Motor-control circuits may be very low voltage; they may be in the 12- to 24-volt (V) alternating current (AC) or direct current (DC) range or as high as 600V. The type of control system used, the source of that system, and the size of the control system will be dictated by the number and types of control devices that connect to the control circuit. Holding coils, pilot lights, relays, solenoids, small torque motors, control transformers and other control devices connected to the circuit must be figured as load for the control circuit. Both inrush current (pull-in current, the amount of current necessary to pull contacts together) and holding current (the amount of current necessary to hold the contacts together after initial pull-in or the sealed position of the contacts) must be calculated for most control-switching devices. What these and other load values are and how long the current is present in the system will determine the size of the overcurrent device protecting the circuit, the size of the control transformer and the size of the control conductors.
Where a control transformer—as permitted in 430.72(C) in the 2011 National Electrical Code (NEC)—or a control coil is used, the designer or installer must first understand that there is inrush current involved when the transformers or coils are energized. The initial energizing of the transformer or the coil is usually relatively quick, about 5 to 20 milliseconds, but can be from five to 12 times the normal holding current or sealed position of other control devices. This transformer current, as well as the pull-in current or holding current, whichever is higher, must be used for sizing any overcurrent protection provided for the control circuit. Section 430.72(C)(4) states, “Where the control circuit transformer rated primary current is less than 2-amperes, an overcurrent device rated or set at not more than 500 percent of the rated primary current shall be permitted in the primary circuit.” This oversizing of the protective device permits the pull-in current plus any transformer or coil inrush current to occur without opening the protective device.
The de-energizing or dropout time for many motor-control devices can be as little as 7.5 to 12 milliseconds, depending on the type of device. Some devices incorporate a spring to help return the device to the open or closed position, depending on the application. Manufacturer data sheets are available that show electrical specifications of particular control devices, such as motor starters, electrical relays, solenoid-actuated valves or similar devices. Larger starters, relays or transformers will have much higher coil inrush current than smaller devices. The amount of energy necessary to hold the contacts together (called sealed volt-ampere load or sealed VA) or to maintain the coil’s magnetism is usually minimal. Remember that each manufacturer’s coil may have different inrush and sealed VA based on the characteristics of the coil used in the device.
A small size-zero starter has about 192 VA of inrush load with about 29 VA of sealed VA when the contacts are closed. A size-five starter might have 1,950 VA inrush with approximately 98 VA sealed load. These figures will obviously vary with the particular manufacturer of the device, so before you figure the load of a particular control circuit, always check the manufacturer’s specification sheet for all devices. Since the normal spec sheet will provide the VA value for both inrush and sealed VA, to convert these values into ampacity, divide the VA by the voltage of the control circuit, and the result will be the ampacity or the current of both sealed and inrush values.
For example, if the control voltage is 120 and a size-five starter is used, the inrush current would be 1,950 VA divided by 120V or 16.25A. The holding current would be 98 VA divided by 120V or 0.81A. If both the inrush VA and the sealed VA are not taken into account, the result could be a hot control transformer that could fail, or the life expectancy could be dramatically reduced.
For a control transformer supplying multiple starters, calculate the total inrush VA and the total sealed VA for all three starters, and use the larger value of the two.
For example, where other devices are connected into the control circuit, these values are added to the overall totals above. By carefully calculating both the inrush and holding current of each control device, a safe and reliable control circuit will be provided.
ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and email@example.com.