Wire Gauge and Ampacity Guide: Charts and Rules for Sizing Wire by Amps

Picking wire from a single ampacity number is how a 50-amp circuit ends up on conductors that run hot. The right size depends on the conductor metal, the insulation and termination temperature ratings, the breaker, how many conductors share a raceway, the air temperature around them, and the length of the run.

Ampacity is the maximum current a conductor can carry continuously under its conditions of use without exceeding its temperature rating. The NEC's starting values come from Table 310.16, and the usable result is then limited by termination ratings, cable type rules, small conductor breaker limits, and correction or adjustment factors. Section numbers here follow the 2023 edition of the National Electrical Code; the edition and amendments your jurisdiction enforces govern, and line voltage work should be done or inspected by a licensed electrician.

How the AWG Wire Gauge System Works

American Wire Gauge (AWG) numbers run backward: the larger the number, the smaller the wire. Every three gauge numbers smaller roughly doubles a conductor's cross-sectional area, and every six roughly doubles its diameter. Sizes larger than 4/0 AWG are named in kcmil, meaning thousands of circular mils, a unit identical to the older MCM label.

The diameters in the chart below are for bare solid conductors, calculated from the AWG definition. Stranding and insulation make finished wire larger, which is why conduit fill calculations use each wire type's actual area. Sheet metal gauges follow a different system, so 14 gauge wire and 14 gauge steel are not the same thickness.

Wire Gauge and Ampacity Chart for Copper and Aluminum

The chart lists NEC Table 310.16 ampacities, as published in Cerrowire's ampacity chart, for copper and aluminum conductors with no more than three current-carrying conductors in a raceway or cable and an ambient temperature of 86°F (30°C). The 60°C column covers the sizes made as NM-B cable, which stops at 2 AWG.

Aluminum typically needs a conductor about two AWG numbers larger than copper for the same ampacity: 8 AWG copper and 6 AWG aluminum are both 50 amps at 75°C, and 3 AWG copper and 1 AWG aluminum are both 100 amps. The tradeoffs beyond sizing are covered in aluminum vs copper wiring.

Four Rules That Decide the Final Wire Size

The chart value is only the starting point. Four rules can lower a conductor's usable ampacity or the breaker allowed on it, and the smallest result governs.

1. Termination Temperature Ratings

A conductor is used at the temperature rating of the weakest part of the circuit. Under Section 110.14(C)(1), terminations for circuits rated 100 amps or less, or for 14 through 1 AWG conductors, are treated as 60°C unless the equipment is listed and marked for 75°C, while larger circuits and conductors default to 75°C. A 90°C THHN conductor landing on 75°C breaker terminals is therefore sized from the 75°C column, and its 90°C rating stays useful for derating math.

2. Cable Type Limits

Some wiring methods add their own cap. NM-B cable, sold under the Romex brand, uses 90°C conductors, but Section 334.80 limits its ampacity to the 60°C column, so 10 AWG NM-B is a 30-amp cable even though 10 AWG THHN is listed at 35 amps at 75°C. The NM-B cable guide explains where that cable can be used.

3. Small Conductor Breaker Limits

For 14, 12, and 10 AWG copper, Section 240.4(D) caps the breaker or fuse at 15, 20, and 30 amps after any correction and adjustment factors, whatever the table allows. A 12 AWG THHN conductor rated 25 amps at 75°C still gets a 20-amp breaker, the same line drawn in 12 vs 14 gauge wire. The rule also covers 12 and 10 AWG aluminum and copper-clad aluminum, with separate limits.

4. Correction and Adjustment Factors

Table ampacities assume no more than three current-carrying conductors and 86°F surroundings. Section 310.15 reduces ampacity when more current-carrying conductors share a raceway or cable, from 80 percent for four to six conductors down to 35 percent for 41 or more, and it corrects again for hotter surroundings. Equipment grounding conductors do not count toward the total, and spare conductors meant to be energized later do.

Example: four current-carrying 14 AWG THHN conductors share a raceway in a 95°F space. Start from the 90°C value of 25 amps, multiply by the 0.96 temperature correction and the 0.80 adjustment, and the conductors are good for about 19 amps. Section 240.4(D) still limits the breaker to 15 amps.

What Size Wire Do I Need? Breaker to Wire Size Table

Under standard conditions, the minimum copper wire is 14 AWG for a 15-amp breaker, 12 AWG for 20 amps, 10 AWG for 30 amps, and 8 AWG for 40 amps. Above 40 amps the answer splits by wiring method and termination rating.

The table assumes copper, no more than three current-carrying conductors, 86°F surroundings, noncontinuous loads, and a conductor ampacity at least equal to the breaker, without relying on the NEC's limited allowance to round up to the next standard breaker size. Long runs and continuous loads can push the answer up a size. Once the conductors are chosen, match them to circuit breakers and equipment terminals rated for the same temperature.

Continuous loads need 125 percent. A continuous load is one expected to run at maximum current for three hours or more. Sections 210.19(A) and 210.20(A) require the conductors and breaker to be sized at 125 percent of it, so a 24-amp continuous load needs a 30-amp circuit and at least 10 AWG copper. The guide to Level 2 EV charger circuits applies the same math to charging equipment.

Services and subpanels. Subpanel feeders follow the same ampacity, termination, and derating rules. A dwelling's main service is handled differently, because the NEC includes a separate sizing allowance for dwelling service and main feeder conductors, which the comparison of 100 vs 200 amp service puts in context. Equipment grounding conductors are also sized from their own NEC table based on the breaker rating, not from this chart.

Voltage Drop: When Distance Requires a Larger Wire

A long run can need a larger conductor than ampacity alone suggests, because resistance grows with length and the voltage at the load falls. The NEC treats voltage drop as a recommendation in informational notes rather than an enforceable requirement, and 3 percent is a common design limit for branch circuits.

Cerrowire's 3 percent voltage drop tables show how quickly distance adds up. For a 120-volt, 20-amp copper circuit, 12 AWG covers runs to 50 feet, 8 AWG is listed at 100 feet, and 6 AWG at 150 feet. At 240 volts the same 20 amps stays on 12 AWG out to 100 feet. Those tables assume steel conduit, a 0.9 power factor, 86°F, and no more than three current-carrying conductors.

Why Undersized Wire Is a Fire Risk

Conductors pushed past their temperature rating break down their own insulation, and that insulation is a common first fuel in electrical fires. NFPA's February 2022 analysis of U.S. fire department data estimated an average of 32,620 home fires per year from 2015 through 2019 involving electrical distribution and lighting equipment, and electrical wire or cable insulation was the first item ignited in 32 percent of them. The guide to electrical fire causes and prevention lists the warning signs of overheated wiring.

How to Size Wire Step by Step

1. Find the load from the nameplate or a load calculation, and multiply continuous loads by 125 percent.
2. Choose the breaker that protects the circuit at that load.
3. Pick the wiring method. NM-B is limited to the 60°C column; THHN/THWN-2 can use higher columns when terminations allow.
4. Check every termination rating at the breaker, panel, and equipment, and size from the lowest.
5. Apply the small conductor limits for 14, 12, and 10 AWG.
6. Derate for more than three current-carrying conductors or hot surroundings.
7. Check voltage drop on long runs and upsize when needed.
8. Match the terminations. Lugs, connectors, and breaker terminals must be rated for the conductor's size and metal, such as the compression and mechanical lugs listed by conductor size.