Ecmweb 8318 Code Requirements Motors Pr
Ecmweb 8318 Code Requirements Motors Pr
Ecmweb 8318 Code Requirements Motors Pr
Ecmweb 8318 Code Requirements Motors Pr
Ecmweb 8318 Code Requirements Motors Pr

Critical Code Requirements for Motor Applications

July 18, 2016
Some common violations set motor installations up for increased danger and decreased reliability.

Given the title of this article, you might expect the citations to all be in Art. 430. But we begin with Art. 110.

Poor workmanship [110.12] is often a factor in motor failures. For example, an electrician troubleshooting a “failed” motor found the cause inside the weatherhead. The wires had been twisted together by hand and then wrapped in tape. That unreliable assembly method is a failure waiting to happen, and, in this case, it occurred.

The amount of space around these motors appears to be adequate for routine maintenance activities (Hramovnick/iStock/Thinkstock).

The electrician fixed this by crimping lugs on the wires and making bolted connections. Properly used solderless connectors also work well in motor applications.

Another workmanship error is pulling flexible conduit at a tight angle so there’s constant strain on it. Cut it long enough and use a fitting (e.g., elbow) for large direction changes rather than forcing flexible conduit into a high-stress position; don’t exceed the bend radius.

Working space

Is the space around the motor adequate for maintenance [110.26]? In many cases, the answer is simply no, and there’s no fixing that because of the location. For example, a motor under a paper machine isn’t going to get adequate clearance around the motor because the machine is in the way. But you don’t need to complicate the problem by locating other electrical equipment in that space.

When you can control space around a motor, allow for lifting equipment. Can a lift truck or “cherry picker” boom access the space? What about a path to the main aisle? In the paper machine example, the motor was in a bad location. But the engineer designed a 5-ft-wide path all the way to the main aisle to facilitate getting that motor out and a replacement in.

Granted, 110.26 is intended to cover electrical distribution equipment rather than loads. However, the same “worker’s safety” principles logically apply to motors. In many cases, you won’t have much choice in how much space you can provide. But where you do have a choice, make a smart one.

Related

Grounding

A manufacturing plant in northwest Tennessee was experiencing high rates of motor failure. Each of its production lines was driven by a 50-hp motor. The maintenance manager was proud of the fact he had driven ground rods into the cement floor at each motor, “thus solving any electrical cause of failure.” Or so he mistakenly thought.

A consultant used nothing more complicated than a DMM to show the maintenance manager what the problem was. The potential between one ground rod and a nearby I-beam was 90V. This meant undesired current flowed through the motor bearings, therefore explaining the high failure rate.

A ground rod on the load side of a distribution system doesn’t really help. The soil has relatively high resistance, so this just provides a high-resistance path back to the source. Undesired current will seek all paths in reverse proportion to the resistances of each. The path through motor bearings is fairly low-resistance.

The solution is to provide a low-resistance path, namely through the equipment grounding conductor (EGC) [Art. 250, Part VI]. An EGC must be provided to a stationary motor frame if certain conditions exist. One of those is when the motor is supplied by metal encased (e.g., EMT) wiring. Another is when any terminal is over 150V to ground [430.242].

This brings us to the question of what an EGC is. It’s not a ground rod. You’ll find 14 kinds of EGC listed in 250.118. The first one listed is a metal conductor in the form of a wire or busway. Many of the others are types of metallic raceway. If you use metallic raceway, then you must ensure its electrical integrity to maintain the EGC [250.96]. All EGCs eventually terminate to the grounding connection at the power source (e.g., the electrical service).

To get all the undesired current into the EGC and to provide an equipotential plane, you must bond [Art. 250, Part V]. That means making a metallic path between non-current-carrying metallic objects, such as motor frames, equipment enclosures, and cabinets.

Here again, Art. 110 comes into play. How well you connect that bonding jumper is critical. If it’s a high-resistance connection because you used worn-out crimpers, applied the wrong torque to bolted connections, or failed to clean the mating surfaces, then the bonding jumper won’t serve its purpose. Bonding jumper requirements are in 250.102. These aren’t complicated, and Table 250.102(C)(1) will help you correctly size the bonding jumper for the application.

More requirements

We could go over the requirements for surge arresters [280] and surge protective devices [285] because inadequately protected electrical infrastructure guarantees you’ll have motor failures. But that’s moving out of our scope. Let’s just sum up the other requirements of the first three Chapters by saying many of them contribute, directly or indirectly, to premature motor failure.

You can’t install a motor in a system rife with Code violations and expect it to last, even if you comply with every Art. 430 requirement. And that brings us to our next topic.

Article 430 highlights. Article 430 is the largest Article in the NEC. We could do a 12-part series on it alone. And for the motor itself, some major mistakes aren’t even NEC-related. For example:

  • Wrong NEMA type for the application.
  • Wrong insulation rating for the environment.
  • Mismatched to variable frequency drive.
  • Poor alignment.

As for Art. 430 violations, let’s start at the same place motor replacement typically starts: at the motor disconnect. Article 430 devotes Part IX to this one topic alone.

A requirement that appears multiple times in Part IX is the disconnect must be in sight of the motor. Yet in many industrial applications, there’s a tank or other equipment located between the motor and its disconnect. Or, the disconnect is mounted on a column in sight of the motor but not on the side facing the motor.

Part IX does not explicitly require mounting the lever switch type of disconnect in such a way as to facilitate its safe operation. That’s really covered under 110.12 (workmanship) and 110.3(8) (practical safeguarding of persons). The safest way to operate this type of disconnect is to stand with your body to the right of it and push/pull the handle (on the far right side) with your left hand. This puts you out of the blast path. Yet many disconnects are mounted in a way that requires standing in the blast path.

The correct sizing of feeders, branch circuit protection, and motor overloads continues to baffle people who only occasionally perform these calculations. Why? With non-motor circuits, the functions of short circuit protection, ground-fault protection, and overload protection are combined in a single overcurrent protection device (OCPD) but with motor circuits, this job is (usually) split among two devices. The split is necessary due to the inrush current when starting a motor (typically five times the running current).

This confusion results in violations of requirements in Parts IV and V (short circuit protection, ground-fault protection for branch circuits and feeders, respectively) and in violations of Part III (motor overloads). These violations can, at one extreme, impede operation. At the other extreme, the system fails to provide adequate protection.

To prevent such mistakes, go to Informative Annex D and carefully read through Example D8. Then, use it as a guide for your motor supply calculations. If you have a variable-speed drive, do the same thing with Example D10.

Beyond the NEC

The NEC does not stop you from supplying other equipment from motor branch circuits. But when you do this with 3-phase motors, you’re inviting voltage imbalance. Lighting is a single-phase load; add some lights to that branch circuit, and you’re asking for problems.

The NEC doesn’t require you to account for voltage drop, either. If the motor is at the end of a long run, it may draw excess current (and run hotter) if the conductors aren’t sized to account for voltage drop.

Other power quality issues adversely affect motors. Low power factor (at the motor) means the motor draws more current (and runs hotter) to do the same work. Power factor correction capacitors at the motor solve this problem, as does a power factor corrected motor drive. Caution: The required size of power factor capacitor is different for a motor that’s running under full load than one that isn’t. Know your resistive load in each case and engineer the correction accordingly.

These and other issues not covered by the NEC can result in premature failure of the motor, so mere NEC compliance is no guarantee of adequate motor life. However, NEC violations can defeat good design decisions and render the best maintenance efforts moot.        

Lamendola is an electrical consultant located in Merriam, Kan. He can be reached at [email protected].

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

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