Ecmweb 8266 Code Quandaries June 2016 Pr
Ecmweb 8266 Code Quandaries June 2016 Pr
Ecmweb 8266 Code Quandaries June 2016 Pr
Ecmweb 8266 Code Quandaries June 2016 Pr
Ecmweb 8266 Code Quandaries June 2016 Pr

Stumped by the Code? Code Rule for Sizing Short Circuit and Ground Fault Protection for a Motor Feeder

June 20, 2016
Your most pressing National Electrical Code (NEC) questions answered

Q. What is the Code rule for sizing short circuit and ground fault protection for a motor feeder?

A. Feeder conductors must be protected against short circuits and ground faults by a protective device sized not more than the largest rating of the branch-circuit short-circuit and ground-fault protective device for any motor, plus the sum of the full-load currents of the other motors in the group [430.62(A)].

Depiction of the equipment arrangement described in the example problem in the first Q

Let’s run through a specific example problem to highlight how this would be done.

What size feeder protection (inverse time breakers with 75°C terminals) and conductors are required for the following two motors (Figure)?

Motor 1 — 20 hp, 460V, 3-phase, 27A FLC [Table 430.250]

Motor 2 — 10 hp, 460V, 3-phase,  14A FLC

Step 1: Determine the feeder conductor size [430.24]: (27A × 1.25) + 14A = 48A

An 8 AWG conductor is rated 50A at 75°C [110.14(C)(1) and Table 310.15(B)(16)]

Step 2: Feeder protection [430.62(A)] isn’t greater than the largest branch-circuit ground-fault and short-circuit protective device plus the other motor FLC.

Step 3: Determine the largest branch-circuit ground-fault and short-circuit protective device [430.52(C)(1) Ex]:

20-hp motor = 27A × 2.50 = 68A, next size up = 70A

10-hp motor = 14A × 2.50 = 35A

Step 4: Determine the size of the feeder protection:

Not more than 70A + 14A = 84A,  next size down = 80A [240.6(A)]

The “next size up protection” rule for branch circuits [430.52(C)(1), Exception No. 1] doesn’t apply to a motor feeder protection device rating.

Q. What are the Code requirements for motor disconnects?

A. A disconnecting means is required for each motor controller, and it must be located within sight from the controller [430.102(A)]. According to Art. 100, within sight means that it’s visible and not positioned more than 50 ft from one to the other.

A motor disconnect must be provided in accordance with Sec. 430.102(B)(1) or (B)(2).

1) Separate motor disconnect. A disconnecting means is required for each motor, and it must be located in sight from the motor location and the driven machinery location.

2) Controller disconnect. The controller disconnecting means [430.102(A)] can serve as the disconnecting means for the motor, if the disconnect is located in sight from the motor location.

There are exceptions to (1) and (2) above. A motor disconnecting means isn’t required under either condition (a) or (b), if the controller disconnecting means [430.102(A)] is lockable, as described in 110.25.

a) If locating the disconnecting means is impracticable or introduces additional or increased hazards to persons or property.

b) In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure only qualified persons will service the equipment.

Informational Note for Item 2: For information on lockout/tagout procedures, see NFPA 70E, Standard for Electrical Safety in the Workplace.                           

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.