Wire Gauge Calculator
Find the smallest wire size that can carry your load. Enter the amps, or the watts and voltage, and get the AWG size from NEC Table 310.16 for copper or aluminum. A metric mode sizes in mm² by two standards side by side. It's a quick table lookup for reference. It doesn't replace a licensed electrician or your local code, and it doesn't check voltage drop on long runs.
Sizing standard
What do you know?
Supply
Conductor material
Before you buy wire
- Use the 60°C column for NM-B (Romex) cable and for most breakers and devices rated 100 A or less, unless they're marked for 75°C wire. Only pick 75°C when both the wire and every termination are rated for it.
- For a continuous load that runs 3 hours or more, like a water heater or an EV charger, the NEC sizes the circuit at 125% of the load. Multiply the amps by 1.25 before entering them.
- The table assumes 30°C (86°F) ambient and no more than three current-carrying conductors together. Hot attics, conduits packed with several circuits and long runs all need a larger wire than this shows.
- 14, 12 and 10 AWG are capped at 15, 20 and 30 A (copper) by NEC 240.4(D), even where the table lists more. The calculator applies this cap and tells you when it does.
Ampacity 30 A
How this is calculated
If you enter power, the calculator first converts it to current: amps = watts ÷ (volts × power factor) for single-phase, and amps = watts ÷ (√3 × line volts × power factor) for three-phase. That's the standard load-current formula from electrical design handbooks. It then goes through NEC Table 310.16 from the smallest size up and picks the first wire whose allowable ampacity in the column you chose (60°C or 75°C, copper or aluminum) is at least that current. For 14–10 AWG it also applies the small-conductor limits of NEC 240.4(D). The table values are for 30°C ambient and up to three current-carrying conductors, with no correction factors. In metric mode the same current is looked up in two mm² tables: the older PUE tables and IEC 60364-5-52 (methods B1, B2 or C). The calculator recommends the larger of the two sizes. Voltage drop, conduit fill, derating and breaker selection aren't covered.
Common questions
What size wire do I need for a 30 amp circuit?
With copper and the 60°C column (NM-B cable, standard breakers), 10 AWG: it's rated 30 A, and 12 AWG is only 20 A. In the 75°C column 10 AWG is listed at 35 A, but NEC 240.4(D) still caps it at a 30 A breaker. Aluminum needs 8 AWG for 30 A at 60°C. Long runs or hot locations may need a bigger size.
Why does the calculator say 12 AWG for 20 A when the table lists 14 AWG at 20 A?
That 20 A figure is in the 75°C column, but NEC 240.4(D) limits 14 AWG copper to a 15 A overcurrent device no matter what the table says. A 20 A circuit needs 12 AWG copper. The result shows both numbers when this limit applies.
Is this enough to wire my house?
No. It's a reference lookup for one circuit under standard conditions. Real installations also depend on voltage drop, ambient temperature, how many circuits share a conduit, the breaker and equipment ratings, and the code edition your area has adopted. Check with a licensed electrician or your local inspector before installing anything.
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