As manufacturers begin to see the value of plug-and-play applications, it has become common for users to run into requirements for plugs on motors. Full load amps (FLA) are mentioned at the beginning of each conversation between OEMs and users. It is time to start providing the voltage and horsepower (hp) and forego shortcutting to FLA.
It is understandable why specifiers simply provide the FLA; when supplying many types of equipment for the motor branch circuit, users need only know the full load current to specify ratings of the equipment. The National Electrical Code (NEC) states:
Motor circuit conductors—430.22—shall have an ampacity of not less than 125% of the FLA.
Motor overcurrent protective device (OCPD)—430.32—shall be rated at no more than the following percent of the FLA.
Motor ground fault interrupter (GFI)—430.52—shall comply with 430.52(B) (starting current) and 430.52(C) (FLA chart) or 430.52(D) (torque motor nameplate), as applicable.
Motor controller—430.82—shall have a rating as specified by 430.83(A) (hp rating, molded case circuit breaker [MCCB] OCPD rating, switch rating), unless otherwise permitted in 430.82(B) (>1/3 hp motors) or (C) (FLA rating) or as specified in (D) (torque nameplate rating), under the conditions specified.
Disconnecting means—30.110—shall have an ampere rating no less than 115% of the FLA.
Now for those that understand the NEC, there is more to specifying devices than the general rules shorthanded above. Those responsible for providing the equipment to the specifier are not always aware of all the intricacies of the application.
This often leads to shortcuts being taken during the specification process. A common one with plugs and receptacles is “the rating is 125% of the FLA.” A best guess of where that shortcut came from is that the majority of plugs and receptacles are specified around the requirements of the wire, and wire “shall have an ampacity not less than 125% of the FLA.”
But a plug and receptacle combination is not a wire. Granted, when the device is assembled, it operates much like wire but with one big difference: wire cannot be broken and made, over and over again. Therefore, there are additional requirements when specifying a plug and receptacle in a motor branch circuit. Those requirements stem from where the plug and receptacle will be in the motor branch circuit and how it will be operated.
If the plug is going to be located on the motor in a general purpose branch circuit but not used while the circuit is energized, then there is a reference to how to specify the OCPD that confusingly provides guidance as to how the cord connector should be specified.
Part III: Motor & Branch-Circuit Overload Protection
430.42 Motors on general-purpose branch circuits—Overload protection for motor used on general-purpose branch circuits as permitted in Article 210 shall be provided as specified in 430.42(A), (B), (C) or (D).
(C) Cord-and-plug connected—The rating of the attachment plug and receptacle or the cord connector shall determine the rating of the circuit to which the motor may be connected, as provided in 210.21(B).
210.21(B) Receptacles—A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit. Exception No. 1: A receptacle installed in accordance with 430.81(B) (<1/3 hp motors).
Following the logic of the code, the requirements are circular. The rating of the plug and receptacle determines the requirement of the OCPD, and the rating of the receptacle is determined by the rating of the branch circuit, which is determined by the OCPD rating. In these situations, default to the code that helps determine the OCPD for the application, which was already covered earlier in Part III.
If users are asked to help specify a plug and receptacle for this application, they would want to know what was specified for the OCPD upstream.
However, the more common application of providing a plug and receptacle in a motor branch circuit is to allow it to be used as another device in the motor branch circuit, the disconnecting means.
If the plan is to install the plug and receptacle as the disconnecting means, it is advisable to seek out another section of the code.
Part IX: Disconnecting Means
430.108 Every disconnecting means—Every disconnecting means in the motor circuit between the point of attachment to the feeder or branch circuit and the point of connection to the motor shall comply with the requirements of 430.109 and 430.110.
430.109 Type—The disconnecting means shall be a type specified in 430.109(A), unless otherwise permitted in 430.109(B) through (G), under the conditions specified.
(F) Cord-and-plug connected motors—For a cord-and-plug connected motor, an hp-rated attachment plug and receptacle, a flanged surface inlet and cord connector, or attachment plug-and-cord connector having a rating no less than the motor ratings shall be permitted to serve as the disconnecting means.
430.110 Ampere rating & interrupting capacity—The disconnecting means for motor circuits rated 1000 volts (V), nominal or less, shall have an ampere rating not less than 115% of the full-load current rating of the motor.
This could bring about more confusion. This section of the code points to two different ratings to determine the rating of the plug and receptacle disconnecting means: motor ratings (broad term) and 115% FLA. The rating depends on the specifier’s interpretation. Because there are three types of connectors that could be used in this application, each of these devices must have equivalent ratings or at least have ratings determined by the motor manufacturer to be safe. Plugs and receptacles having hp ratings have been required to pass a test in which the plug and receptacle is cycled 50 times at the locked rotor condition of the motor.
Flanged surface inlets are likely to be supplied by the motor manufacturer and tested as a unit before it is sold. The remaining type of plug and receptacle needs to be rated per the motor ratings, which should include locked rotor amperes since it is a specification on the motor’s nameplate.
If a user is going to use 115% of the FLA to specify the plug and receptacle, they will likely miss the locked rotor amperes to be a type of disconnecting means. Users also will likely not have the appropriate ratings to meet the OCPD rating as described in the first specification when the device is not being used as a disconnecting means.
At the end of the day, most specifiers are providing the FLA of the motor to the manufacturer because they expect their representative to be informed on the details of the applications in order to help them specify the appropriately rated device, and they want it to be simple.
Unfortunately, there are lots of details that are hard to remember when working with plugs and receptacles in motor branch circuits. This can lead to a requirement for a plug and receptacle where the only information provided is the FLA of the motor being specified as the rule of thumb at 125%, but in the worst case to the rating of the FLA.
A simpler option for specifiers is if they are looking for a plug and receptacle for a motor application, to provide the hp and voltage of the motor. This will ensure that the right solution is specified.