Wire Ampacity and Voltage Drop Calculator

Current rating of a copper or aluminum conductor (AWG, kcmil or mm²) with temperature and grouping corrections, and the voltage drop and power loss along the run.

How it works

A conductor is limited by two things: heating and voltage drop. The ampacity is the current at which the conductor reaches the temperature rating of its insulation in the stated conditions; it is read from a code table and then reduced for a high ambient temperature and for several current-carrying conductors bundled together. The voltage drop comes from the conductor resistance.

ΔV = k × L × I × Rm, Rm = ρ ÷ A, Ploss = k′ L I² Rm

The ampacity tables are NEC Table 310.16 for AWG and kcmil sizes (not more than three current-carrying conductors in a raceway, 30 °C ambient) and the IEC 60364-5-52 table for metric sizes (copper, method C, two loaded conductors). Corrections for ambient temperature follow NEC 310.15(B)(1) and IEC Table B.52.14, and for grouping NEC 310.15(C)(1) and IEC Table B.52.17. The wire size gauge, tinning and joins are in Wires, Connectors and Cables and the system view in System Integration.

Worked example

A 10 AWG copper conductor with 75 °C insulation carries 20 A for 30 m (one way) on a 230 V single-phase circuit. The ambient temperature is 40 °C and there are five current-carrying conductors in the conduit.

  • Table ampacity (75 °C column) = 35 A. Ambient correction at 40 °C for 75 °C insulation = 0.88; five conductors need 0.80. Ampacity = 35 × 0.88 × 0.80 = 24.64 A: the 20 A load is within it (81 %).
  • 10 AWG has a diameter of 0.127 × 92(36 − 10)/39 = 2.588 mm and an area of 5.261 mm². At 75 °C ρ = 2.097×10⁻⁸ Ω·m, so R = 3.985 mΩ/m.
  • ΔV = 2 × 30 × 20 × 0.003985 = 4.78 V, which is 2.08 % of 230 V: within a 3 % allowance. The power lost in the conductors is 96 W.
  • The same run in aluminum (ρ = 3.45×10⁻⁸) would drop 7.86 V (3.4 %) and its ampacity in 10 AWG is only 30 A at 75 °C.

Things to know

Overcurrent protection has its own rules: NEC 240.4(D) limits 14 AWG copper to 15 A, 12 AWG to 20 A and 10 AWG to 30 A regardless of the table, and the protective device must protect the conductor after the corrections.

For long runs, voltage drop governs the size long before heating does. Low-voltage circuits (12 or 24 V) are especially sensitive: a 3 percent drop at 12 V is only 0.36 V.

Alternating-current circuits with large conductors (above about 2/0 AWG or 70 mm²) also have inductive reactance, which this calculator ignores: use the exact formula with the impedance for large conductors and low power factor.

Continuous loads (three hours or more) are limited to 80 percent of the rating of the protective device in the NEC; a motor or LED driver may have inrush current that affects the drop at start.

The ampacity of a conductor in free air, or buried, or in a cable tray, is different: use the appropriate code table. Flexible cords and welding cables have their own ratings.

These are design aids, not a substitute for the electrical code in force where you work. Have electrical work checked by a qualified person.

Common questions

What is ampacity?

The maximum current a conductor can carry continuously without exceeding its insulation temperature rating, under stated conditions of ambient temperature, grouping and installation.

How do I reduce the voltage drop?

Use a larger conductor, shorten the run, raise the supply voltage or split the load. Doubling the area halves the drop.

What is the difference between 60, 75 and 90 °C ratings?

The temperature limit of the insulation. Higher ratings give higher ampacity but most terminals and breakers are rated for 60 or 75 °C, so the lower rating of the conductor and terminal governs.

Why does bundling reduce ampacity?

Conductors heat each other and the heat cannot escape. The code requires a reduction when more than three current-carrying conductors share a conduit or cable.

Sources

  • NFPA 70, National Electrical Code, Tables 310.16, 310.15(B)(1), 310.15(C)(1), 240.4(D) and Chapter 9 Table 8 (conductor properties).
  • IEC 60364-5-52, Low-voltage electrical installations: selection and erection of wiring systems, Tables B.52.4, B.52.5, B.52.14 and B.52.17; IEC 60228 (conductor sizes).

Last reviewed: November 2, 2026