Overcoming Junction Box-to-Motor Mounting Deficiencies

Overcoming Junction Box-to-Motor Mounting Deficiencies

Poor J-box connections violate NEC requirements and create a fire and safety hazard.

When swapping out a replacement motor, it’s tempting to unbolt the J-box (junction box/terminal box/ T-box) from the motor and leave it attached to the conduit. The problem with doing so is that the motor manufacturers have never standardized the bolt-hole spacing for the J-box. If you are installing a “Brand A” motor in place of “Brand B” — even though the frames are exactly the same — this common shortcut puts you at risk of an explosion or fire.

This is the type of damage that can occur due to improper grounding. Arcing between this box and the motor frame it was connected to eventually caused a fire.

The problem

Electrical service centers frequently receive motors for repair without the J-box, which means the connection box is still “out there” in your plant. When someone’s in a hurry, and the J-box bolt holes don’t match those on the replacement motor, it’s not uncommon to see a J-box attached by a single bolt. Besides the risk of water intrusion into the motor (and subsequent failure), this increases the chance of a ground failure due to unreliable ground fault protection.

Some plants ground the motor frame; others only connect the ground to the J-box. So, if the J-box is solidly grounded but not bonded to the motor, the motor is not really grounded per the National Electrical Code (NEC). The Photo illustrates what can happen when a J-box is not properly grounded to the motor frame (i.e., if both are not properly grounded per the NEC).

The solution

Because technicians are under pressure to change motors quickly — and it’s much easier (and faster) to unbolt the J-box than to disconnect the incoming conduit — why not solve the real problem?

For each occurrence of interchangeable motors, determine which brand has the highest representation and then sketch the J-box bolt pattern for the dominant motor. Have your service center drill and tap new holes in the correct locations on the rest of the motors, so that your now-standardized J-box can be solidly bolted to each one.

Tapped holes that are covered by the J-box can be left as is, or filled with epoxy or silicone. Those that won’t be covered by the J-box can be filled with set screws that are secured in place with a thread-locker product. In some cases, the service center may need to machine the side of the frame flat, to ensure a proper contact between the J-box and frame.

Finally, buy a spare J-box from the manufacturer you select, and send it along for fitting purposes each time you send a motor out for repair.

Yung is a senior technical support specialist at the Electrical Apparatus Service Association (EASA) in St. Louis.

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