Voltage Drop Calculator

Enter the supply voltage, the current, the one-way length and the conductor. You get the volts lost, the percentage, and, if you set a limit, the smallest size in the list that meets it.

Circuit

Supply voltage

The voltage at the start of the run. Single-phase: across the two conductors, such as 120, 240 or 230. Three-phase: line to line, such as 208, 400, 480 or 600. A DC run is fine too, such as 12 or 48.

Load current

What the load actually draws, from the nameplate or a clamp meter. Use the breaker rating instead only if you want the worst case the circuit allows.

One-way length of the run

From the panel or source to the load, along the route the cable actually takes, not a straight line. Count it once: the return conductor is already allowed for.

Length in

Conductor size

The cable you are checking. AWG and kcmil sizes show their area so you can see what the answer is built on. Choose "Enter my own area" for any other size.

Conductor material

Result

Fill in a few more boxes and the answer will show here.

Opens your print dialog. Choose Save as PDF. Your figures stay on this device.

Loading the calculator.

This is an estimate from conductor resistance at 75 °C, not a code calculation. It does not check ampacity, conduit fill, temperature or bundling correction, or reactance on large AC conductors. Rules differ between Canada, the US, Australia and New Zealand, and the inspection authority where you work has the final say.

Voltage drop is resistance times current, and the length counts twice

Every conductor has resistance, and current through resistance costs volts. The drop is the current times the resistance of the run, and the resistance is set by three things: the metal, the length and the cross-section. Double the length and the drop doubles. Go up one AWG size and it falls by about a fifth. On a single-phase or DC circuit the current goes out and comes back, so the formula counts the length twice. On a balanced three-phase circuit it multiplies by 1.732 instead.

The figure most people check against is a percentage of the supply. The Canadian Electrical Code (CSA C22.1, Rule 8-102) sets 3% for a feeder or branch circuit and 5% from the service to the load. AS/NZS 3000 sets 5% from the point of supply. The NEC puts the same 3% and 5% in informational notes, as advice rather than a rule. Enter the one you work to and the tool finds the smallest size in the list that stays inside it.

The common mistake is treating that size as the answer. Voltage drop and ampacity are separate checks, and the bigger of the two wins. A long run to a small load is sized by drop; a short run to a big load is sized by ampacity, which comes from the code table that applies where the job is, with its temperature and bundling corrections. This tool does not do that part.

It also uses resistance alone, so on large AC conductors at a poor power factor the real drop is somewhat higher. Write the run length down on the walk-through. In Zeus it stays with that address, so the quote is priced on the cable you measured.

Built for Electrical work: see the Electrical job app.

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