When I was new to the electrical construction industry, I worked with a journeyman electrician on a job site who asked me to connect a new electric motor in the building’s electrical equipment room. I told him I needed to get my National Electrical Code book from the truck to install the motor correctly. He told me to use the motor nameplate full-load ampacity (FLA). I explained that, unless it was a specialty motor, I needed the full-load current (FLC) values from NEC tables 430.247, 430.248, 430.249 or 430.250. He insisted I use the nameplate FLA to size the conductors, circuit breaker—the branch-circuit short-circuit and ground-fault protective device—and any switches in the motor circuit. 

I followed his instructions, but when the electrical inspector arrived, his first question was whether I had used the nameplate FLA or the NEC table FLC values. When I told him I used the FLA, he told me to review NEC 430.6(A)(1), which requires using the FLC values from NEC tables 430.247 through 430.250, and he said he would return after lunch to inspect the installation. Upon review, the installation was acceptable, but it was a good lesson for a new electrician to not necessarily listen to someone you work with when you know better.

So, what’s in the code?

The requirements in NEC 430.6(A)(1) and 430.6(A)(2) provide us with clear information on how to calculate motor circuits. Article 430 uses FLC based on the values given in NEC tables 430.247, 430.248, 430.249 and 430.250, and FLA on the nameplate of the motor for overload protection. 

The table values must be used, rather than the actual current rating marked on the motor nameplate, to determine the ampacity of conductors, current ratings of switches and current ratings of branch-circuit short-circuit and ground-fault protection. FLA is to be used only for overload protection unless the nameplate ampacity value on the motor is larger than the NEC Table 430.247, Table 430.248, Table 430.249 and Table 430.250 values. 

The tables are used due to the availability of different power factors for the same size horsepower motor. For example, a new 10-horsepower motor could be purchased with a 60% power factor (not usually available anymore due to efficiency requirements) but are also available in 80% or 90% for single-phase motors and are available exceeding 90% in three-phase motors. If the nameplate FLA was used initially for sizing the conductors, the motor disconnecting means and the branch-circuit short-circuit and ground-fault protective device, any replacement motor would require the same power factor or better.

Section 430.7 provides the marking requirements for motors and multiple motor applications. The manufacturer’s name, the rated voltage and FLA, the rated frequency and number of phases for AC motors, the rated temperature rise or insulation class and the rated ambient temperature are extremely important markings so an installer can match the motor to the conditions at the location where the motor will be installed. 

The time rating on the motor nameplate indicates the duration the motor can operate safely at its rated load without overheating. Many motors are rated for continuous use and are so marked, but if the marking is for intermittent use, such as 5 minutes, 15 minutes, 30 minutes or 60 minutes, then ensuring the motor is used for the maximum time for safe temperature limits will prevent damage and provide reliability for the installation. 

In addition, NEC 430.22(E) and Table 430.22(E) provide the current-rating percentages for sizing the motor supply conductors based on the time rating shown on the nameplate to the motor and the classification of service (how the motor will be used). Conductors used for short-term, intermittent, periodic or variable-duty applications must have an ampacity of not less than the percentage of the motor nameplate current rating shown in NEC Table 430.22(E). 

For example, the conductor sizing for a motor used for raising or lowering rolls of paper at the printing press at a book publishing company is based on 110% for a 5-minute-rated motor, 120% for a 15-minute-rated motor and 150% for 30- or 60-minute-rated motors based on the length of time the motor will be used. This ensures the proper conductor size based on the motor’s use.

Next month, I will cover the code letter requirements, how to determine the locked-rotor values and additional information on new motor designs and new NEC Table 430.251(A) that provides single-­phase locked-rotor currents; NEC Table 430.251(B) that provides locked-rotor currents for three-phase for Design B, C and D motors; and NEC Table 430.251(C) that provides locked-rotor currents for three-phase Design BE and CE motors. 

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