Voltage Drop Calculator

This calculator estimates the voltage drop of an electrical circuit. The NEC data tab works from the resistance and reactance tables in the National Electrical Code. The Estimated resistance tab works from the resistance implied by the wire's cross-section. The Other tab takes a resistance or impedance figure of your own, from a different standard or a manufacturer's datasheet.

What voltage drop is

When current moves through a wire it is pushed by electrical potential, and it has to overcome a certain amount of contrary pressure from the wire itself. Voltage drop is the potential lost to that pressure.

With direct current the contrary pressure is called resistance. With alternating current it is called impedance, which is a vector quantity made of resistance together with reactance, the reaction of a built-up electric field to a change of current. That distinction is why the NEC tab asks for a power factor and the estimated-resistance tab does not.

Why it matters

Excessive voltage drop makes lights flicker or burn dimly, heaters heat poorly, and motors run hotter than they should. A motor is the expensive case: it draws more current to make up the lost voltage, heats further, and eventually burns out.

The commonly quoted target is to keep the drop under 5% at full load. In the National Electrical Code that figure is split: informational notes suggest no more than 3% on a branch circuit, and no more than 5% for the feeder and branch circuit combined. Neither is a mandatory rule, but they are what inspectors and designers work to, and the calculator flags both.

Meeting them is a matter of choosing the right conductor for the run, and of care with extension leads and similar temporary wiring, where a long thin cord can lose more voltage than the entire fixed installation behind it.

The four major causes

Conductor material

Silver, copper, gold and aluminium are among the metals with the best electrical conductivity. Copper and aluminium are what wires are actually made from, because silver and gold are far too expensive. Copper is the better conductor, so for a given length and size it drops less voltage than aluminium: at 1 AWG the difference is about 0.41 ohms per kilometre against roughly 0.67.

Wire size

Larger wire drops less voltage over the same length. American Wire Gauge runs backwards, so a smaller number means a thicker wire, and the scale is geometric rather than linear. Every 6-gauge decrease doubles the diameter, and every 3-gauge decrease doubles the cross-sectional area, which is the figure resistance actually depends on.

The metric gauge scale works differently: the gauge is ten times the diameter in millimetres, so a 50 gauge metric wire is 5 mm across.

Wire length

Shorter runs drop less than longer runs of the same size, in direct proportion. This is rarely a problem inside a house and becomes one when wiring reaches an outbuilding, a well pump, a gate motor or a car charger at the end of a drive. Note that the calculator asks for the one-way distance and doubles it internally, since the current has to return.

Current

More current through the same wire means more voltage drop, again in direct proportion. The amount a conductor can carry safely is its ampacity, short for ampere capacity, and it is limited by the material, by the frequency where the current alternates, and by the temperature the wire works in.

Bundling matters too. Cables run together cannot shed heat as easily, so the heat they generate collectively reduces the ampacity of each. There are strict rules about bundling for exactly this reason, and derating for conductor count is part of any real design.

Choosing a cable

Two principles govern the choice. First, the cable has to carry its load current without overheating, under the most extreme conditions it will meet in service rather than the conditions on the day it is installed. Second, it has to offer sound earthing, both to limit the voltage a person could be exposed to and to let fault current rise high enough to trip the protective device quickly.

Voltage drop sits alongside those two rather than replacing them. A cable can be perfectly adequate on ampacity and still be the wrong choice for a long run, which is the situation this calculator exists to catch.

Working the drop out

Ohm's law is the whole basis:

$$V_{\mathrm{drop}}=I·R$$

where I is the current in amperes and R is the resistance of the wires in ohms.

Wire resistance is normally published per unit length, in ohms per kilometre or ohms per 1000 feet, and the current has to travel out and back. So for a single-phase or direct-current circuit:

$$V_{\mathrm{drop}}=2·I·R·L$$

and for a three-phase circuit:

$$V_{\mathrm{drop}}=\sqrt{3}·I·R·L$$

where L is the one-way length. Three phase uses root three rather than two because the return current is shared between phases rather than travelling back along a dedicated conductor.

On the NEC tab there is one more step, because alternating current sees impedance rather than plain resistance. The effective figure at a given power factor is:

$$Z_e=R·\mathrm{cos\theta }+X·\mathrm{sin\theta }$$

Take 3 AWG copper in steel conduit: the code gives 0.25 ohms and 0.059 ohms of reactance per 1000 feet, so at a power factor of 0.85 the effective impedance is 0.2436 ohms per 1000 feet, or 0.799 per kilometre. Over a 500 metre single-phase run at 1 amp that is a drop of 0.80 volts, which is 0.67% of a 120 volt supply.

Which tab to use

NEC data is the one to use for a real installation in North America. It accounts for reactance and for the conduit material, since steel conduit raises the reactance noticeably against PVC or aluminium.

Estimated resistance is the quick answer, based only on the conductor's cross-section and the resistivity of the metal. It ignores reactance, temperature and conduit, so it reads lower than the NEC figure for the same wire. Useful for direct current, for low power factors, and for sanity checks.

Other is for working to a datasheet. Manufacturers publish impedance per kilometre for their own cables, and other standards tabulate different values, so this tab takes the figure directly in whichever unit it was published in.

Typical AWG wire sizes

American Wire Gauge is the system used predominantly in North America for the diameters of round, solid, non-ferrous conducting wire. The resistances below are for copper at 20°C and are computed from the gauge geometry rather than looked up, so they follow the definition exactly.

Common mistakes

The first is entering the round-trip length. The distance field wants the one-way run; the calculator doubles it for single-phase and direct current.

The second is using resistance where impedance belongs. On a long alternating-current run at a poor power factor, reactance is a real part of the drop, and the estimated-resistance figure will read optimistically low.

The third is forgetting temperature. Published resistance is quoted at a reference temperature, 20°C for the geometric figures here and 75°C for the NEC table. Copper resistance rises roughly 0.4% per degree, so a conductor running hot in a loft drops more than the table suggests.

The fourth is treating voltage drop as the only test. Ampacity and fault protection come first; a cable that satisfies the 3% guidance can still be undersized for its load.