A 480V 3phase squirrelcage motor is a common piece of equipment in many facilities

Motor Nameplate Current vs. NEC Full Load Current Values

May 23, 2017

Let’s explain the differences between the two options.

When sizing circuit breakers, fuses, and conductors for a general AC motor branch circuit, National Electrical Code (NEC) Sec. 430.6(A)(1) stipulates that the currents indicated in the current tables at the end of Art. 430 must be used to determine the ratings instead of the actual nameplate current rating. Do you know why we can’t use the nameplate rating of motors when making these calculations? If we did, it would typically result in smaller branch-circuit, short-circuit, and ground-fault protective devices and conductors.

The full-load current values for NEMA motor horsepower ratings found in Tables 430.248, 430.249, and 430.250 are typically higher than the nameplate value found on the actual motor. The table value ensures that no matter what motor is installed, the conductor and branch-circuit, short-circuit, and ground-fault protective devices are sized to safely operate the motor.

For example, the nameplate rating of a 480V, 3-phase, 50-hp motor may only be 60A, taking into account the lowest running power factor and efficiency level. However, the NEC table value is 65A. The table values account for a power factor (PF) and efficiency level somewhere less than 100% (among other characteristics) between different motors. Remember, a motor’s PF and efficiency will vary with the mechanical load applied. An under-loaded squirrel cage motor has a lower PF; whereas, a highly loaded motor has a higher PF.

To illustrate this concept, let’s take a 50-hp motor at 100% PF and 100% efficiency and solve for current.

Beta Ecmweb Com Sites Ecmweb com Files Motor Nameplate Equation 1

A quick check of the NEC table, however, indicates that we should be using a value of 65A.

Why is the NEC value nearly 20A more than our calculated value? Let’s run through the calculation again using a 0.85 PF and a 0.85 efficiency.

Beta Ecmweb Com Sites Ecmweb com Files Motor Nameplate Equation 2

As you can see, this result is much closer to the NEC table value. Do you know why? This tells us that the current values shown in the NEC tables already factor in some level of PF and efficiency.

While branch-circuit, short-circuit, and ground-fault protective devices and conductors must use the NEC table current values, thermal overloads should be calculated using actual nameplate data.

About the Author

Eddie Guidry | Senior Fellow

Eddie Guidry is a senior fellow with Fluor Enterprises, Inc., Sugar Land, Texas. He is highly skilled in electrical and control systems for industrial construction, design, and engineering (both domestic and abroad). The majority of Eddie’s 38 years of experience includes upstream and downstream portions of petrochemical plants and refineries. He is also heavily experienced in water and wastewater treatment facilities. Eddie is very active in the development of U.S. national (ANSI) and international codes and standards, and has been a principal member of the National Fire Protection Association (NFPA) National Electrical Code (NEC) committee since 1999 and NFPA’s National Advisory Committee on Electrical Safety Research since 2008. Eddie, who has also developed and conducted many electrical courses and seminars over the years, currently holds the corporate Master Electrician license for Fluor Corporation in the State of Texas and is also an ICC/IAEI certified electrical inspector.