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Understanding Voltage Drop: NEC Requirements & How to Calculate It

SignedBid Team2 min read
voltage-dropNECwiring

When wiring a new circuit, particularly one that spans a long distance, it’s not enough to simply consult an ampacity chart and choose a wire size. As electrical current travels through a conductor, the natural resistance of the copper or aluminum causes some of the electrical energy to be lost as heat. This loss of voltage over distance is known as voltage drop.

Why Voltage Drop Matters

If voltage drop is too severe, the equipment at the end of the line won't receive the voltage it was designed for. This can lead to sluggish motors that overheat, flickering lights, and sensitive electronics failing to operate properly. A significant drop in voltage causes the equipment to pull more amperage to compensate, which further drives up heat.

The NEC Recommendation

While the National Electrical Code (NEC) is primarily focused on safety rather than efficiency, it does provide clear guidance on voltage drop. According to an informational note in NEC 210.19(A), branch circuit conductors should be sized to prevent a voltage drop exceeding 3% at the farthest outlet. Furthermore, the combined voltage drop for both the feeder and the branch circuit shouldn't exceed 5%.

For example, on a standard 120V circuit, a 3% drop means you can lose up to 3.6 volts. The equipment at the end of the line should still receive at least 116.4V under full load.

How to Calculate Voltage Drop

The basic formula for single-phase voltage drop is:

VD = (2 × K × I × D) / CM

  • K = Specific resistance (often 12.9 ohms for copper and 21.2 for aluminum)
  • I = Current in Amperes
  • D = Distance in feet one way
  • CM = Circular Mils of the wire (found in NEC Chapter 9, Table 8)

For three-phase systems, you use a slightly different multiplier (1.732 instead of 2).

The Easy Way

Calculating this by hand for every long run is tedious. That's why we built our Voltage Drop Calculator tool. Simply input your voltage, current, material, and distance, and it will automatically calculate the drop and recommend the correct wire size to keep your installation compliant and functioning properly.