Estimate an AWG or kcmil conductor size using explicit ampacity checks, with separate insulation and terminal limits.

Wire size by load and ampacity

Scope: insulated single conductors in raceway, general branch circuits up to 600 V. This model does not cover NM-B cable, free-air ratings, motors or special load rules.

Total load current (A)Total actual current, including the continuous portion.

Load typeGeneral branch-circuit model; motor and special-load rules are outside scope.

Continuous portion (A)Included in total current, not added to it.

Conductor material

Insulation temperature rating (°C)Use the rating applicable to the installation conditions, including wet or dry location.

Lowest terminal temperature rating (°C)Higher insulation temperature does not establish a higher terminal rating.

Ambient temperature (°C)Supported input range: −40 to 85°C; unsupported table cells return no size.

Current-carrying conductor countEnter a positive whole number after applying the relevant neutral-counting rules. Do not count equipment grounding conductors.

Minimum size to considerOptional constraint; the result still must meet both checks.

Use a decimal point or comma. A single separator is decimal: write 1000 for one thousand.

Upload an equipment nameplate or wiring diagram photo to suggest realistic input values (load current, voltage, etc.).

Verify any suggested values against the source document.

Compare cable scenarios

Calculate an option, save it, then change the inputs and save another. Compare up to three snapshots with their assumptions.

Scenario name (optional)

No scenarios saved yet. Comparisons stay in this tab until you reload.

Plan one circuit across ampacity and voltage drop — enter shared inputs once and open both checks with those values.

Cable planning: choose your next check

Use ampacity and voltage drop as separate checks. Choose the tool for the information you have; review each model before using its result.

Voltage dropCheck DC, single-phase or balanced three-phase voltage drop from cable area and current.

Wire ampacity — current toolCheck temperature, conductor grouping and terminal limits. Voltage drop is a separate check.

Cable length and areaCompare cable areas and estimate the maximum distance for your voltage-drop target.Three-phase cable impedanceUse electrical input kW or line current, with reference or custom cable resistance and reactance.

How this wire size calculator works

Enter actual load current, the portion that is continuous, conductor material, insulation rating, terminal rating, ambient temperature and current-carrying conductor count. The tool returns the smallest listed AWG or kcmil size satisfying two ampacity checks. It shows the previous size so you can see why it failed.

The base ampacity values and correction factors were checked against the HELUKABEL reference reproducing NEC 2023 tables, using Table 310.16 rather than the free-air Table 310.17. This is a defined reference model, not a claim that every requirement of the edition adopted in your jurisdiction has been checked.

Two load checks, before and after adjustment

Let C be continuous current and N be noncontinuous current. The first check requires a pre-adjustment ampacity of at least N + 1.25 × C. The second requires corrected and adjusted usable ampacity of at least N + C. Both must pass. This distinction is explained in Schneider Electric’s branch-circuit conductor guidance.

The mixed-load entry includes continuous current within the total. For example, total 50 A with continuous portion 20 A means 30 A noncontinuous: the first target is 30 + 1.25 × 20 = 55 A, and the second is 50 A. The model does not implement the exception for a complete assembly listed for 100% operation.

Insulation and terminals are separate limits

Temperature and grouping factors apply to the ampacity column matching the usable insulation rating. The terminal column is a separate cap. For example, 90°C insulation can provide a higher starting value for adjustment, but it does not permit a 60°C termination to be treated as 90°C. The Cerrowire methodology also distinguishes insulation, terminations and cable-specific limits.

This tool assumes individual insulated conductors in raceway. NM-B, UF-B, cable trays, free-air arrangements, parallel runs, dwelling service allowances, motors, welders and other special circuits need their own rules and are outside this model. Use the lowest verified temperature rating of all terminations; the initial 60°C setting is conservative.

Correction, grouping and small conductors

The corrected insulation ampacity is the base value multiplied by an ambient-temperature factor and a conductor-count factor. The result is limited by the terminal value and the general small-conductor protection ceiling. A blank cell in the temperature table produces no recommendation; the tool does not reuse the last factor for arbitrarily high temperatures.

For general small-conductor protection, the ceilings applied here are 15/20/30 A for copper 14/12/10 AWG, and 15/25 A for aluminum 12/10 AWG. These limits and the existence of special exceptions are summarized by Encore Wire. The calculator does not select a breaker or apply next-size-up and special-equipment exceptions.

Six worked examples

Unless changed in the row, these examples use copper, 30°C ambient, three current-carrying conductors and no minimum-size constraint. They demonstrate the stated model, not complete circuit designs.

Inputs	Checks	Smallest listed size
20 A noncontinuous; 90°C insulation; 60°C terminals	20 A before and after; 14 AWG is limited to 15 A	12 AWG Cu
20 A continuous; 90°C insulation; 60°C terminals	25 A before adjustment; 20 A after	10 AWG Cu
40 A continuous; 90°C insulation; 75°C terminals	50 A before; 40 A after; 8 AWG has a 50 A terminal limit	8 AWG Cu
Same 40 A case with 60°C terminals	8 AWG is limited to 40 A and fails the 50 A pre-adjustment check	6 AWG Cu
60 A continuous; 75°C insulation and terminals; 10 current-carrying conductors	75 A before; 60 A after; 1 AWG gives 130 × 0.50 = 65 A	1 AWG Cu
50 A total, including 20 A continuous; 90°C insulation; 75°C terminals; 50°C ambient; 6 conductors	55 A before; 50 A after; 4 AWG gives 95 × 0.82 × 0.80 = 62.32 A	4 AWG Cu

Frequently asked questions

Does a higher insulation rating always allow a smaller wire?

No. Temperature adjustment may improve, but terminals or small-conductor limits can still determine the result. The output states which usable-ampacity limit controls.

Which conductors count for grouping?

Enter the applicable current-carrying count for the actual circuit arrangement. Equipment grounding conductors do not count. Neutral treatment depends on the load and circuit, so the tool does not infer it from “single-phase” or “three-phase”.

Why does the calculator sometimes return no size?

The selected temperature/insulation combination may be unsupported, the inputs may be incomplete, or no listed size through 500 kcmil may meet both checks. Those cases must not produce a fabricated recommendation.

Is this also a voltage-drop calculator?

No. Ampacity concerns thermal limits; voltage drop also depends on route length and circuit impedance. Use the cable length and voltage-drop calculator for that separate check.