Electrical design as easy as N-E-C

A step-by-step look at how to rework a motor branch circuit as well as tips when a motor control center (MCC) is not involved in the project.


The new Size 6 starter is ready to be attached to the side of the motor control center. Courtesy: Robert BarnettAlthough the use of the National Electrical Code (NEC) is mandated by OSHA, many plant electrical engineers whose background is control systems are unaware of how it affects their work.

What follows is a look at how to design a motor branch circuit based on NEC regulations. For this example, we're looking at one specific aspect of motor circuit design-a single motor on a branch circuit. The references to NEC articles cited below can be used as a starting point for other motor circuits that feed more than one motor on a set of fuses, for instance.

The project:

A PLC programmer is presented with the following project:

A pump has been upgraded to allow for delivery of more power. The motor driving the pump will need to be increased from 200 HP to 250 HP. The programmer must design the necessary modifications to the existing electrical system to accommodate the new motor and intend to use the existing hardware as much as possible.

The first steps:

The programmer needs to know what is required for the new 250 HP motor, then he can determine what can be reused. First the programmer must determine the motor characteristics, either from a motor data sheet for this specific motor, or from the motor nameplate. In this case, the motor specifications are:

  • 250 HP
  • 1200 rpm
  • 460 V
  • 3 PH Squirrel Cage Induction Motor with a 1.15 service factor.

This is all the programmer needs to begin the design. The frame size and the enclosure type are not important when designing the circuit components. The steps involved in the process can be ordered whichever way works best for the project.

In this case, the programmer begins by determining the motor full load current, or FLA. Section 430.1 in the NEC shows a diagram of a motor branch circuit and which sections in the code apply to which part of the circuit. A branch circuit starts at the final overcurrent device—in this case, a fuse—and extends to the "outlet"-in this case, a motor.

1. Find the full load current (FLA). Use NEC Article 430.6(A)1 and Table 430.250. This section requires us to use this table in determining the FLA and not the motor nameplate date. If a motor is not in the table you can use the nameplate FLA. For a 250 HP motor the FLA is 302 amps.

2. Multiply the FLA by 1.2. Use Article 430.22 to get 378 Amps. This is the beginning of a series of derating steps to adjust the values given in tables to the various situations encountered in the environment for which we are designing. In this case, the multiplier of 1.25 is actually a derating of 80%. This takes into account such things as the motor's service factor, which really is the use of the motor above its HP rating and other deviations from rated conditions.

The existing Size 5 motor starter is being abandoned, and a new starter is being designed in line with the National Electrical Code. Courtesy: Robert Barnett3. Choose the wire. Use Table 310.15(B)(16). Note that this table applies to a run of not more than three conductors in a 30° C (86° F) ambient. Use 90° C wire, but choose the ampacity from the 75° C column. This column is used because the terminals of most motor control centers (MCCs) are rated for 75° C. Consult the manufacturer's data to confirm the terminal temperature rating for the MCC being used. Terminals must not be operated above their temperature rating (Article 110.14(C). A conductor chosen from the 90° C column could reach or at least approach, 90° C and cause the terminal to which it is connected to run above its rated temperature.

The programmer tentatively chooses a 500 kcmil (500MCM) conductor with THHW insulation in the form or a 3-conductor plus ground armored cable. This size is good for 380 amps under the not more than 3 conductor and 30° C ambient provisions.

4. Check voltage drop. Next the voltage drop must be checked. Refer to Article 210.19(A) No. 4. From the source (the MCC) to the motor it must be less than 3%. There are dozens of voltage drop calculators on the Internet, and in this case, the voltage drop is well below 3%. This is often the case with large wires at distances less than 500 feet. A check of voltage drop should always be made for motor branch circuits, especially because of the sensitivity of motor torque to the voltage applied to it. If the percentage is above 3%, a large conductor is called for. A long run may also require increasing the conductor size to reduce voltage drop and allow the motor to start properly.

5. Check ambient temperature. This motor will be operating in an ambient temperature of 100° F (38° C) for the summer months. Since Table 310.15(B)(16) was based on a 30° C ambient we will have to derate the conductors. Refer to Table 310.15(B)(2)(a). This shows that for a range of 36-40° C, a 90° C conductor must be derated by a factor of 0.91. Since the 90° 500 MCM cable is good for 430 amps in a 30° C ambient, when derated for a 40° C environment it can carry only 430 x 0.91 = 391 amps. Because the motor only requires 378 amps, the 500 MCM conductor is still good in 40° C ambient.

With the above design the programmer has kept both the insulation and terminal temperature within ratings and has a cable capable of handling 391 amps in that environment.

6. Choose branch circuit protection. Having chosen the wire size (500 MCM, 90° C), the programmer now selects the branch circuit overcurrent protection (use Section 430). For a motor circuit, the protection is based on the motor FLA and not the wire size. In this case, the programmer uses a dual element fuse. Literature from the fuse manufacturer is used to choose the fuse size. Table 430.52 limits a dual element fuse size to 175% of the motor FLA. So, 1.75 x 302 = 528 A, which means a 500 amp dual element fuse is what should be used.

7. Select the motor starter. A combination starter has the motor branch circuit disconnect, fuses, contactor and overload relay designed and built as one unit. The overload relay is used to protect the motor from an overload situation and the resulting thermal degradation of the windings. Most motors have relays that are set at no more than 125% of full load current. Refer to 430.32(A)(1). Use of an electronic overload relay makes good sense. It is easy to adjust and can be used to monitor the circuit as well.

The contactor is used to connect and more important, to disconnect the motor from the power source. It must be chosen to be able to carry and interrupt the current that the motor could draw under normal operation as well as overload and short circuit conditions. The manufacturer's catalog suggests using a size 6 starter. The catalog is based on NEMA specifications.

8. Check the existing equipment. The existing starter is size 5, so a new starter is needed. The existing cable is 500 MCM and can be used if it proves to be in good condition when tested.

This single-line electrical drawing cites the pertinent NEC codes discussed in the article. Courtesy: Robert BarnettA size 6 starter is a full section for the MCC in use. The programmer can add a full section to the existing MCC and, with a little help from a good electrician, can reconnect the existing cable using the space where the abandoned size 5 starter was located to mount power terminal blocks.

The new starter is added as a section that connects to the MCC buss.

It is assumed here that the MCC buss and the circuit components feeding that buss can handle the new load. It also is assumed the short circuit current rating (SCCR) of the MCC and its "buckets" are matched to what the supply can deliver under short circuit conditions. This is a reasonable assumption for an existing MCC.

However, the fault level at the MCC should be confirmed to determine if the new starter is suitable for use at the MCC's short circuit rating. This is mandated in NEC 670.5, although this is outside the scope of this example.

What if an MCC is not involved?

Article 240.21 requires that overcurrent protection (fuse or breaker) be provided in each ungrounded conductor at the point where it receives its supply. Exceptions are allowed for unprotected conductors of a limited length. These are also called feeder tap conductors. For most industrial motor circuit uses, there two exceptions—for feeder taps not over 10 feet long and feeder taps not over 25 feet long. This requirement is called out for motors in Article 430.28(1) and (2).

The 10-foot tap conductors must be rated to carry the load and be at least the amp rating of the equipment and the overcurrent device they feed. They must be enclosed in a raceway and not extend past the load equipment. They also must terminate in an overcurrent device. The previously selected 3C/500 MCM cable (380A) would be acceptable if a combination starter located less than 10 feet from the splitter is used.

The 25-foot tap conductors must have an amp capacity greater than 33% of the overcurrent device protecting the feeder. They must terminate in a set of fuses or circuit breaker that limit the current to less than the ampacity of the tap conductors. They must also be in a raceway. Here the 350 MCM cable wouldn't work. Assuming the 1200 amp splitter is protected by a 1200 amp fuse or breaker, the cable would have to be at least one-third of 1200 amps. So, a 400 amp, 600 MCM cable is needed.

A MCC is always the best solution when more than two or three motors are involved. The above construction method using tap conductors is not recommended but sometimes is needed when an old system is modified. Be careful when modifying this type of outdated construction. The short circuit current rating of this system may not be high enough to provide a safe installation.

Robert Barnett PE, is an electrical engineer for Cascades Containerboard Packaging Inc., Niagara Falls, N.Y.

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