. For ordinary branch circuits that is what the code's own examples do. For long AC feeders at low power factor, and for large conductors in steel conduit where inductive reactance starts to matter, the fuller Table 9 impedance method gives a larger and more accurate drop.

It also does not size overcurrent protection, does not apply the small-conductor rule of 240.4(D), does not check conduit fill, and does not know about the many specific exceptions the code carries for motors, air conditioning, welders, ranges and services. It is a design aid for working out roughly what you need and understanding why. Local amendments vary, the NEC edition your jurisdiction has adopted varies, and the person who signs off on the work is an electrician and an inspector — not a web page. Ampacities here are the 2020 NEC Table 310.16 values; later editions revised some cells.