Electrical equipment that operates below its voltage rating can overheat or power down. Here's how to determine your load's operating voltage.


Voltage drop across switchgear and feeder and branch circuit conductors can quickly reduce the output voltage of the power supply to an unacceptable limit. Although UL doesn't specifically require equipment manufacturers to specify an acceptable operating voltage range for a given piece of equipment, manufacturers typically recommend that the electrical circuit operate at no less than 90% of the equipment voltage rating. But why should you concern yourself with low-voltage situations? Because operating electrical equipment outside of its acceptable voltage rating can lead to premature equipment failure and hazardous situations.

Inductive loads like motors and ballasts can overheat, shortening equipment life and increasing energy consumption if they operate below their voltage rating. In addition, undervoltage can cause sensitive electronic equipment like computers, PLCs, and copy machines to lock up or suddenly power down. This can result in lost data, increased production costs, and the increased probability of equipment failure due to excessive heat.

Resistive loads that operate at less than their rated voltage simply won't provide the rated power output. For example, a 10kW heater rated 230V will provide less than 8.2kW of power (work) at 208V (P5E24R). This may not damage the equipment or endanger your workers, but by causing the process to not function as originally designed, it could delay production and raise production costs. Reduced circuit voltage can cause incandescent lighting to flicker when other appliances, office equipment, or heating and cooling systems are cycled on.

You can determine the actual operating voltage for a load by subtracting the conductor voltage drop from the open circuit voltage. For example, you can determine the voltage drop of the circuit conductors for a single-phase load by multiplying the current of the circuit by the total impedance of the circuit conductors.

VD (single-phase) = I x Z x 2

VD (3-phase)= I x Z x 1.732

In this equation, I is the load in amperes and Z is the impedance of the conductor as listed in Chapter 9, Table 9 of the NEC.

The NEC contains many specific and detailed requirements on the topic of voltage drop. For example, you should also take into consideration the presence and operational effectiveness of additional circuit components like buck-boost transformers and power factor correction capacitors. Remember that while economics affect all design decisions, the goal is to ensure that the equipment is supplied with a voltage that meets the equipment manufacturer's requirements.

Are you still confused by the Code? For additional information on Code-related topics please visit www.mikeholt.com or send an e-mail directly to the author at mike@mikeholt.com.