Measuring Voltage Drop

Voltage drop under typical operating conditions can easily be measured. If excessive voltage drop is suspected in a circuit, follow these steps:

1. Turn on all the electrical equipment which is normally in operation at the time excessive voltage drop is suspected to be a problem.

2. Measure the voltage at the service panel that supplies the circuit in question. It should be 234 volts or more between hot conductors and 117 volts or more between hot and neutral of a 120/240 volt, single phase system (maximum of 3% voltage drop on service drop). If not, call the utility.

3. Measure the voltage at the service panel board with the problem circuit. It should be 227 volts or more between hot conductors and 113.5 volts or more between hot and neutral of a 120/240-volt, single phase system (maximum of 3% voltage drop on feeders, 2% maximum recommended).

4. Measure the voltage at the problem piece of equipment. It should be 220 volts or greater between hot conductors of a 240-volt circuit or 110 volts or greater between hot and neutral of a 120-volt circuit (maximum of 3% voltage drop on the branch circuit back to the service panel board).

Results
If a problem with the voltage exists at the main service - call the utility .

If voltage at main service was fine but low at service panel - check feeder from main service for problems.

If voltage at service panel was fine but low at outlet or controller, check branch circuit for problems.

Remember
A 480 volt circuit should be a minimum of 480 volts at the transformer secondary and a minimum of 440 at the equipment outlet or controller.

A 240 volt circuit should be a minimum of 240 volts at the transformer secondary and a minimum of 220 at the equipment outlet or controller.

A 120 volt circuit should be a minimum of 120 volts at the transformer secondary and a minimum of 110 at the equipment outlet or controller.