NEC Requirements

The National Electrical Code (NEC) requires motor overloads to be sized according to the motor's nameplate full-load amps (FLA). However, it does not explicitly address the case of a missing nameplate.

Industry Practice

In the absence of a nameplate, it is common industry practice to estimate the FLA using one of the following methods: Table FLA

Use the FLA value specified in NEC Table 430.247 for the motor's horsepower, voltage, and speed. Multiply the FLA by 1.15 to account for a 15% safety factor.

Measured Current

Use an ammeter to measure the current drawn by the motor at full load. This method is more accurate but requires special equipment and expertise.

Recommended Approach

It is advisable to consult with the motor manufacturer or a qualified electrician to determine the appropriate overload size. However, if this is not possible, the following approach is recommended:1. Use Table FLA

Multiply the NEC Table FLA value by 1.15.
2.

Consider Actual Current

If the motor has been previously operated and its actual current is known, use this value to calculate the overload size. However, be sure to consider any potential load changes or aging effects that may have altered the current draw.3.

Safety Factor

Always include a safety factor when sizing overloads to account for variables such as temperature, voltage fluctuations, and aging. A 15% safety factor is common.

Example Calculation

For a 5 horsepower, 460-volt, 1800 rpm motor with a missing nameplate, the estimated FLA using NEC Table 430.247 is 6.2 amps. Multiplying by a 15% safety factor gives an overload size of:

6.2 amps x 1.15 = 7.13 amps

Therefore, a motor overload with a setting of 7.13 amps would be appropriate. Flag for review