Voltage Drop Calculator

This tool estimates the voltage drop across an electrical circuit. The "NEC data" pane uses resistance and reactance values taken from the National Electrical Code. The "Estimated resistance" pane works with resistance approximated from the selected wire gauge. Choose the "Other" pane to input your own resistance or impedance numbers, for example values from other codes or manufacturers.

When current flows through a conductor it is driven by voltage and must overcome the opposition presented by the wire. The resulting loss of voltage is called voltage drop. For AC the opposition is termed impedance, a vector composed of resistance and reactance (the reaction of an electric field to changes in current). For DC the opposition is simply resistance.

There are four major causes of voltage drop:

The conductor material is the first variable. Silver, copper, gold and aluminum rank highest in conductivity, but copper and aluminum dominate the market because they are far cheaper than silver or gold. Between them, copper conducts better and therefore yields a lower voltage drop for a given length and size.

Length is another key contributor. Short runs lose less voltage than long runs of the same size. Voltage‑drop concerns usually don’t arise in typical house wiring, but they become significant when running cable to outbuildings, wells, or other distant equipment.

The magnitude of the current itself influences the drop – more amperage means a larger loss. The term ampacity describes the maximum current a conductor can safely carry, essentially the amount of charge that can be pushed through at once.

Several variables dictate a wire’s ampacity. The base material is primary, while the frequency of an AC signal can also affect it. Operating temperature is another critical factor.

When multiple cables are grouped together they generate more heat, which in turn reduces both ampacity and increases voltage drop. Because of this, codes impose strict limits on how cables may be bundled.

Choosing a cable follows two main rules. First, it must be capable of handling the expected load without exceeding its temperature limits, even under the most extreme conditions it will face. Second, it must provide a reliable grounding system that (i) keeps exposed voltages within safe bounds and (ii) lets a fault current trip protective devices quickly.

Voltage drop calculation

Ohm's Law is a very basic law for calculating voltage drop:

V_drop = I·R

where:

Wire resistance is usually expressed as a length‑dependent value, commonly quoted in ohms per kilometre or ohms per thousand feet. Because a conductor’s current travels out and back, the total resistance accounts for the round‑trip length. This leads to the standard equation used for single‑phase or direct‑current circuits:

V_drop = 2·I·R·L

The formula for a three-phase circuit becomes:

V_drop = √3·I·R·L

where:

Typical AWG wire sizes

The American Wire Gauge (AWG) system is the primary method in North America for specifying the diameters of solid, non‑magnetic conductors. Below you’ll find a table showing typical AWG sizes and their corresponding dimensions: