Cables & ProtectionSafety-sensitive
Cable Ampacity & Sizing Assistant
Suggested conductor size from load current, ambient temperature, and raceway grouping derating.
About this calculator
A conductor's tabulated ampacity — the current it can carry indefinitely without exceeding its insulation temperature rating — is only valid at the table's base ambient temperature (typically 30°C) with no more than a few current-carrying conductors sharing the raceway. Outside those conditions, code-style tables apply correction factors: one for ambient temperature above or below the base, and one for the number of current-carrying conductors grouped together, which reduces each conductor's ability to shed heat. This calculator divides your load current by both factors to find the minimum table ampacity a conductor needs, then picks the smallest standard size that clears that bar.
Use it as a first pass when sizing a feeder or branch circuit conductor, to see how much a hot ambient or a crowded raceway shrinks your effective ampacity, or to sanity-check a conductor size you've already selected. The result shows the exact correction factors applied and the resulting margin above your load current.
This calculator uses representative 75°C-column ampacity, ambient-temperature correction, and grouping-adjustment values consistent in shape with NEC Table 310.16 and the associated 310.15(B)(1)/(C)(1) correction tables — not a substitute for the governing code table edition and jurisdiction-specific amendments. It does not apply continuous-load 125% sizing, does not check voltage drop (use the voltage drop calculator separately), and does not account for conduit fill, cable tray fill rules, or terminations rated below 75°C, all of which can further limit the final selection.
Assumptions
- Representative 75°C-column ampacity, ambient-temperature correction, and grouping-adjustment values, consistent in shape with NEC Table 310.16 and 310.15(B)(1)/(C)(1) — verify against the governing code edition and jurisdiction.
- Does not apply 125% continuous-load sizing — apply that separately if the load is continuous.
- Does not check voltage drop, conduit fill, or termination temperature rating — verify each separately.
When to use this calculator
Appropriate for
- A first-pass estimate of the derated ampacity a conductor needs given ambient temperature and conductor grouping
- Seeing how a hot ambient or a crowded raceway shrinks effective ampacity
- Sanity-checking a conductor size already chosen against representative table values
Not suitable for
- Final conductor selection or code-compliance sizing — the table values here are representative, and the governing code edition and jurisdiction amendments always control
- Continuous-load 125% sizing, voltage-drop limits, conduit fill, cable-tray rules, or terminations rated below 75 °C, none of which this applies
- Installation methods or insulation types outside the representative 75 °C copper/aluminum basis used here
What this calculator does not cover
- Uses representative 75 °C copper/aluminum table values — always verify against the governing CEC/NEC edition, insulation type, and installation method for your project.
- Applies ambient and grouping derating only — rooftop temperature adders, the 125% continuous-load factor, and terminal temperature limits are not applied.
- Thermal ampacity only — voltage drop and short-circuit withstand of the conductor must be checked separately.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
Why does ambient temperature change the required conductor size?
Ampacity tables are only valid at their stated base ambient (typically 30°C). A hotter environment reduces how much current a conductor can carry before exceeding its insulation temperature rating, so the correction factor derates the table value downward.
Why does grouping conductors together reduce ampacity?
Each current-carrying conductor generates heat, and conductors bundled together in a raceway can't shed that heat as effectively as an isolated conductor, so code tables reduce allowable ampacity as more current-carrying conductors share the same raceway.
Does this include the 125% continuous-load factor?
No — enter the load current you want the conductor to carry. If the load is continuous (operates at maximum current for 3 hours or more), size the conductor for 125% of that current before running it through this calculator, per code.
References
- NEC Table 310.16 and Table 310.15(B)(1) / 310.15(C)(1) (or the equivalent CEC Table 2/5A/5C for Canadian installations)
- IEEE Std 835 — Power Cable Ampacity Tables
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