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Wire Ampacity Chart

Common ampacity reference with a simplified derating calculator for conductor count and ambient temperature.

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Derating Factor Calculator

Base Ampacity (1-3 Conductors) 100% At 75°C Derating Examples 80% 4-6 conductors 70% 7-9 conductors Temperature Correction 95°F: 94% | 104°F: 88% | 122°F: 75% Both factors apply: multiply ampacity × adjustment × temperature Allowable Ampacity = Base Ampacity × Adjustment Factor × Temperature Factor

Multiple conductors in the same raceway require derating to account for heat buildup. Temperature also reduces safe ampacity.

Wire Ampacity Reference for Sizing Conductors Correctly

A wire ampacity chart is used to answer a basic but important question: how much current can a conductor carry under the conditions of the installation. That answer depends on conductor material, insulation rating, terminal temperature limits, conductor count, and ambient temperature. NEC Table 310.16 is the reference many electricians start with before applying any corrections.

NEC Table 310.16 as a Starting Point

The table gives base ampacity values for copper and aluminum conductors under standard conditions. Actual allowed ampacity may still be reduced by terminal limits, small-conductor rules, conductor count, or ambient temperature.

When Derating Applies

Derating applies when multiple current-carrying conductors share the same raceway or cable and when ambient temperature exceeds the base condition used by the code table. Both adjustments may apply to the same circuit.

Conductor Derating

When the number of current-carrying conductors increases, the allowable ampacity is reduced by the applicable adjustment factor. This check is separate from conduit fill.

Temperature Correction

Ambient heat reduces usable ampacity. Attics, rooftops, and sun-exposed raceways may require a temperature correction factor before final conductor sizing is confirmed.

60C, 75C, and 90C Ratings

Conductors may be listed with 60C, 75C, or 90C insulation ratings, but usable ampacity is limited by the lowest-temperature termination in the circuit.

Example Condition Check

A conductor installed in a hot attic with several other current-carrying conductors may require both conductor-count adjustment and temperature correction before breaker size is finalized.

Reference: Ampacity Table

AWG / kcmil Copper Reference Values Aluminum Reference Values Common Application
60°C 75°C 90°C 60°C 75°C 90°C
14 20 20 25 15 15 20 Lighting circuits
12 20 25 30 15 20 25 General purpose (20A)
10 30 35 40 25 30 35 Heavy loads (30A)
8 40 50 55 35 40 45 Large circuits (50A)
6 55 65 75 40 50 55 Subpanel feeders
4 70 85 95 55 65 75 Large feeders (100A)
2 95 115 130 75 90 100 Service entrance (125A)
1 110 130 150 85 100 115 Service entrance (150A)
1/0 125 150 170 100 120 135 Service entrance (150-175A)
2/0 145 175 195 115 135 150 Service entrance (200A)
3/0 165 200 225 130 155 175 Service entrance (225A)
4/0 195 230 260 150 180 205 Service entrance (250A)
250 kcmil 215 255 290 170 205 230 Large feeders (300A)
300 kcmil 240 285 320 190 230 255 Utility feeders (300A+)
350 kcmil 260 310 350 210 250 280 Utility feeders (350A+)
400 kcmil 280 335 380 225 270 305 Utility feeders (400A+)
500 kcmil 320 380 430 260 310 350 Utility feeders (500A)

Conductor Derating Factors — NEC Table 310.15(B)(2)(a)

Number of Current-Carrying Conductors Adjustment Factor Practical Example
1-3 100% (no derating) Standard residential circuits; 3 wires in conduit
4-6 80% 4-6 circuits sharing a large conduit
7-9 70% Large industrial panel feeder with multiple circuits
10-20 50% Large cable tray with many feeder circuits
21-30 45% Massive feeder system with 25+ circuits
31-40 40% Utility distribution or data center feeder
41+ 35% Extreme bundling; rarely seen in practice

Frequently Asked Questions

It's how many amps your wire can safely carry without the insulation melting. Blow past it and you start a fire. That's why every breaker you install has to match or be smaller than your wire's ampacity. NEC 310.16 is your reference—check it before you size anything.
12 AWG copper is commonly listed at 25A in the 75°C column, but small-conductor branch circuits are typically protected at 20A maximum. If the conductors are bundled or installed in hot ambient conditions, apply the required adjustment and correction factors before choosing overcurrent protection.
Multiple wires in one conduit heat each other up, so you reduce (derate) their safe current. Same with hot environments. Derate when: (1) more than 3 wires in one conduit, or (2) ambient temperature above 86°F. Multiply both factors if both apply.
Hotter air means the wire can't cool as fast, so it can't carry as much current safely. Using a 75°C base ampacity, 95°F applies a 94% factor, 104°F an 88% factor, 113°F an 82% factor, and 122°F a 75% factor.
They are insulation and termination temperature ratings. Many modern conductors are dual-rated, but you still size from the lowest-temperature termination in the circuit. That is why the 75°C column is commonly used in practical work.

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