Wire size calculator

National Fire Protection Association, NFPA 70, National Electrical Code — Article 310, Conductors for General Wiring (2023) · checked 2026-09-21

1. 01Circuits at 100 amps and belowUse the 60 °C column, unless the equipment at both ends is marked for 75 °C — and most modern panels and breakers are. The label inside the panel door tells you.
2. 02Circuits above 100 ampsUse the 75 °C column. Equipment at this size is listed for it as a matter of course.
3. 03The 90 °C columnOnly ever as the starting point for derating. Apply your temperature and bundling factors to it, then cap the result at the number in your terminal column. It is headroom, not capacity.

National Fire Protection Association, NFPA 70, National Electrical Code — 110.14(C), temperature limitations at terminations (2023) · checked 2026-09-21

National Fire Protection Association, NFPA 70, National Electrical Code — Chapter 9, Table 8, conductor properties (2023) · checked 2026-09-21

1. 01Sizing for the current and ignoring the distance

   Ampacity does not change with length and voltage drop does nothing but. A 12 AWG that is correct at forty feet is the wrong answer at a hundred and forty, and every table that stops at ampacity will tell you it is fine.

   Instead — Take the larger of the two answers, and find out where the crossover is — the calculator above prints it, so you know whether a change of route matters.

2. 02Reading the 90 °C column because the wire says 90 °C

   110.14(C) caps the conductor at the terminal rating, which is 60 or 75 °C. The 90 °C column is a starting point for derating and never a final answer. This is how terminations overheat.

   Instead — Use the 60 °C column at 100 amps and below unless both ends are marked 75 °C, and check the label inside your panel door before assuming they are.

3. 03Forgetting the 125 per cent on a continuous load

   A car charger, a water heater or a run of lighting works for hours without a break, and a breaker in a warm panel needs headroom above that. Forty amps of charging needs a fifty amp circuit.

   Instead — Ask whether it runs three hours or more. If it does, multiply by 1.25 before you size anything — and set the appliance to 80 per cent of the breaker afterwards.

4. 04Setting an EV charger to the breaker rating

   The breaker is 50 because the load is 40. Setting the charger to 50 puts a continuous 50 amps on a circuit designed for 40 and defeats the whole rule.

   Instead — Set the charger to 40, or to 80 per cent of whatever breaker is fitted. It is a commissioning setting and it is routinely left at the default.

5. 05Never derating for the attic

   An unvented attic under a dark roof reaches 110 or 120 °F on a summer afternoon, which takes ampacity to 87 per cent or less. The correction applies to the whole run, not to the hot part of it.

   Instead — Apply the ambient band honestly, or route the cable below the insulation where the temperature is the room's rather than the roof's.

6. 06Bundling circuits in a conduit without adjusting

   Four to six current-carrying conductors in one raceway take every one of them to 80 per cent. Two 20 amp circuits sharing a pipe is already four.

   Instead — Count the current-carrying conductors — the neutral of a shared circuit usually counts, the ground never does — and apply the factor, or run a second pipe.

7. 07Sizing an air conditioner circuit from the tonnage

   Tonnage is cooling output and tells you nothing about current. The nameplate gives a minimum circuit ampacity and a maximum overcurrent device, and the second one is a ceiling you may not exceed even if a bigger breaker would be convenient.

   Instead — Size the conductor to the minimum circuit ampacity and the breaker to no more than the maximum on the plate.

8. 08Upsizing the ground along with the conductors

   Not wrong, just unnecessary and expensive. The equipment grounding conductor is sized from the breaker in Table 250.122, and going up two sizes for voltage drop does not change what the breaker is.

   Instead — Size the ground off the overcurrent device. The one real exception is when the phase conductors are upsized for voltage drop on a feeder, where 250.122(B) asks for a proportional increase — worth checking on a long feeder.

1. 01Find the current, not the wattsOff the nameplate. For air conditioning and heat pumps use the minimum circuit ampacity printed on the plate, which is already worked out for you, and never the tonnage.
2. 02Decide whether it is continuousThree hours or more of running means the conductor and the breaker are sized at 125 per cent. Car chargers, water heaters and lighting are continuous; ranges and dryers are not.
3. 03Measure the route, not the distanceThe cable goes along joists and around corners, not through the middle of the house. Pace the route it will actually take, one way — the formula counts the return trip itself.
4. 04Size it for the currentTable 310.16 in the column your terminals are rated for, then apply the temperature and bundling factors if either applies.
5. 05Size it again for the distanceAnd take the bigger of the two answers. This is the step most calculators leave out, and it is the one that matters on any run over about fifty feet.
6. 06Check the small conductor rule14, 12 and 10 AWG copper are capped at 15, 20 and 30 amps of protection no matter what the ampacity column says.
7. 07Size the ground from the breakerTable 250.122, off the overcurrent device rating. It does not change when you upsize the conductors for distance.

AWG

American Wire Gauge. The numbers run backwards — smaller number, thicker wire — and each three steps down roughly doubles the cross-section. Above 1 AWG the sizes continue as 1/0, 2/0, 3/0 and 4/0, spoken as one-aught and so on.

Ampacity

The current a conductor can carry continuously without its insulation exceeding its temperature rating. It comes from Table 310.16 and is then corrected for how hot the surroundings are and how many conductors share the raceway.

Voltage drop

The voltage lost to the resistance of the wire on the way to the load. It is not a code requirement in the NEC — it lives in an informational note that recommends 3 per cent for a branch circuit — which is why a run can exceed it and still pass inspection.

Terminal temperature rating

The temperature the lugs and breaker terminals are listed for, 60 °C or 75 °C. Under 110.14(C) it caps the ampacity you may claim, which is why 90 °C insulation almost never buys you a smaller wire.

Continuous load

One expected to run for three hours or more. The conductor and the breaker are sized at 125 per cent of it, which is why a 40 amp car charger needs a 50 amp circuit.

Derating

Reducing the table ampacity for ambient temperature or for bundling more than three current-carrying conductors together. It is applied to the 90 °C column, then the result is capped at the terminal rating.

Small conductor rule

240.4(D). Whatever the ampacity table says, overcurrent protection on 14, 12 and 10 AWG copper may not exceed 15, 20 and 30 amps. It is why a 12 AWG house circuit is always twenty amps.

Equipment grounding conductor

The protective conductor that carries fault current back so the breaker trips. Sized from the breaker rating in Table 250.122, not from the conductors it runs beside — so it does not grow when you upsize for voltage drop.

Minimum circuit ampacity

The figure printed on an air conditioner or heat pump nameplate. It is what the circuit is sized to, already worked out by the manufacturer, and it is not the same as the running current or the tonnage.

THHN / THWN-2

The common building wire insulation, rated 90 °C dry and wet. The 90 °C matters for derating headroom and almost never for the final answer, because the terminals it lands on are not rated that high.

NM-B

Non-metallic sheathed cable, what most people call Romex. Its conductors are 90 °C but 334.80 requires it to be used at the 60 °C column, so an NM-B run gets the conservative answer whether or not the panel is marked 75.

Aught

How the sizes above 1 AWG are spoken: 1/0 is one-aught, 4/0 is four-aught. Four-aught aluminium is the usual 200 amp residential service conductor.