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Wire Size & Ampacity Calculator

A tabulated ampacity is only the starting point. Ambient temperature, conductor count, termination ratings, small-conductor overcurrent limits and voltage drop can each govern the final size. This tool applies all of them and identifies which constraint governed the result and the table it came from.
Code basis
NEC 2023, Art. 310 / 240
Editions
2017, 2020, 2023
Conductor sizes
14 AWG – 1000 kcmil
Materials
Copper, aluminum
Constraints
Ampacity, OCPD, voltage drop

Circuit details

Every field affects the result. Defaults describe a common 240 V branch circuit.

Load

Panel to load, not the total conductor length.

Conductor

Read from the 90 °C column.

The lower-rated end of the two. Most breakers and lugs are 75 degrees C.

Match the edition your jurisdiction enforces.

Installation conditions

Air around the raceway, not the room thermostat.

Excludes a neutral carrying only unbalanced current and the ground.

A recommendation from an informational note, not a rule.

Minimum conductor

Pass

8 AWG

Governed by the temperature rating of the terminations.

Derated ampacity
50 ARequired: 40 A
Overcurrent device
40 A
Grounding conductor
10 AWG
Voltage drop
1.9%4.7 V lost, 235 V at the load

Required current

StepBasisEffectValue
Load current——40 A
Derivation of the current the conductor must be sized for

Conductor ampacity

StepBasisEffectValue
Base ampacity, 8 AWG copperTable 310.16—55 A
Termination rated 75 CGoverning110.14(C)cap 5050 A
Governed by Termination rated 75 C (110.14(C)), not by the load itself.
Derivation of the selected conductor's usable ampacity

Calculated against the NEC 2023 edition using 310.16, 310.15(B)(1), 310.15(C)(1), 110.14(C), 240.4, 250.122. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Conductors in free air, which are sized from a different ampacity table.
  • Raceways exposed to direct sunlight on or above a rooftop.
  • Motor and air-conditioning circuits, which have their own sizing rules.
  • Parallel conductor sets and taps.

Data last verified

Calculation Method

Six steps, in the order the code applies them.

  1. Step 1

    Required current

    A continuous load is taken at 125 percent; a non-continuous load at its actual current.

  2. Step 2

    Tabulated ampacity

    Read from the column matching the conductor insulation, based on 30 degrees C ambient and no more than three current-carrying conductors.

  3. Step 3

    Ambient correction

    Multiplied by the correction factor for the actual ambient temperature. Above the published range the conductor is not permitted, and no extrapolation is performed.

  4. Step 4

    Bundling adjustment

    Multiplied by the adjustment factor for the number of current-carrying conductors in the raceway: 80 percent for four to six, 50 percent for ten to twenty.

  5. Step 5

    Termination limit

    Capped at the ampacity in the column matching the lowest-rated termination. This is a cap, not a multiplier, and does not enter the preceding arithmetic.

  6. Step 6

    Overcurrent and voltage drop

    The device is selected from the standard ratings subject to the small-conductor limits and the next-size-up allowance. If drop over the run exceeds the target, the conductor is upsized and the result is reported as drop-governed.

The equipment grounding conductor is sized from the overcurrent device rating, not from the ungrounded conductors. Where the ungrounded conductors are upsized for voltage drop, the grounding conductor must be increased proportionally.

Governing Constraint by Scenario

The binding constraint varies by installation, which is why a single lookup table cannot answer this.

ScenarioUsually governed byBasis
Short 20 A receptacle circuit in conditioned spaceSmall conductor overcurrent limitThe table allows more, but 12 AWG cannot be protected above 20 A.
50 A range feeder, 30 ft, three conductorsTermination temperatureThe 90 C column reads higher, but the 75 C lugs set the ceiling.
Six circuits sharing one raceway in an atticAmbient correction and bundling adjustment togetherCompounding 0.91 and 0.80 removes over a quarter of the ampacity.
20 A circuit to a detached garage, 200 ftVoltage dropAmpacity allows 12 AWG; staying under 3 percent needs 8 AWG.
200 A aluminum service conductorsTabulated ampacityConditions are benign, so the table value decides it directly.
Typical governing constraint by scenario.

Ampacity Reference

Tabulated values before correction. The calculator applies ambient and bundling factors; this table does not.

SizeCu 60 °CCu 75 °CCu 90 °CAl 75 °CMax breaker
14 AWG152025—15 A
12 AWG2025302020 A
10 AWG3035403030 A
8 AWG40505540By ampacity
6 AWG55657550By ampacity
4 AWG70859565By ampacity
3 AWG8510011575By ampacity
2 AWG9511513090By ampacity
1 AWG110130145100By ampacity
1/0 AWG125150170120By ampacity
2/0 AWG145175195135By ampacity
3/0 AWG165200225155By ampacity
4/0 AWG195230260180By ampacity
Ampacity at 30 degrees C ambient, no more than three current-carrying conductors. Derived and reorganised from the NEC ampacity table; consult the published code for the authoritative values.
Full ampacity table

Input Guidance

The four fields most often entered incorrectly.

  • Ambient temperature

    Use the air temperature surrounding the raceway, not the room setpoint. Attics, boiler rooms and south-facing exterior walls sit well above the 86 degrees F the tables assume. An optimistic value here is the most consequential input error on this page.

  • Current-carrying conductors

    Count only conductors carrying current. On a single-phase multiwire branch circuit the neutral carries unbalanced current only and is excluded; the equipment grounding conductor is always excluded. Three two-wire circuits in one raceway is six current-carrying conductors, not nine.

  • Termination rating

    Use the lower rating of the two ends. Equipment listed for 75 degrees C is typical, but unless marked otherwise, terminations on equipment rated 100 amperes or less are treated as 60 degrees C. Verify against the equipment label.

  • Conductor material

    Aluminum requires roughly one to two sizes more than copper for the same load at significantly lower cost. It is standard for services and large feeders and uncommon for branch circuits. Confirm terminations are listed for aluminum before specifying it.

FAQs

Why does this calculator give a larger wire than the ampacity chart?
A chart shows the tabulated ampacity at 30 degrees C with no more than three conductors in the raceway. Real installations rarely match that. Once the ambient correction and the adjustment for additional conductors are applied, and once the termination temperature and the small conductor overcurrent limits are enforced, the usable ampacity is often well below the chart value. The breakdown on this page names the specific step that reduced it.
Can I use the 90 degree C column?
You can use it for the derating arithmetic, but the final ampacity may not exceed the value in the column matching the lowest-rated termination at either end of the circuit. Most breakers and equipment lugs are listed for 75 degrees C, so a THHN conductor is derated from the 90 degree C value and then capped at the 75 degree C value. Skipping that cap is one of the most common sizing errors.
Why is 12 AWG copper limited to 20 amperes when the table says 30?
The small conductor rule limits overcurrent protection for 14, 12 and 10 AWG regardless of what the ampacity table says. A 12 AWG copper conductor has a tabulated 90 degree C ampacity of 30 amperes but cannot be protected above 20 amperes. Because the breaker sets the practical ceiling on the circuit, the calculator treats that limit as binding.
Does the neutral count as a current-carrying conductor?
Not when it carries only the unbalanced current of the other conductors, which is the usual case in a multiwire branch circuit on a single-phase system. The equipment grounding conductor never counts. A neutral does count on a three-phase, four-wire system supplying substantial nonlinear load. The calculator takes the current-carrying count directly rather than guessing from the conductor count.
Is exceeding 3 percent voltage drop a code violation?
No. The 3 percent branch-circuit and 5 percent combined figures come from informational notes, which are recommendations rather than enforceable requirements. The calculator reports a caution rather than a failure when they are exceeded. Some jurisdictions and some other standards do enforce limits, so confirm with your authority having jurisdiction.
Which NEC edition should I select?
The one your jurisdiction currently enforces, which is often not the newest. Adoption is by state and sometimes by municipality, and local amendments can modify what the published edition says. For the tables this calculator uses, the values are the same across the 2017, 2020 and 2023 editions; what changed is the section numbering, so selecting your edition makes the citations shown match the book on your desk.
Should I use copper or aluminum?
Aluminum carries roughly two thirds the current of copper at the same size, so it needs one to two sizes larger for the same load, but it costs substantially less per foot and weighs less on a long feeder. Aluminum is common for services and large feeders and uncommon for branch circuits. Terminations must be listed for aluminum, and an antioxidant compound is generally specified.