Voltage Drop Calculator

Why voltage drops along a wire

Every wire resists the flow of electricity a little, and that resistance turns a small amount of electrical energy into heat, which shows up as a voltage drop between the source and the load. Over a short run inside a house, the drop is negligible, but over a long run to a garden shed, a gate motor, a well pump, or an outbuilding, it can become large enough to dim lights, overheat motors, and trip sensitive electronics. The drop grows with current, with distance, and with wire resistance, and it shrinks when you use thicker wire, which is why long circuits need larger gauges than short ones carrying the same current. The calculator on this page applies the standard single-phase formula, voltage drop equals two times length times current times resistance divided by one thousand, where the factor of two accounts for the out-and-back path of the circuit.

The percentage drop matters more than the raw volts, because a five-volt drop on a twelve-volt system is catastrophic, while the same five volts on a two-hundred-and-forty-volt system is barely noticeable. Electrical guidance in most countries recommends keeping voltage drop below three percent for branch circuits and below five percent overall from source to load, and many designers aim for three percent total on long runs to leave margin for future loads. A circuit that drops eight percent may still work after a fashion, but motors will run hot, draw more current to compensate, and fail early, and LED drivers and electronic controls may behave erratically. Sizing wire for voltage drop, not just for ampacity, is the difference between a circuit that works and one that works well.

The formula, explained plainly

Voltage drop in a single-phase circuit is two times the one-way length in feet, times the current in amps, times the wire resistance in ohms per thousand feet, divided by one thousand. The factor of two exists because current travels out along one conductor and back along the other, so the total wire length is twice the distance to the load. Resistance per thousand feet depends on gauge and material: for copper, fourteen gauge is about two point five two five ohms, twelve gauge about one point five eight eight, and ten gauge about zero point nine nine nine, with thicker wire having lower resistance. Aluminium has higher resistance than copper for the same gauge, so it needs a larger gauge for the same drop, and temperature also raises resistance, which is why calculations use values at operating temperature rather than at room temperature.

Worked through, a one-hundred-and-twenty-volt circuit carrying fifteen amps over one hundred feet on twelve-gauge copper drops two times one hundred times fifteen times one point five eight eight, divided by one thousand, which is four point seven six volts, or just under four percent. That is within the five percent overall limit but above the three percent branch-circuit guideline, so a careful designer would consider ten-gauge wire for that run, which drops only three volts, or two and a half percent. The same circuit at two hundred feet on fourteen-gauge wire drops ten point one volts, or eight point four percent, which is plainly too much, and no amount of hoping will fix it: the wire must get thicker or the circuit must get shorter.

Wire gauge: ampacity is not the whole story

Most DIYers size wire by ampacity, the maximum current it can carry safely without overheating, and stop there, but ampacity and voltage drop are separate limits, and the stricter one governs. Fourteen-gauge copper is rated for fifteen amps, so it passes the ampacity test for a fifteen-amp circuit, yet over a long run it can fail the voltage-drop test badly, as the example above shows. The rule is simple: size for ampacity first, then check voltage drop, and if drop exceeds your target, go up a gauge or two until it passes, even though the thicker wire costs more. The extra copper is cheap compared with a motor replaced early or a shed circuit that dims every time a tool starts.

What voltage drop does to motors and lights

Motors suffer most from low voltage, because a motor that receives less voltage draws more current to deliver the same power, which heats its windings, shortens its insulation life, and can stall it on startup, when current is already highest. A well pump, air compressor, or table saw at the end of a long, thin circuit will start hard, run hot, and hum, and owners often blame the motor when the wire is the culprit. Lights show the problem differently: incandescent and halogen lamps dim visibly, while LEDs may flicker or shift colour as their drivers struggle. Electronics are the most sensitive of all, with computers, routers, and control boards resetting or behaving strangely at voltages that lights tolerate, which is why data and control circuits deserve the tightest drop budgets.

Three-phase and direct current differences

The formula on this page is for single-phase alternating current, which covers most household circuits. Three-phase circuits use a different factor, the square root of three instead of two, because of how the phases share conductors, and the drop is correspondingly lower for the same load. Direct current circuits, such as solar panel runs, battery cables, and low-voltage garden lighting, use the same out-and-back factor of two, but the percentage matters far more, because system voltages are low: a two-volt drop on a twelve-volt solar run is nearly seventeen percent, which can prevent batteries from charging properly. For low-voltage DC, keep runs short, wires thick, and check the drop at maximum current, not at average current.

Common long-run scenarios

Garden sheds and workshops are the classic problem, because owners run a standard gauge for a hundred feet or more and then wonder why the saw bogs down. Outbuildings, gate motors, pond pumps, electric vehicle chargers, and landscape lighting all create the same issue, and each has its own target: EV chargers and motors deserve under three percent, while lighting can tolerate a little more if brightness is acceptable. In each case, measure the actual one-way distance along the cable route, not the straight-line distance on a map, since cables follow walls, trenches, and conduits and are always longer than they look. Add ten percent for routing slack when estimating, and if you are near a threshold, go up a gauge: copper is a one-time cost, while voltage problems are forever.

Fixing an existing circuit with too much drop

If a circuit already drops too much, the fixes are limited but real. Upsizing the wire is the proper cure, though it means re-pulling cable, which is disruptive but permanent. Moving the load closer to the source, splitting loads across two circuits, or raising the supply voltage, for example by using a two-hundred-and-forty-volt circuit instead of one-hundred-and-twenty where the equipment allows, all reduce drop. Transformers and voltage regulators can compensate in specific cases but add cost and losses of their own. What does not work is ignoring it: a circuit that drops eight percent today will drop more as connections age and resistance rises, and the motor it feeds will pay the price first.

Measuring drop on a real circuit

You can verify a calculation with a multimeter by measuring voltage at the source and at the load under full load, not with the circuit idle, since drop only appears when current flows. The difference is the actual drop, and it should roughly match the calculator's figure; a much larger measured drop suggests a loose connection, a damaged conductor, or a longer route than assumed, all of which deserve investigation. Never work on live circuits beyond your competence, and for any circuit that supplies a building or a large load, have a qualified electrician confirm the design. The calculator is a planning tool, not a substitute for code compliance, which also covers protection, earthing, and installation methods.

Designing it right the first time

The cheapest time to fix voltage drop is before the cable is buried or plastered in. Decide the load, measure the route, pick a target percentage, and run the calculation for two or three gauges, choosing the smallest gauge that meets the target with margin. Note the calculation in your project file, alongside the cable route and the load, so that a future addition, a bigger saw or an EV charger, can be checked against the same numbers rather than guessed. Good electrical design is rarely visible, which is exactly the point: a circuit sized for voltage drop simply works, quietly, for decades, while an undersized one announces itself every time a motor starts.

• Size wire for voltage drop as well as ampacity, and take the stricter result.
• Use one-way route length, doubled by the formula, not straight-line distance.
• Aim for under 3 percent on branch circuits, under 5 percent overall.
• Motors and electronics suffer first from low voltage — protect them with margin.
• Verify with a multimeter under full load, and call an electrician for building circuits.

Voltage drop is invisible until it bites, but it is entirely predictable. Enter your voltage, current, distance, and wire gauge above, and size the circuit once, properly, instead of troubleshooting it for years.