voltage drop

Voltage Drop is the loss of voltage that occurs due to the resistance or impedance through the conductor in an electrical circuit.

Items to consider when reducing voltage drop is decreasing the current load, increasing wiring size or decreasing the length of the conductor. The current load location and size for most circuits are fixed so reducing the current is usually not an option. Increasing the conductor size or shortening the conductor run are typical ways to lower it. Another item to consider when increasing conductor sizes is the lug or terminal size of the feed and load devices.

Recommendations for voltage drop percentages issued by the National Electrical Code (NEC) are as such:

  1. The maximum voltage drop on the feeder or branch circuit should not exceed 3%.
  2. The maximum combined voltage drop for both the feeder and branch circuit should not exceed 5%.

See the following NEC Articles: 210.19(A), 215.2(A)(4), 230.31(C), and 310.15(A)(1) for more information.

There are a number of on-line voltage drop calculators that can assist the engineer with calculations. Here is a link to one that the Electrical Engineering team at Pearl has used.

Below is a practical example of controlling voltage drop:

We recently encountered a 240-volt single phase power distribution network where the feed devices were 20-amp circuit breakers and the loads were each 10-amp. Typically a #12 AWG wire @ less than 100’ would be adequate for this circuit but because of the long wire lengths the load location needed to be considered. Per the NEC recommendation of a voltage drop less than 3%, the wire length from the feed to the load needed to measure 573’ or less. Some of our feed to load distances were over 1000’ which would require a #6 AWG wire to meet the less than 3% voltage drop recommendation. The issue with this size wire is that it would not fit under the 2 pole Square D 20-amp circuit breaker lug. The largest size wire that this circuit breaker lug can accommodate is a #8 AWG wire. Since the load positions and wire size could not change, the only variable that could be changed were the wire lengths. Our successful solution was to add two distribution panelboards and install them in a central location so the feed to load distance would be less than the 573‘ recommendation.

Pearl Engineering is proud of our culture of: ‘Creating Innovative Solutions to Complex Problems’. Please let us know if we can help you with your Electrical Engineering and Designer needs.