When properly applying the National Electrical Code to electric motor installations, it is important to determine if Article 430 or 440 must be applied. The differentiation is related to the motor in each application.

If the motor operates in the refrigerant, then it is a hermetic motor refrigerant compressor and Article 440 applies. If the motor does not, then Article 430 applies to the installation. This article will focus on 430.

Safe wiring installations of electric motors and motor circuits depend on understanding some basic requirements in Article 430 of the NEC, particularly those in Section 430.6(A) for general motor installations. Instead of the nameplate full-load current, Section 430.6(A) requires the tables in Article 430 be used to size circuit conductors, branch-circuit short-circuit and ground-fault protective devices, and ampere ratings of disconnect switches. The actual full load current for different motors of the same size and type may vary.

The tables are used to ensure that if a motor needs to be replaced, the components of the motor circuit will not also need to be replaced. This requirement applies to general motor applications. There are specific rules for torque motors and alternating-­current adjustable voltage motors, which are different. The actual nameplate current is used to size these circuit components. This article deals with requirements for general motor applications.

To achieve proper motor and motor circuit overcurrent protection, a systematic approach usually works best. Three important elements of the installation typically include (1) the branch-circuit sizing (conductors), (2) the overload protection and (3) the branch-circuit short-circuit ground-fault protective device. These three certainly are not all-inclusive as installations differ.

Motor nameplate information is important because nameplate voltage and horsepower ratings are needed to use the tables in Article 430. The horsepower rating at the applied voltage is used with the appropriate table to determine the full-load current rating of the motor. This value must be used to size the conductors and the branch-circuit short-­circuit and ground-fault protective device.

Motor branch-circuit conductors

Part II of Article 430 covers the requirements for sizing the branch-circuit conductors for single and groups of motors. As an example applying Section 430.22, a single motor used in a continuous-duty application (three hours or more as defined) is required to have an ampacity of not less than 125% of the motor full-load current as determined by Section 430.6(A)(1).

Branch-circuit short-circuit and ground-fault protection

Branch-circuit short-circuit and ground-fault protective devices must generally be sized in accordance with the values given in Table 430.250. The branch-circuit short-circuit ground-fault device sizing requirements are located in Part IV of Article 430. Section 430.51 indicates that the rules included in Part IV amend or add to the requirements of Article 240.

A few different types of branch-circuit short-circuit and ground-fault protective devices may be used for protecting motor branch-circuit conductors, motor control apparatus and the motor against overcurrent due to short-circuits or grounds. Section 430.52(C) requires that the rating of the protective device must not exceed the value calculated according to the percentage values given in Table 430.52. 

Higher percentages are permitted to account for starting current (inrush or locked rotor current) of motors.

Overload protection

Overload protection devices are intended to protect motors, motor-control apparatus and motor branch-circuit conductors against excessive heating due to overloads and failures to start. An overload in an electrical motor circuit is an operating current that, if it persists for a sufficient length of time, would cause apparatus damage or dangerous overheating. 

Overload protection does not include protection against short circuits or ground faults. The combination of the overload protective device and the branch-circuit short-circuit ground-fault protective device provide the total overcurrent protection for the motor and motor circuit.

Overload protection for motors can be provided in a few different forms. If the motor itself is thermally protected, it is required to be marked with the words “Thermally Protected” or “TP.” If the motor is not marked as such, then overload protection must be installed.

The total overcurrent protection for a motor, motor branch-circuit and motor control apparatus is provided by the combination of the motor branch-circuit short-circuit and ground-fault protective device (fuses, circuit breakers or motor circuit protectors) in accordance with Part IV of Article 430, used in combination with an overload protective device meeting the requirements of Part III of Article 430. 

The Code also references Example D8 in Appendix D, which greatly assists inspectors and installers in understanding and applying the requirements for overcurrent protection of motors and motor circuits.

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