Wire Size and Ampacity Calculator

About this calculator

How to use the ampacity calculator

Start with the load current, then pick a conductor that stays at or above that current after you apply the temperature and bundling rules. The calculator reads NEC Table 310.16 and lets you check voltage drop on the same size.

• Use the 75 °C column only when the termination is listed for 75 °C. Otherwise, equipment rated 100 amperes or less uses the 60 °C column under 110.14(C).
• If the insulation is 90 °C, you may start the adjustment from the 90 °C column, then the result cannot exceed the termination column.
• More than three current-carrying conductors in the same raceway use the adjustment factors in 310.15(C)(1).

Worked example

Four AWG copper is 85 amperes in the 75 °C column of Table 310.16 and 95 amperes in the 90 °C column. Six current-carrying THHN conductors use the 80 percent adjustment for 4–6 conductors:

95 A × 0.80 = 76 A from the 90 °C column
Termination limit at 75 °C is 85 A, so 76 A governs

That 4 AWG run is a 76 ampere conductor after bundling, not an 85 ampere conductor. Upsize if the load is higher, then run the voltage-drop check.

Common questions

Which NEC table is the ampacity calculator using?

NEC Table 310.16 for copper and aluminum at 60 °C, 75 °C, and 90 °C, not more than three current-carrying conductors. Temperature and bundling adjustments are applied on top of that table.

Is this ampacity calculator free?

Yes. It runs in the browser with no account and no signup. It is a design aid, not a code-compliance determination.

For design, installation, or safety decisions, verify assumptions against the applicable code edition, equipment documentation, and site requirements.