What is the acceptable range of rate voltage readings at the electrical motor?

• A. 5% above 5% below
• B. 5% above 10% below
• C. 10% above 5% below
• D. 10% above 10% below

Answer: (C)

Voltage levels at a motor should stay within a defined window around its rated value to ensure proper torque, speed, and thermal performance. The standard tolerance used in most industry practice is +10% / -5%. That means readings can be up to 10% higher than the rated voltage or up to 5% lower.

So, the acceptable range is from 95% to 110% of the nameplate voltage. For example, a motor rated at 460 V should be read between about 437 V and 506 V; a 230 V motor would be acceptable between about 218.5 V and 253 V.

If voltage falls below the lower limit, the motor may draw more current to try to deliver the same torque, leading to overheating and potential overload. If voltage rises above the upper limit, insulation and winding stress increase, which can shorten life or cause tripping. The specified range balances reliable operation with protection of the motor.