Voltage Loss Calculator

Think of a garden hose: the longer and narrower the hose, the more pressure you lose by the time water reaches the sprinkler. Electrical wire works the same way — every conductor has resistance, and resistance steals voltage before it reaches your load. A light bulb, motor, or appliance at the end of a long run gets slightly less voltage than the panel delivers, and that deficit is voltage drop.

The calculator uses Ohm's Law (voltage equals current multiplied by resistance) applied to the full round-trip path of the circuit. You enter the one-way cable run in feet; the tool doubles it because current must travel to the load and return through the return conductor, both of which add resistance. The resistance per foot depends on the American Wire Gauge (AWG) number and whether the conductor is copper or aluminum. Lower AWG numbers mean physically thicker wire and lower resistance per foot — 4 AWG is far thicker than 14 AWG, which is why large motors use large conductors.

The percent voltage drop is the calculated drop divided by your supply voltage, expressed as a percentage. This percentage is what really matters to your equipment: a 5-volt drop on a 480 V circuit is trivial, but the same 5-volt drop on a 120 V circuit takes away over four percent of the supply. The compliance verdict shown below the result tells you directly whether your configuration meets, approaches, or exceeds the commonly used 3 percent guideline for branch circuits.

Use this tool whenever you are planning a new circuit run, troubleshooting unexplained equipment faults, or checking whether an existing wire can handle a load increase. It is directly applicable to residential branch circuits, outbuilding subpanel feeds, EV charger installations, and commercial lighting or HVAC feeds where the panel is far from the load. The tool is also useful for solar and battery system DC wiring, where voltage drop at low voltages has a proportionally larger impact on system efficiency.

The tool is not appropriate as a substitute for a full load analysis by a licensed electrician when the circuit feeds life-safety equipment, medical devices, or large industrial machinery. It also does not account for derating due to conduit fill, high ambient temperatures, or continuous loads (which the NEC requires to be derated to 80 percent of breaker capacity). For those scenarios, calculate the derated current first and then use this tool with the derated figure.

A good rule of thumb: if this tool shows you at or above the 3 percent guideline, go up one AWG size and recalculate before ordering wire. The cost difference between 12 AWG and 10 AWG on a typical run is small compared to the cost of rewiring after the fact.

Mistake 1 — measuring one-way and forgetting to double. The most common error is entering the total round-trip distance as the one-way run, which doubles the calculated drop. The calculator already doubles the value you enter. If you measure 85 feet from panel to outlet, enter 85 — not 170. Entering the round-trip length will make a compliant circuit look like it fails.

Mistake 2 — using nameplate watts instead of actual running amps. A motor's rated wattage represents its output power, not its input current demand. A 1-horsepower motor draws more amps than its output wattage implies because of efficiency losses. Always use the measured or nameplate amperage, not a watt-to-amp conversion from output power, or the voltage drop calculation will understate the true drop during operation.

Mistake 3 — ignoring starting current on motor loads. The tool calculates drop at the steady-state running current you enter. Induction motors draw five to seven times running current during startup, briefly deepening the voltage drop for every other load on the same circuit. If the calculated drop is near the guideline at running current, starting transients will push it well above it. Size the conductors to hold drop well below the guideline if the circuit feeds motors.

The core formula is: Voltage Drop = 2 × L × I × R_ft, where L is the one-way cable length in feet, I is the current in amps, and R_ft is the resistance per foot of one conductor in ohms per foot. The factor of 170 feet (for the example run of 170 ft total) accounts for the complete circuit path. The result is a voltage in volts.

For the example circuit — copper 12 AWG, 120 V supply, 15 A, 170 ft total path — the resistance of the full run is 0.3366 Ω. Multiplying by 15 A gives a drop of 5.05 V. Dividing by the 120 V supply and multiplying by 100 yields 4.21% percent drop. The voltage remaining at the load is 114.95 V.

Percent drop = (Voltage Drop / Supply Voltage) × 100. This normalisation is what makes the 3 percent guideline universal across 120 V, 240 V, and 480 V systems. The absolute drop number varies widely with supply voltage; the percent captures how much of the available pressure you are losing regardless of system voltage level.

The resistivity values in this tool are for conductors at 75 degrees Celsius — the standard temperature for NEC ampacity tables. A conductor running near its ampacity limit will be hotter, and resistance rises with temperature (roughly 0.4 percent per degree C for copper), so the actual drop at a loaded conductor will exceed the calculated value. For circuits loaded above sixty percent of ampacity in high-ambient environments, apply a temperature correction factor before trusting the result. Additionally, the formula assumes DC resistance; for AC circuits at power frequency (50 or 60 Hz), skin effect and proximity effect add a small impedance increment that becomes relevant for conductors larger than 2/0 AWG — the Chapter Nine of the NEC AC impedance values supersede the DC figures at those sizes.