Wire Ampacity Calculator

What ampacity means and what the base table assumes

Ampacity is the current a conductor can carry for a long period without pushing its insulation past its temperature rating. Push more current through the same metal and it runs hotter, the insulation ages faster, and at some point the heat becomes a fire risk rather than a nuisance.

Table 310.16 in the National Electrical Code lists ampacities for insulated conductors under three fixed conditions: an ambient temperature of 30 degrees C (86 degrees F), no more than three current-carrying conductors in a raceway or cable, and a system voltage of 2,000 volts or less. Those conditions are the baseline, not a promise about your installation.

Two multipliers move the number away from the table: the ambient correction factor for hot locations and the adjustment factor for bundled conductors. Ampacity = table value x ambient factor x bundle factor. The result also has to respect the temperature rating of the terminations at each end, which is often lower than the wire rating.

The numbers on this page come from NFPA 70, National Electrical Code, 2023 edition, Tables 310.16, 310.15(B)(1)(1) and 310.15(C)(1). The NFPA 70 page at NFPA lists the editions in force, and your state or city decides which one applies to a job.

Table 310.16 ampacities for copper and aluminum

The table below covers 14 AWG through 4/0 in both metals and all three insulation classes. Read across a row to see what the insulation rating buys: 12 AWG copper is 20 amps at 60 degrees C, 25 amps at 75 degrees C, and 30 amps at 90 degrees C.

Two footnotes matter in practice. 14 AWG aluminum is not listed, so an aluminum circuit starts at 12 AWG. And the small conductor rule in 240.4(D) caps the overcurrent device at 15 amps for 14 AWG copper, 20 amps for 12 AWG copper and 30 amps for 10 AWG copper, with 15 amps for 12 AWG aluminum and 25 amps for 10 AWG aluminum, unless a specific article permits more.

Those caps answer a different question from ampacity. A 12 AWG copper conductor in the 90 degree column shows 30 amps, yet a standard 20 amp branch circuit is still the normal use, because the breaker protects the wire and the small conductor rule sets the ceiling.

Hot air lowers the number: ambient correction

The base table assumes 30 degrees C. An attic, a boiler room, a rooftop conduit in the sun or a shop in Phoenix runs hotter, and hot air pulls heat out of the conductor more slowly, so the same wire reaches its limit at a lower current. Table 310.15(B)(1)(1) supplies the multiplier.

Work in Celsius when you read the table, because the bands are defined in Celsius. 104 degrees F is 40 degrees C, which lands in the 36 to 40 band: 0.82 for 60 degree insulation, 0.88 for 75 degree insulation and 0.91 for 90 degree insulation.

The 90 degree column loses the least capacity in heat, which is why THHN and XHHW-2 wire is common in hot locations. That advantage holds only when the terminations are rated for it too.

Ambients below 70 degrees F are outside this calculator. The printed table allows factors above 1.00 there, so cold weather work should read the table directly instead of assuming a bonus.

Bundling: more than three current-carrying conductors

Conductors packed into one raceway heat each other, so the code steps the allowed current down as the count rises. Table 310.15(C)(1) gives the percentages.

Count only current-carrying conductors. Equipment grounding conductors do not count, and the neutral of a balanced three-phase wye circuit does not count under the conditions in 310.15(E). Two hots and a neutral on a single-phase circuit do count as three.

The steps are steep, and that surprises people. Nine 12 AWG conductors in one conduit keep 70 percent of 30 amps in the 90 degree column, which is 21 amps before any heat correction. Add the 0.91 factor for a 104 degree F attic and the same conductor is down to 19.1 amps.

Terminations decide which column you can use

Section 110.14(C) limits a conductor to the temperature rating of the terminals it lands on. Most breakers, receptacles and switches are rated 75 degrees C, and equipment rated 100 amps or less is often rated 60 degrees C unless marked otherwise. A 90 degree conductor on a 75 degree terminal is treated as a 75 degree conductor for ampacity.

That is why the 90 degree column is a starting point for derating rather than a final answer. You may start at 30 amps for 12 AWG THHN, apply the correction factors, and then cap the result at the 75 degree value of 25 amps.

The same logic runs through aluminum. A 4/0 aluminum conductor shows 205 amps in the 90 degree column and 180 amps in the 75 degree column, and a 75 degree lug makes 180 the ceiling before any derating.

Copper against aluminum

Aluminum has higher resistance than copper for the same cross section, so the table gives it less current: roughly 76 to 86 percent of the copper value at the same size. The gap is widest at 6 AWG and settles near 78 to 80 percent from 4 AWG up.

Aluminum costs less and weighs less, which is why service entrance and feeder conductors are often aluminum. It also needs terminals listed for aluminum, an antioxidant where the manufacturer requires one, and correct torque. A loose aluminum termination heats up, and that is a real failure mode rather than a myth.

Never swap a copper size for the same aluminum size and assume the circuit still works. Read the aluminum column, then apply the same correction and adjustment factors.

Worked example: 12 AWG THHN in a 104 degree F attic

A 20 amp branch circuit runs through an attic that reaches 104 degrees F, and eight current-carrying conductors share one conduit for part of the run. The wire is 12 AWG copper THHN, so the 90 degree column applies as a starting point.

• Base ampacity at 90 degrees C: 30 amps.
• Ambient factor for 104 degrees F: 0.91.
• Adjustment for 8 conductors: 0.70.
• Derated: 30 x 0.91 x 0.70 = 19.11 amps, which rounds to 19.1 amps.
• Termination ceiling at 75 degrees C: 25 amps, so it does not bind.
• Small conductor cap from 240.4(D): 20 amps, so it does not bind either.

The usable ampacity is 19.1 amps, which is below the 20 amp breaker. That circuit now needs either a smaller load, a lower ambient, fewer conductors in the raceway or a larger wire.

Each step up in size buys about 6 amps here, because the derating factors stay the same and only the base value moves. A second case shows how aluminum behaves. A 6 AWG aluminum feeder at 75 degrees C, in the same 104 degree F space, with five conductors in the raceway: 50 x 0.88 x 0.80 = 35.2 amps, and the 75 degree terminal ceiling of 50 amps does not bind.

What this page does not cover, and when to call an electrician

Ampacity is one input to a circuit design, not the whole design. This page does not size conductors for continuous loads, which take a 125 percent multiplier under 210.19 and 210.20, and it does not touch motor circuits, where Article 430 has its own rules for overload and branch circuit protection. It also skips voltage drop, conduit fill from Chapter 9, and the rooftop temperature adder for conduits exposed to sunlight.

Voltage drop is the other half of wire sizing. A conductor can pass the ampacity check and still deliver poor voltage at the far end of a long run, which is why a separate voltage drop calculator exists on this site. Ampacity asks whether the wire survives the current, and voltage drop asks whether the equipment at the end still works.

Code editions vary by location. The 2023 NEC is the edition used here, and by 2026 many states have adopted it, while others still enforce the 2020 or 2017 edition. Local amendments are common, and the authority having jurisdiction makes the final call. OSHA's electrical safety material covers the workplace side of the same rules.

This page is a reference and a teaching tool, not electrical construction advice. Actual wiring has to follow the code edition your local authority enforces, the listing instructions on the equipment, and the judgment of a licensed electrician who can look at the actual installation. If a number here decides whether a circuit is safe, confirm it against the printed code book and a qualified professional before you pull wire.