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Voltage drop on a normal home circuit
Most household circuits are single-phase alternating current, at 120 volts in North America or 230 volts in much of the rest of the world. Voltage drop still applies, but the numbers feel kinder than in a 12-volt DC system because the supply voltage is large. A 3-volt loss on a 230-volt circuit is only 1.3 percent, and even a long garden run often stays inside the usual 3 percent planning target for branch circuits, with 5 percent as the combined limit for feeder plus branch in many guides.
This calculator uses a simplified resistive model for single-phase AC: drop equals twice the current times the one-way length times resistivity, divided by cross-section, multiplied by the power factor. The factor of two covers the out-and-back path, just as in DC. Power factor enters because loads like motors draw current slightly out of step with voltage, and only the in-step part produces the resistive drop this model counts. For resistive loads such as heaters and incandescent lamps the power factor is 1.0, while for motors a value around 0.85 to 0.9 is a reasonable planning figure.
Why long runs still cause trouble at mains voltage
A long run to a garden office, a workshop, or a pool pump is the classic case. A 16-amp load, 30 metres away, on 10 AWG copper at a power factor of 0.9 loses about 2.8 volts, which is only 1.2 percent of 230 volts and perfectly fine. Stretch that run to 60 metres and the loss doubles to about 5.6 volts, or 2.4 percent, still acceptable but no longer comfortable. Stretch it to 90 metres, or drop to 14 AWG, and the same load crosses 5 percent, where motors start sluggishly, lights flicker when a large load switches on, and electronics may reset.
A worked example
Consider a small workshop fed from the house, 40 metres away, drawing 20 amps on 230 volts through 8 AWG copper at a power factor of 0.9. The calculator gives a drop of about 2.95 volts, or 1.3 percent. That is a healthy result with room for a future load increase. Now change only the cable to 14 AWG, the size used for many indoor lighting circuits. The drop jumps to about 11.8 volts, or 5.1 percent, which is beyond the usual combined target and would show up as dimming lights and slow motor starts whenever the workshop load peaks.
A North American example at 120 volts tells the same story more sharply. A 16-amp load, 25 metres away, on 12 AWG copper at a power factor of 0.9 loses about 2.5 volts, or 2.1 percent of 120 volts. That passes the 3 percent branch target but leaves little margin. The same load on 10 AWG loses about 1.6 volts, or 1.3 percent. Because the supply voltage is lower, every volt of drop counts for roughly twice the percentage it would at 230 volts, which is why long 120-volt runs deserve the thicker cable sooner.
Common mistakes with AC voltage drop
The first mistake is entering the total cable length instead of the one-way length. This calculator doubles the one-way length for you, so entering the round trip doubles the answer. The second mistake is using the breaker rating as the current. A 32-amp breaker does not mean the circuit draws 32 amps; use the actual load, or the design load for a planned circuit, because an inflated current inflates the drop and can push you to cable you do not need, or hide the real problem.
Frequently asked questions
What is a good power factor to use? Use 1.0 for heaters and lamps, and 0.85 to 0.9 for motors and mixed loads if you do not know the exact figure. Does this calculator work for 120 volts and 230 volts? Yes, enter your actual system voltage and read the percentage, which is what matters. Is 5 percent always acceptable? No, 5 percent is a common combined ceiling for feeder plus branch, while 3 percent is the usual target for a branch alone, and sensitive equipment may need less. Check your local code and the equipment manual.
Safety first: an estimate, not a code approval
Every electrical tool on this page is an educational estimator only. It is never professional advice and never a substitute for a licensed electrician. Electrical codes differ by country, region and building type, and they change over time. A result that looks fine here can still fail a local inspection, and a result that looks tight here can still be acceptable under a rule this simple calculator does not model, such as temperature correction, grouping, or a special location rule. Before you buy cable, cut a wall, or touch a panel, check your local electrical code and talk to a licensed electrician. Breaker, conduit and box-fill results are estimates, not code-compliance guarantees. If you are unsure about any part of a job, stop and ask a professional. Electricity can injure or kill, and no free online calculator changes that fact.
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