Voltage drop calculator with wire size

Every wire has resistance, so some voltage is lost pushing current along it. The longer the run, the more current, and the thinner the wire, the more is lost.

Single phase: VD = 2 × K × I × L ÷ CM

Three phase: VD = 1.732 × K × I × L ÷ CM

• K is the resistance of a wire one circular mil in area and one foot long at 75°C: about 12.9 ohms for copper and 21.2 for aluminum.
• I is the load current in amps.
• L is the one-way length in feet, from the panel to the load.
• CM is the conductor's area in circular mils, from NEC Chapter 9, Table 8.

The 2 counts the wire out and the wire back. In a balanced three phase circuit the return is shared, so the multiplier is √3, or 1.732. Percent drop is VD divided by the system voltage.

Ampacities are the 75°C column of NEC Table 310.16. The breaker limits on 14, 12 and 10 AWG come from the small-conductor rule in NEC 240.4(D).

A 120 V circuit in a detached garage. 16 A of load, 100 ft away, 12 AWG copper:

VD = 2 × 12.9 × 16 × 100 ÷ 6,530 = 6.32 V. That is 6.32 ÷ 120 = 5.27%, leaving 113.7 V at the load. 12 AWG carries the current, but the drop is well over 3%. 10 AWG gives 3.31%, still over. 8 AWG gives 2.08%, which works.

A 240 V EV charger. A 48 A charger, 150 ft from the panel. The NEC treats EV charging as a continuous load, so the circuit is 60 A and the wire has to carry it. 6 AWG copper (65 A at 75°C) gives 2 × 12.9 × 48 × 150 ÷ 26,240 = 7.08 V, 2.95%. That just makes 3%. 8 AWG would give 4.69%, and it is too small for a 60 A circuit anyway.

A 480 V three phase feeder. 100 A on 200 ft of 1/0 aluminum: 1.732 × 21.2 × 100 × 200 ÷ 105,600 = 6.95 V, 1.45%.

These assume the circuit is loaded to its full rating. Real loads are often lower, so real drops are often smaller. The calculator works from the current you enter.

The NEC's voltage drop guidance sits in informational notes after 210.19 (branch circuits) and 215.2 (feeders). They say that conductors sized to keep voltage drop under 3% on a branch circuit, and under 5% for the feeder and branch together, give reasonable efficiency of operation.

Informational notes explain; they are not requirements. An inspector cannot fail a job on the 3% figure unless the local jurisdiction has adopted it, though some energy codes and specifications do make voltage drop limits mandatory. A few NEC sections do set real limits for specific equipment, such as fire pumps and sensitive electronic equipment.

So why bother? Low voltage makes motors draw more current and run hot, shortens their life, dims lights, and can trip electronics and make compressors struggle to start. On long runs, it also wastes power as heat in the wire, every hour the load runs.

Size every conductor for two things, then use the bigger of the two:

1. Ampacity. The wire has to carry the current. The calculator uses the 75°C column of Table 310.16, the usual rating for terminations on 100 A and larger equipment and on most breakers listed for 75°C, plus the 240.4(D) caps on 14, 12 and 10 AWG. If your terminations are only rated 60°C, use the 60°C column instead, which is lower for small wire.
2. Voltage drop. The wire has to be big enough that the drop stays under your target over the run.

The calculator finds both and suggests the larger. It also shows the longest run your chosen wire can go at the drop limit, which is handy for quoting a detached garage, a well pump or a landscape lighting run.

Ampacity can be reduced further for things this calculator does not check: more than three current-carrying conductors in a raceway, high ambient temperatures such as a hot attic or a rooftop in the sun, and the 125% rule for continuous loads (three hours or more), which applies to EV chargers, water heaters and many HVAC loads. Apply those corrections and adjustments before you settle on the size.

Some runs come up again and again, and they are where voltage drop catches people out:

• Detached garages and shops. A 120 V circuit to a garage 75 ft away is already past the 3% figure at 20 A on 12 AWG. Run a 240 V feeder to a subpanel instead, or upsize the branch circuit.
• Well pumps. Long runs to a submersible pump drop voltage at the moment the motor starts, when current is highest. Pump makers publish maximum cable lengths by wire size and motor size; follow those, since they account for starting.
• Outdoor and landscape lighting. Low-voltage lighting runs at 12 V, so a drop of a single volt is over 8%. Use the transformer maker's guidance, or center-feed long runs.
• EV chargers. A 48 A charger is a 60 A circuit and a long run from the panel to a driveway is common. The 150 ft example above needed 6 AWG copper to stay at 3%.
• HVAC condensers. Compressors are sensitive to low voltage at start. The nameplate gives the minimum circuit ampacity to size the wire; check the drop on long runs to rooftop or remote units.

In each case the fix is the same: a bigger conductor, a higher voltage, or a shorter run. Raising the voltage is the most powerful: the same load at 240 V draws half the current of 120 V, and the same volts lost are half the percentage.

Under the answer, the calculator lists every size of the conductor you picked, from the smallest to 750 kcmil, with the maximum current each can be protected at, the drop in volts, the percent, and a verdict for your run:

• Works: carries the current and stays under the drop you chose.
• Too small for the current: the ampacity, or the 240.4(D) breaker cap for 14, 12 and 10 AWG, is below your load.
• Over 3% (or 5%): carries the current but loses more voltage than you set as the limit.

The first "Works" row is the smallest wire that does both jobs, which is what the suggested wire shows. Use the table to see the cost of going one size up: often the next size buys a much lower drop for a modest price difference, which is worth it on a load that runs many hours, such as a well pump, a shop compressor or an EV charger.

If you are quoting a feeder plus branch circuits, work out the feeder at its own load and length first, then each branch. Keep the two drops added together under 5%, for example 2% on the feeder and 3% on the farthest branch.

• Large conductors. The K method uses DC resistance. For big AC conductors, especially in steel conduit, reactance adds to the drop, and NEC Chapter 9, Table 9 gives the AC values. Above about 1/0 the simple method starts to understate the drop.
• Temperature. K is for 75°C. A lightly loaded wire runs cooler, with a little less resistance, so the formula is slightly conservative.
• Motor starting. A motor draws several times its running current for a moment when it starts. Check the starting dip separately for long runs to well pumps and compressors.

When you have the size, build the estimate in Koira with the wire, conduit, breaker and labor as their own lines. For labor rates, see the electrician labor rate calculator.