Voltage Drop Calculator

About the Voltage Drop Calculator

Voltage drop is the quiet performance killer on long circuits: push current down enough copper and the load at the far end sees noticeably less voltage than the panel supplies. Motors run hot, LED drivers flicker, and heaters underperform. This calculator lets electricians, low-voltage installers, and solar techs check the drop on a run before pulling wire, using the circular-mils method that underlies the NEC's own examples.

Every conductor has resistance, and that resistance turns into lost volts whenever current flows. The longer the run and the smaller the wire, the bigger the loss. The National Electrical Code does not outright forbid excessive drop, but it recommends keeping branch-circuit drop under 3% and total feeder-plus-branch drop under 5% for reasonable efficiency and equipment performance.

Enter your system voltage, load current, one-way distance, conductor material, and wire gauge, and the tool returns the drop in volts, the drop as a percentage, and the actual voltage the load will see. It flags any result over 3% so you know immediately when a run needs a heavier conductor.

How the Voltage Drop Calculator works

The calculation uses the classic formula VD = (2 x K x I x L) / CM. K is the resistivity constant in circular-mil-ohms per foot, 12.9 for copper and 21.2 for aluminum. I is the load current in amps, and L is the one-way distance in feet. The 2 accounts for the current traveling out and back on a single-phase circuit.

CM is the cross-sectional area of the conductor in circular mils, drawn from a lookup table: #14 AWG is 4,107 CM, #12 is 6,530, #10 is 10,380, #8 is 16,510, #6 is 26,240, #4 is 41,740, #2 is 66,360, and 1/0 is 105,600. Larger wire means more circular mils, which shrinks the drop.

Once the drop in volts is known, the tool divides it by the system voltage and multiplies by 100 to get the percentage, then subtracts the drop from the source voltage to show the voltage delivered at the load. Anything above 3% is highlighted as exceeding the recommended limit so you can size up the conductor.

Worked example

Consider a 240-volt circuit carrying 20 amps over a 150-foot one-way run in #10 copper. Plugging in: VD = (2 x 12.9 x 20 x 150) / 10,380 = 77,400 / 10,380 = 7.46 volts of drop.

As a percentage, 7.46 / 240 x 100 = 3.11%, which the calculator flags red because it edges past the 3% guideline. The load only sees 240 - 7.46 = 232.5 volts.

Upsizing to #8 copper (16,510 CM) recalculates to VD = 77,400 / 16,510 = 4.69 volts, or 1.95%, comfortably inside the limit with 235.3 volts at the load. That one gauge change is the difference between a marginal run and a solid one.

Tips & common mistakes

• Always enter the one-way distance, not the round-trip loop length. The formula already doubles it with the factor of 2 to account for the return conductor, so entering the full loop would double-count the run.
• Switch the material to aluminum when pricing large feeders. Aluminum's K of 21.2 versus copper's 12.9 means it drops about 64% more voltage at the same size, so aluminum runs usually need to be one gauge larger.
• Use 3% as your target for branch circuits and keep feeders under 2% so the combined feeder-plus-branch drop stays within the NEC's recommended 5% total.
• Voltage drop is not the same as ampacity. A wire can be big enough to carry the current safely yet still drop too much voltage on a long run, so check both before finalizing a size.
• For motor and welder circuits, remember that starting inrush current is several times the running amps; a run that looks fine at running current can sag hard on startup, so consider the momentary drop too.

Voltage Drop Calculator: frequently asked questions