A Limited Study of  Electrical Designs for Motors: Understanding the Basic Concepts

by Ken Lovorn

We just introduced ‘Electrical Design for Motors‘, a 6 CEU course on designing electrical systems for various types of motors.  In our 50 person sampling of the electrical industry, we found that some of the basic concepts from the National Electrical Code that were presented in the course are not being addressed in electrical design.  Some of these issues include:

  • The proper sizing of components for fire pump installations
  • How the temperature correction factors for various ambient conditions are applied?
  • Applying the NEC requirements to group motor installations.
  • How can motor branch circuit conductors be sized at 125% and motor branch circuit breakers be sized at 250% and still meet the NEC requirements?
  • What are the special requirements for elevator motor installations?


There are two Codes that govern the proper installation of a fire pump and the associated equipment.  Article 695 of the NEC and NFPA 20 both are critical for design of the fire pump and the ancillary equipment.  If you size the normal power feeder breaker at over 600% of the motor full load current, how do you justify sizing the conductors at 125% of the full load?  Why would you size the emergency feeder breaker at 250% of the motor full load current and the normal breaker at 600%?  What ancillary equipment is required for a fire pump installation and where should it be located to meet all of the Code requirements?


In earlier issues of the NEC the temperature correction factors were listed adjacent to 310.16 for the allowable conductor ampacities.  In the recent NEC issues, these factors were separated into their own sections, in an attempt to emphasize their importance.  In addition, Article 310.15 (B)(2) Ambient Temperature Correction Factors states, “Ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(2)(a).”  In the Header for Table 310.15(B)(16) for Allowable Ampacities of Insulated Conductors, the last sentence states very clearly: “…based on Ambient Temperature of 30 degrees C (86 degrees F).”  So, unless the mechanical engineer provides air conditioning for all of the electrical closets and mechanical spaces and all conduits are run inside the airconditioned areas of the building, these conductors are not installed in an 86 degree ambient condition.  Therefore, for every circuit within these infrastructure areas, the conductors must be de-rated in accordance with aforementioned table.  For example, a 4/0 THW copper conductor has an allowable ampacity of 230 amps when installed in a 86 degree F ambient condition.  When you apply the mandatory temperature correction factor from table 310.15(B)(2)(a), you must multiply this allowable ampacity by 0.88 so a 4/0 copper conductor is really rated at 202 amps.  

In a similar manner, the standby 500 kcmil copper conductor, which has always been considered “good” for 380 amps up to 400 amps, is really only rated at 334 amps in a 40 degree C (104 degree F) ambient condition.  If you decide to round the conductor ampacity up to the next standard breaker trip rating, you are permitted to only go to 350 amps, not 400.  When you parallel conductors, two sets of 500 kcmil are really not rated at 800 amps, but only 668 amps.  The best you might achieve with these two sets of 500 kcmil is 700 amps, not the 800 amps at which they were applied.  


When an engineer designs a group of motors, very often the breaker feeding the group of motors (perhaps a motor control center) ends up twice as large as it needs to be just because Article 430.53 (C) is not correctly applied to this group of motors.  We address how you design the feeder breaker and how to size the feeder conductors so that you keep the excess fat out of your electrical design and still meet all of the NEC requirements.


Motor circuits are always somewhat confusing, as the breaker must be sized large enough for the motor to start while still protecting from both motor overloads and short circuits.  How many times have you been asked about sizing the motor branch circuit at 125% of the full load amps while the breaker is sized at 250%?  Or do you match the conductor ampacity to the motor branch circuit breaker size and spend way too much money on the motor circuit conductors?


Have you ever designed an elevator penthouse and received major change orders from the contractor to modify the machine room layout so the elevator inspector’s requirements will be met?  Have you had to add fire alarm devices to your elevator electrical design drawings as an addendum after missing them during the design process?  In this course, you will learn how to apply the requirements from the NEC article 620 and the National Elevator Code to make sure your elevator installation passes the elevator inspector’s scrutiny… without any comments!

How would you like to have your submittals to the Authority Having Jurisdiction pass the permit review process the first time? In Electrical Design for Motors, the engineer or designer will learn how to apply the widely diverse articles of the NEC Code to their particular application so they are fully compliant.  

Enroll in Electrical Design for Motors today and join your expert instructor for 3 weeks of intensive training!