by Dick Statham

Puzzled by voltage drop?
*What is the voltage drop on a 100 ft. long, 30 amp circuit?
*What is the maximum distance of a 20 amp circuit before voltage drop becomes a problem?
*On any circuit, what size wire is required to minimize voltage drop?

See how a major wire manufacturer provides the answers...

Click here to see the Voltage Drop Calculator from Southwire®.

Enter the following values and select "calculate".

A #6 conductor is needed to supply a 30 amp load which is 100 feet away from the source. Without voltage drop, a #10 conductor would carry the load. A larger conductor has less resistance to current flow and therefore less voltage drop.

The NEC recommends keeping voltage drop to 3% on branch circuits and feeders and not more than 5% for combined feeder and branch circuits (210.19 & 215.2). Keeping voltage drop within these limits is not enforceable, because it is discussed in Fine Print Notes.

The higher the voltage is, the less the voltage drop. Double the voltage on the above circuit to 240 volts.

The wire size can drop from a #6 to a number #10 AWG because of the higher voltage.

Since a 3-phase circuit delivers more power than a single phase circuit at the same voltage and current, there is less voltage drop with 3-phase circuits than single phase circuits. Notice the difference in wire size in the following example:

The Southwire® calculator can also be used to determine the maximum distance of the load from the source. Try these values:

Beyond the calculated distance, the voltage drop would be greater than 3%.

 

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When selecting wire size there are many considerations besides voltage drop. Ambient temperature, numbers of conductors in conduit, continuous loading, and the temperature rating of terminals must all be taken into account.

Too much voltage drop causes lights to dim and appliances to run below their rated wattage. Motor loads are especially sensitive. At 10% below rated voltage motors have less starting and running torque. The temperature rise on a motor is greater when the voltage is reduced. When operated below their rated voltage, motors pull higher than normal currents which can overheat windings and damage the insulation.

Try other examples using the Southwire® Calculator to see the effects of voltage, phase, conductor type, distance and current on voltage drop.

 

About the author:

Dick Statham is a former instructor and department chairman in the Electrical and Electronic department at Guilford Technical Community College in Greensboro, NC. He is also a contributor to JADE Learning's 2005 and 2008 NEC Changes courses.

Dick Statham (in rear).