Voltage Drop Calculator

What Voltage Drop Is

Every conductor has resistance. Push current through it and some of the supply voltage is consumed by the wire itself instead of reaching the load. That loss is voltage drop. A 120 volt circuit with 8 volts of drop delivers 112 volts at the far end, and the missing 8 volts becomes heat in the conductor.

Drop grows with current and with distance, and shrinks as the conductor gets larger. Short runs at modest current rarely matter. Long runs, high current, or both together are where drop turns into a design problem.

The inputs, one at a time

• K, the resistivity constant. 12.9 for copper, 21.2 for aluminium. These are ohms per circular mil foot at typical operating temperature. Aluminium's higher value is the reason aluminium always drops more than copper at the same size.
• I, the current. Use the actual load current in amps, not the breaker rating. A 20 amp breaker feeding a 12 amp load should be calculated at 12 amps unless the design intent is to size for the full breaker.
• L, the length. One-way distance from the source to the load, measured along the actual cable route, not straight line across the yard. The formula already doubles it.
• CM, circular mils. The cross sectional area of the conductor. Larger conductors have more circular mils and less drop.

Circular Mils by Wire Size

Circular mils is the area unit used for conductors in the US. One circular mil is the area of a circle 0.001 inch in diameter. The values below are the standard figures for each AWG size and are the numbers to put into the formula.

Two things about that table. First, ampacity and voltage drop are separate checks, and a conductor can pass one and fail the other. Second, small conductors carry an additional restriction: overcurrent protection for 14, 12 and 10 AWG copper is generally limited to 15, 20 and 30 amps respectively regardless of the ampacity column, subject to the exceptions in the code. Ampacity figures also require correction for ambient temperature, for conduit fill, and for the temperature rating of the terminations at each end, which is commonly 60 C on residential equipment.

Worked Examples

A 20 amp circuit run 100 feet

A 20 amp load on a 120 volt circuit, 100 feet one way, wired in 12 AWG copper: 2 x 12.9 x 20 x 100 equals 51,600, divided by 6,530 circular mils equals 7.90 volts. That is 6.59 percent of 120 volts, and the load sees about 112 volts. Well past the 3 percent recommendation.

Step up to 10 AWG: 51,600 divided by 10,380 equals 4.97 volts, or 4.14 percent. Better, still over. Step up to 8 AWG: 51,600 divided by 16,510 equals 3.13 volts, or 2.60 percent. That one passes.

The same run in aluminium

Swap to aluminium and K becomes 21.2. For 8 AWG aluminium: 2 x 21.2 x 20 x 100 equals 84,800, divided by 16,510 equals 5.14 volts, or 4.28 percent. It fails where copper passed. Moving to 6 AWG aluminium gives 84,800 divided by 26,240, which is 3.23 volts or 2.69 percent. In practice aluminium needs roughly two AWG sizes larger than copper to reach the same drop.

A three phase feeder

A 100 amp three phase load at 480 volts, 250 feet away, fed with 1/0 copper: 1.732 x 12.9 x 100 x 250 equals 558,570, divided by 105,600 equals 5.29 volts. On 480 volts that is 1.10 percent. Higher system voltage is the single most effective way to control drop, because the same volts lost represent a much smaller percentage.

Working backward to a wire size

Rearranging the formula gives the minimum conductor area directly.

The 3 Percent and 5 Percent Figures

The National Electrical Code recommends a maximum of 3 percent voltage drop on a branch circuit to the farthest outlet, and a maximum of 5 percent total for the combination of feeder and branch circuit. On a 120 volt circuit, 3 percent is 3.6 volts and 5 percent is 6 volts. On 240 volts they are 7.2 and 12 volts.

These figures appear in informational notes rather than in enforceable code text, which means they are recommendations for reasonable efficiency of operation and not hard requirements in general branch circuit wiring. Some specific applications, such as certain sensitive equipment, fire pumps and some renewable energy circuits, carry their own stricter rules. Local amendments can also make the recommendation mandatory, and many engineered specifications require it by contract. Treat 3 percent as the working target and check the local jurisdiction before assuming it is optional.

Why Long Runs Need Bigger Wire

Drop scales linearly with length. Doubling the run doubles the drop, all else equal. A 12 AWG copper circuit carrying 16 amps drops about 1.26 volts at 20 feet and about 12.6 volts at 200 feet. The conductor that was completely adequate near the panel becomes unusable at the end of a long feeder to a detached garage, a well pump, a barn or a set of yard lights.

Current scales the same way. The wire feeding a 3 amp lighting load can be small at any practical distance. The wire feeding a 40 amp EV charger 150 feet from the panel is a different problem entirely. Multiply length by current and the product tells you roughly how much trouble the run is in before doing any arithmetic.

Copper Versus Aluminium

Copper conducts better per unit of area, and its K value of 12.9 against aluminium's 21.2 makes that concrete: at identical size, aluminium drops about 64 percent more. Aluminium compensates by being much cheaper and lighter per foot, which is why service entrance conductors and long feeders are frequently aluminium while branch circuits are usually copper.

Aluminium brings its own requirements. Terminations must be rated for aluminium, the conductor size must be chosen from the aluminium ampacity column, and antioxidant compound is commonly specified on terminations. Aluminium and copper must not be joined at a termination unless the connector is listed for that combination.

What Excessive Drop Actually Does

• Motors run hot. A motor at reduced voltage draws more current to produce the same mechanical output. Extra current means extra heat, shorter winding life, harder starting and more nuisance tripping. Motors are the loads most damaged by chronic low voltage.
• Heating output falls off sharply. Resistive heat output varies with the square of the voltage, so 10 percent low voltage delivers about 81 percent of rated output. Water heaters, baseboard heaters and stoves take noticeably longer.
• Lighting dims and flickers. Incandescent and halogen lamps visibly dim. LED drivers usually hold output until they drop out, at which point they flicker instead.
• Electronics behave unpredictably. Switching power supplies compensate by drawing more current at low voltage, which increases the drop further.
• Energy is wasted as heat. Voltage lost in a conductor is power dissipated in the wall, paid for and unused.

Practical Notes

• Measure the route, not the distance. Conductors follow walls, joists and conduit paths, and the real length is routinely 20 to 40 percent longer than the straight line.
• Use the actual load current where the load is known, and use the full circuit rating where it is not.
• Add the feeder drop and the branch circuit drop together to check the 5 percent total.
• Raising the system voltage cuts percentage drop hard. A 240 volt circuit carries half the current of a 120 volt circuit for the same power and halves the drop again as a percentage.
• Splitting a long high current run into two circuits reduces current per conductor and can be cheaper than one very large conductor.
• Bad terminations create localized drop and heat that no conductor calculation will predict. Torque connections to specification.