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Wire Size & Voltage Drop Calculator

Find the cable cross-section in mm² and AWG for a current, length and allowed voltage drop.

How to use the Wire Size & Voltage Drop Calculator

  1. 1Choose DC, single-phase or three-phase AC and enter the voltage.
  2. 2Enter the current or the power, the one-way cable length and the conductor material.
  3. 3Set the allowed voltage drop and read the size in mm² and AWG, checked against current capacity.

About Wire Size & Voltage Drop Calculator

The Wire Size & Voltage Drop Calculator finds the conductor cross-section for a DC, single-phase or three-phase AC circuit. Enter the voltage, the current or the power (with the power factor for AC), the one-way cable length, copper or aluminium, the conductor temperature and the voltage drop you allow. The calculator returns the minimum cross-section for that drop and the recommended standard size in mm² and AWG or kcmil, plus the actual drop, the voltage at the load and the power lost in the cable.

The recommendation is also checked against the current the cable can carry, using typical values from IEC 60364-5-52 for metric sizes and the NEC 310.16 table for AWG, and it takes whichever size is larger. A table lists the neighboring standard sizes with their drop and current capacity, and the formula is worked through step by step, including the resistivity at the chosen temperature.

This wire size calculator is handy for 12 V and 24 V systems in campers, boats and solar setups, long runs to garden lighting or a workshop, and for checking a cable gauge before you buy. It ignores cable reactance and installation details, so have mains wiring confirmed by a licensed electrician.

Frequently asked questions

How do I calculate voltage drop?

For DC and single-phase AC: voltage drop = 2 × length × current × resistivity ÷ cross-section. The 2 accounts for the way there and back. For three-phase, use √3 instead of 2. Use the resistivity at the operating temperature, not at 20 °C.

Why does the calculator assume a conductor temperature of 70 °C?

A cable carrying close to its rated current heats up, and common PVC-insulated cable is rated for a conductor temperature of 70 °C. Copper resistance rises about 0.4% per °C, so a hot cable drops about 20% more voltage than the 20 °C textbook value. Sizing at 70 °C keeps the result on the safe side. If the cable is lightly loaded you can pick a lower temperature, or 90 °C for XLPE cable at full load.

What voltage drop is acceptable?

A common limit is 3% for a branch circuit and 5% for the feeder and branch together. Low-voltage DC systems such as 12 V often aim for 3% or less, since every volt counts.

Is the voltage drop size always enough?

No. The cable also has to carry the current without overheating, which depends on the insulation and how it is installed. The calculator checks both and recommends the larger size. Breaker rules can add limits, such as 14 AWG copper on at most 15 A in the US, so have mains wiring checked by an electrician.

How do I convert AWG to mm²?

AWG sizes follow a geometric series: the diameter of gauge n is 0.127 mm × 92^((36 − n) / 39), and the area is π/4 × d². Common values are 14 AWG ≈ 2.08 mm², 12 AWG ≈ 3.31 mm² and 10 AWG ≈ 5.26 mm².

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