Voltage Drop Calculator

How voltage drop works

Every wire has a little resistance. When current flows, part of the voltage is used up pushing that current along the wire, so the load at the far end gets less than the panel or battery sends. That loss is the voltage drop. It grows with the current, the length of the run and the temperature of the wire, and it shrinks as the wire gets thicker.

A small drop is normal. A large one makes motors run hot and slow and upsets electronics, and the lost voltage turns into heat in the wire itself. The calculator shows that heat in watts, so you can see what a thin wire costs you.

The formula, step by step

The calculator works from the physics of the wire, the same method an electrician uses with a pocket calculator:

1. Resistance. It finds the cross-section of the wire from its AWG or kcmil size, then its resistance per 1,000 ft from the metal: copper, or aluminum with about 61% of the conductivity of copper. It corrects for temperature and adds 2% for stranded wire, because the strands spiral and are a little longer than the cable.
2. Drop. Drop = 2 × one-way length × amps × resistance ÷ 1,000. The 2 counts the trip out and back. A balanced three phase circuit uses 1.732 instead of 2, with the volts measured line to line.
3. Percent. The drop divided by the source voltage, times 100.
4. Longest run. The same formula turned around: how far this wire can go before the drop reaches your limit.

The worked example below runs these steps for the numbers the calculator opens with: 20 A over 100 ft of 12 AWG copper on 120 V drops 7.88 V, leaving 112.12 V at the load.

How much drop is acceptable

The National Electrical Code suggests, in informational notes, about 3% on a branch circuit and 5% in total for a feeder and branch together. Those notes are advice, not rules, and most electricians design to them anyway. Equipment makers and local rules can be stricter, so read the nameplate and ask your inspector.

Low-voltage systems use their own figures. Marine practice allows 3% for anything that affects safe operation and 10% for the rest, and solar installers aim for about 2% between the panels and the batteries. The chips under the limit box fill in these figures for you.

Choosing the smallest wire

Distance is only half of the answer. A short run with a heavy load can pass a 3% test on a wire that is far too small to carry the current without overheating. So the smallest wire shown is never below its typical current rating at 60 °C, the figure used for most home circuits of 100 A or less. When that rating sets the size instead of the distance, the result says so. If the wire you picked is below its typical rating for your load, a red line names the limit, such as 15 A for 14 AWG copper.

Above 100 A the calculator still finds the smallest wire for the drop, but you need to check the current rating separately. To size a whole circuit from its load or its breaker, including loads that run for hours, use the wire size calculator.

Low-voltage DC: RVs, vans, boats and solar

At 12 V every tenth of a volt matters: 3% of 12 V is only 0.36 V. That is why battery cables are so thick and the runs so short. Moving the system to 24 V halves the current for the same power and doubles the volts your limit allows, so the same wire can carry the load about four times as far. Pick DC, enter the one-way distance from the battery to the load, and the calculator counts both wires.

What this calculator leaves out

It uses resistance only. On AC, large wires add reactance and skin effect, more so in steel conduit, so for 1/0 AWG and up the real drop can be higher than shown, and the result says so. It does not model parallel sets of wires, the dip when a motor starts, or power factor. For those jobs, and for feeders and services, ask a licensed electrician.

Worked example

1. Inputs: 20 A, 100 ft one way, 12 AWG copper at 75 °C, single phase 120 V, 3% limit.
2. Resistance of 12 AWG copper at 75 °C, stranded: 1.9701 Ω per 1,000 ft
3. Drop = 2 × 100 ft × 20 A × 1.9701 Ω ÷ 1,000 = 7.88 V
4. Drop % = 7.88 V ÷ 120 V × 100 = 6.57%
5. At the load = 120 V − 7.88 V = 112.12 V
6. Heat = 2 × 20 A × 20 A × 100 ft × 1.9701 Ω ÷ 1,000 = 158 W
7. Longest run at 3% = 3% × 120 V × 1,000 ÷ (2 × 20 A × 1.9701 Ω) = 45 ft

Questions people ask