Calculation Results


What exactly is a voltage drop? As an electrical current travels through a wire, voltage (also referred to as the electrical potential) pushes it through. The trouble is, the wire creates contrary pressure that “pushes back,” so to speak. For the current to travel through successfully, it has to surpass that contrary pressure.

When there is an alternating current, the contrary pressure the wire creates is known as impedance (think of impeding something). This is a two-dimensional measure also called a vector; the impedance has both resistance and reactance, the latter being the reaction that occurs when a built-up electrical field encounters a change of current. When there is a direct current, then we simply call the contrary pressure resistance.

A voltage drop, then, is the amount of voltage loss that the contrary pressure in the wire generates, and excessive voltage drops can result in some distinct problems. For example, a motor could run hotter than it usually does, ultimately burning out well before it ever should have. Some other examples of problems stemming from excessive voltage drops are heaters performing poorly and lights flickering or dimming instead of burning steadily and brightly.

Electrical experts recommend that in a circuit with full load, a voltage drop should be under 5%. That’s possible to do by using the right wire for the job or by making sure to keep extension cords and similar items in good condition. And that’s where our AC Voltage Drop Calculator comes in handy.

  • NEC Data: With this tab, the user can calculate an estimated voltage drop by using resistance and reactance date that comes straight from the National Electric Code, or NEC.
  • Estimated Resistance: Using resistance data based on the size of the wire is how this tab determines an approximate voltage drop.
  • Other: Sometimes the source of data is alternate standards or the wire manufacturer. When the user has such customized figures for either impedance or resistance, this tab helps find the needed estimated voltage drop.


  1. Length of the run should only account for distance between the source and the load. Length will be multiplied by 2 for single phase and square root of 3 for three phase automatically.
  2. Calculations are based on voltage drop formulas found in IEEE Std 141 (Red Book).
  3. NEC Informational Note suggests a maximum voltage drop of 3% for branch and feeder circuits with combined voltage drop of feeder and branch circuits limited to 5%. It is important to note that the individual branch or feeders will need to be less than 3% to meet the 5% limit.