Wire Size Calculator — NEC Ampacity and Voltage Drop

Wire sizing has two answers, and you take the larger

When to use this: running a new circuit, feeding a detached building, adding an EV charger, or checking whether existing wire is adequate for a bigger load.

Two independent constraints decide the size, and they behave completely differently:

• Ampacity — can the conductor carry the current without overheating? Set by NEC Table 310.16, then reduced for heat, then capped by the terminals. Does not depend on distance at all.
• Voltage drop — will enough voltage still be there at the far end? Grows linearly with distance and never stops growing.

On a short run ampacity decides and voltage drop is irrelevant. Past roughly 100 ft they swap over, and from then on distance is the only thing that matters. A calculator that checks only ampacity will happily tell you 12 AWG is fine for a 20 A load 300 ft away. It is legal. It is also useless — the load will see about 19% less voltage than it should.

The rule that catches everyone: 110.14(C)

You buy THHN. Its jacket says 90 °C. Table 310.16 says 12 AWG copper carries 30 A in the 90 °C column. So a 30 A breaker, surely?

No. Two separate rules stop you, and both are easy to miss.

1. NEC 110.14(C) — the ampacity of a circuit is limited by the lowest temperature rating of any connected part, and that is the terminals on the breaker and the device. Terminals are listed for 60 °C or 75 °C. Essentially nothing in a panel is listed for 90 °C. Landing 90 °C wire on a 75 °C terminal makes it a 75 °C installation.
2. NEC 240.4(D) — an absolute cap on the overcurrent device for small conductors, regardless of any table: 15 A for 14 AWG copper, 20 A for 12 AWG, 30 A for 10 AWG. No conditions, no exceptions worth relying on.

So 12 AWG THHN is a 30 A wire on paper, a 25 A wire after the terminal rule, and a 20 A wire after 240.4(D). The last number is the one you use.

Then why buy 90 °C wire?

Because derating starts from the 90 °C column. That is the one place the higher rating earns its money. Six conductors in a 50 °C attic get multiplied by 0.82 for ambient and 0.80 for conductor count — starting that multiplication from a bigger number leaves more at the end.

Worked through for 6 AWG copper: from the 90 °C column, 75 × 0.82 × 0.80 = 49.2 A. From the 75 °C column it would be 65 × 0.82 × 0.80 = 42.6 A. Same wire, same conditions, one full conductor size of difference — purely from which column you were entitled to start in.

The order matters and it is worth stating plainly: derate from the insulation column, then cap at the terminal column. Doing it the other way round is a common shortcut and it oversizes every derated run.

How much distance changes the answer

A 20 A load on a 240 V single-phase circuit, copper, 3% drop limit. The load never changes. Only the run does.

The ampacity column never moves. The drop column climbs relentlessly. At 400 ft the same 20 A load wants 4 AWG — a conductor sized for a load four times larger, bought entirely to fight resistance.

Voltage is the cheapest lever here, because drop as a percentage scales with 1/V. That same 20 A at 400 ft needs 2 AWG at 120 V and 4 AWG at 240 V. Running a 240 V circuit and stepping down at the far end is often cheaper than the copper you would otherwise buy.

Common circuits, sized

Copper, 75 °C terminals, 90 °C insulation, 30 °C ambient, three current-carrying conductors, non-continuous. The distance columns show where drop takes over.

These are 240 V figures. At 120 V, halve the distance for the same drop — a 20 A 120 V circuit is already at 2.5% on 8 AWG by 100 ft.

NEC Table 310.16 — allowable ampacities

Not more than three current-carrying conductors in a raceway or cable, 30 °C ambient. These are the values before any derating and before the terminal limit.

Aluminium has no 14 AWG entry — the table starts at 12 AWG, and 14 AWG aluminium is not a branch-circuit conductor you can buy. Across the range, aluminium runs about two sizes behind copper for the same ampacity.

Derating factors

Ambient temperature — Table 310.15(B)(1)

The table is built for 30 °C. Anything hotter costs you ampacity, and the colder-rated the insulation, the harder it is punished.

A dash in the published table means the conductor may not be used at that ambient at all — shown here as "not permitted" rather than quietly treated as no correction, which is the dangerous reading. Note 60 °C wire is finished at 55 °C ambient while 90 °C wire keeps going to 85 °C.

Conductor count — Table 310.15(C)(1)

More than three current-carrying conductors sharing a raceway and they cook each other.

What counts is the subtle part. Ungrounded conductors always count. Equipment grounding conductors never do. The neutral of an ordinary 120/240 V single-phase circuit does not count, because it only carries the unbalance between the two hots. But the neutral of a three-phase four-wire wye circuit feeding nonlinear loads — LED drivers, computer supplies, VFDs — does count under 310.15(E)(3), because triplen harmonics add in the neutral instead of cancelling. That neutral can carry more current than any phase.

Voltage drop: not a rule, still the thing that bites

The 3% branch-circuit and 5% combined figures live in Informational Notes to 210.19(A) and 215.2(A). Informational Notes are explicitly not enforceable requirements. No inspector will fail you for voltage drop on a dwelling branch circuit.

They are still worth designing to, because the failures are real and diffuse:

• Motors draw more current at lower voltage to deliver the same power, so they run hotter and their service life falls. A 10% low-voltage condition is roughly a halving of insulation life.
• Resistive heating falls off with the square of voltage. 10% low is 19% less heat, which reads to the user as an underperforming appliance.
• Starting current makes drop far worse momentarily — which is why lights dim when the compressor kicks in.
• It gets worse over time as connections oxidise and loosen, so a design sitting exactly at the limit will not stay there.

This calculator figures drop from conductor resistivity at 75 °C rather than the K-constant shortcut, so it agrees with our voltage drop calculator exactly. That works out to K ≈ 12.6 for copper against the textbook 12.9 — about 2%, and in the conservative direction against most published tables.

Things that change the answer and are easy to forget

• Continuous loads. Anything running three hours or more is sized at 125% per 210.19(A)(1). EV charging, electric heat and commercial lighting all qualify. This is why a 32 A charger needs a 40 A circuit.
• Motors use table values, not nameplates. NEC 430.6(A) says to size conductors from Table 430.250, whatever the plate says. Then 430.22 applies 125%.
• Aluminium terminations. The metal is fine; the terminations are what failed in 1960s branch wiring. Use listed AL or CO/ALR devices, brush the conductor and use antioxidant compound.
• Stranded and solid are the same ampacity. Table 310.16 makes no distinction. Stranded is slightly fatter overall, which matters for conduit fill, not for current.
• Buried and free-air runs use different tables. Table 310.16 is for raceways and cables. Direct burial and free air have their own tables with higher values.
• Parallel conductors are allowed at 1/0 and above under 310.10(G), and all the parallel conductors must be the same length, material and size.

Once you have a size, check it fits: our conduit fill calculator works to NEC Chapter 9, and the load calculator works out the service size those feeders have to come from.