Motor Calculations are something that is needed practically in field and are also a big part of the licensing exam. This short refresher will help you remember the process to size Overloads, Branch Circuit Conductors, Short Circuit Protection and Feeder Protection for Motors. 

Motor Calculations

Motor calculations are not something that we do everyday. Once we learn them, if we do not practice them, we will forget how. 

PRACTICE., PRACTICE. PRACTICE is the only way to keep these calculations fresh in your mind. Remember, when you are taking the examination, you will be under pressure to complete on time. The best way to be successful is practice these calculations until you are comfortable with them and can do them quickly, accurately, without thinking about it. 

Our three minute drills and practice exams are designed to help you do just that. 

Do them and do them often. Even if you have done them before, repeat them periodically, stay sharp and practiced and you will be confident when you take your test. 

Basic Motor Calculations Refresher

Written By Ted "Smitty" Smith 12/5/2015
Electricians will frequently need to size short circuit protection and overload protection during their careers. This overview of motor calculations should help you understand the requirements of the NEC® for sizing this protection and will also show you examples of each calculation... This module will also be helpful for examination preparation purposes.

The NEC and National Electrical Code ​are registered trademarks of the National Fire Protection Association. See our main home page for contact information.

Motor Calculations                                                                                                              

It is important for all electricians to fully understand motor calculations.

Probably the most important thing to remember is that sizing for Motor’s is done differently than most equipment in the NEC. This is because we typically use a single circuit breaker or set of fuses to provide protection from overloads, short circuits and ground faults. With motors, this is not done because, due to locked rotor current, the means needed to protect from overloads needs to be separate from the means needed to protect from short circuits and ground faults. This is why there is a separate process for sizing items as they apply to motors and none of the methods are dependent on each other.

Also, please remember the importance of 430.6(A)(1) and (2). All motors have two different full load currents. The table value and the nameplate value. The nameplate value is marked by the manufacturer on the nameplate of the motor. This is also the value that is listed in a code question. The table value comes from the tables in the back of Article 430. The nameplate value is only used to size the overload protection. All other sizing will be done with the Table value, unless there is a specific instruction in the code required to use the nameplate value, such as Other than Continuous Duty motors.

This course will cover motor full load currents, branch circuit sizing, branch circuit overload protection, feeder sizing, and feeder overcurrent protection sizing for single-and three-phase, Alternating Current Motors of more than 1 horsepower.

There are six steps involved in calculating motors.  It is a good idea to dog-ear your book and highlight the different steps, so you can easily go through the steps.

Step 1 Single Phase AC – Find the FLC (full load current) – Table 430.248 single phase

Step 1 Three Phase AC – Find the FLC (full load current) – Table 430.250 three phase

Step 2 Sizing Separate Overload Protection – Minimum = 430.32(A)(1) Maximum = 430.32 (C)
You will need the service factor and temperature rise information for this step.
Note:  Use nameplate current rating when calculating overloads.

Step 3 Branch Circuit Conductor Sizing Motor – 430.22 Single Motor:  Calculate 125% of FLC = Ampacity – Turn to 310.15(B)(16) to find the wire size.  Do not forget to apply the requirements of 110.14 (C).

Step 4 Branch Circuit Overcurrent Protection Sizing – 430.52 Select type of motor and type of overcurrent protection device from the chart and multiply the values given by the FLC of the motor.  If the calculated value does not correspond to a standard fuse or breaker size listed 240.6 then you are allowed to go up to the next higher size.

Step 5 Feeder Sizing – 430.24 Several motors or a motor circuit with other loads (s):  Find your largest motor and multiply the FLC of that motor by 125%.  Add the FLC of all the other loads connected to the same feeder.  Turn to 310.16 to find the wire size.

Step 6 Feeder Overcurrent Protection Sizing – 430.62:  Find the largest overcurrent protection device for the branch circuits.  Add the FLC of all the other load(s) on the feeder.  If the calculated value does not correspond to a standard fuse or breaker size listed in 240.6 you must go to the next smaller size.  Important:  You are not allowed to go up in size on feeders, only on branch circuits.

A detailed explanation of steps:

Step 1 – Find the “Full Load Current” of the motor.  Find the phase of the motor, (this will dictate which table you will go to for the FLC.  If the motor is single phase, use 430.148.  If the motor is three phase, use 430.150.  Once you are at the appropriate table.  Find the voltage and horsepower of the motor and use the chart to find the FLC.  Write the FLC down, you will need it throughout each step except Step 2.  If the nameplate FLC rating is given, you will use it in Step 2.
Example:  What is the FLC of a single phase, 2 HP, 115-volt motor?
Answer:  T 430.248 FLC = 24 amps

Example:  What is the FLC of a three-phase, 5 HP, 230-volt motor?
Answer:  T 430.250 FLC = 15.2 amps

Step 2 – 430.32(A)(1) Calculate to find the minimum overload protection.  You must know the service factor and the temperature rise ratings of the motor.

The service factor rating is the amount of output the motor can develop without causing harm to the motor.  If a 5 HP motor has a service factor of 1.15 the motor can produce an output of 5.75 HP without harming the motor.  The service factor could be seen as a safety measure, which would allow the motor to produce extra power if it was needed.

The temperature rise is the difference of the motor winding temperature when running at its full potential and the ambient temperature.  If the temperature rise does not exceed 40 C when running at its full potential, the motor will not be harmed.  This is also a safety margin.

Overloads protect the motor.  This is why the service factor and the temperature rise is important.  Look at 430.32.  If the service factor is not less than 1.15 or the temperature is not more than 40 C then you are allowed to size the overload at 125% of the FLC.  If the motor has less than a 1.15 service factor or the temperature rise is greater than 40 C then it falls into the “all other motors” category and you must size the overload at 115%.

Maximum overload 430.32(C) – If the minimum overload is not of sufficient size to start the motor or carry the load, the next higher size overload can be used, but you can not exceed the percentages listed in 430.32 (C).  You are allowed 140% of the nameplate for motors with service factors of not less than 1.15 or a temperature rise of not over 40 C.  All other motors shall have a maximum overload protection of 130% of the nameplate FLC.

!!! Always use the nameplate FLC if given when calculating OVERLOADS!!!

Example:  What is the minimum overload for a 3hp, single phase, 115 volt motor with a nameplate FLC of 32 amps, with a temperature rise of 40 C?
Answer:  Nameplate rating = 32 amps x 125% (from table 430.32(A) = 40 minimum overload

Example:  What is the maximum overload for the above motor?
Answer:  Nameplate rating = 32 amps x 140% (from table 430.32(C)) = 44.8 maximum overload.

Example:  What if it asked for the maximum overload and did not give the nameplate rating?
Answer:  FLC from T430.248 = 34 amps x 140% = 47.6 maximum overload.
Always use nameplate FLC (430.6)

Example:  What is the minimum overload for a 3hp, single phase, 115 volt motor with a nameplate of 32 amps and a service factor of 1.10?
Answer:  Nameplate rating = 32 amps x 115% (all other motors because the service factor was less than 1.15) = 36.8 minimum overload.

Step 3 – 430.22 Calculating the minimum size branch circuit for a single motor is as simple as multiplying the FLC by 125%.  Conductors supplying a single motor shall have an ampacity of not less than 125%.  Use the FLC rating from Step 1.

Example:  What is the minimum branch circuit ampacity for a 5hp, three phase, 230 volt motor using THW conductors?
Answer:  FLC (T430.150) = 15.2 amps x 125% = 19 amps T310.16 = 14 AWG THW

Step 4 – Table 430.52 – Motors shall have a rating or setting of motor branch-circuit, short circuit, and ground-fault protective devices capable of carrying the motors inrush currents at startup.  430.52 is the maximum allowable ratings of these devices.  You will need to know the type of protective device and the type of motor.  The protective devices are listed in a row at the top of the chart.

If it is a single phase motor, you only have one column on the chart to find the percentages of the FLC.  If it is 3 phase (polyphase = more than one) you have several choices.  You will need to know whether it is a Wound Rotor, other than a wound rotor, Squirrel Cage (Design E, or other than Design E, or a Synchronous type motor.  This will dictate which column you use to find the percentages of the FLC to calculate maximum overcurrent protection.

Example:  What is the maximum size inverse time breaker for a 5hp, three phase, 230 volt, wound rotor motor?
Answer:  FLC = 15.2 amps x 50% (T430.52) = 22.8 go to 240-6.  22.8 is not a standard size.  You are allowed to go up to the next higher size for branch circuits.  Maximum size inverse time breaker = 25.

Step 5 – 430.24 When you have more than one motor or you have one motor and an additional load the feeder conductors shall be equal to the sum of the FLC for all the motors and all additional loads plus 25% of the FLC of the largest motor.  IMPORTANT:  If you have an additional load which is not a motor, and the FLC of the additional load is greater than the largest motor, you do not add 25% to the additional load, you must add the 25% to the largest motor, regardless of the size of the additional load.  Turn to 310.16 to find the wire size.

Example – You have a motor with an FLC of 10 amps and a heat load of 15 amps on a feeder.

Largest motor = 10 amps x 125% = 12.5 amps plus the additional heat load of 15 amps = 27.5 amps.

Even though the heat load FLC was greater than that of the motor, we still added the 25% to the largest motor!

Example:  What is the minimum size THHN feeder allowed for 1 – 3hp, three phase, 208-volt motor and 1 – 2hp, single phase 208 volt motor?
Answer:  3hp motor – 10.6 amps – 2hp motor = 13.2 amps
Largest FLC motor = 13.2 amps x 125% = 16.5 amps plus smaller FLC motor 10.6 amps = 27.1
Go to 310.16 – THHN for 27.1 amps = 12 AWG Feeder Conductor

Step 6 -  430.62 A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating of the largest protective device for any motor supplied by the feeder plus the sum of the FLC of the other motors in the group.

You simply find the motor with the largest overcurrent protective device from Step 4 and add to the rating of that device the full load currents of all the other motors.

Example:  What size feeder overcurrent protection using dual element fuses is required for
1 – 3hp, three phase, 208-volt motor and
1 – 2hp, single phase, 208-volt motor?
Answer:  Branch circuit OCP 3hp, 3 phase motor = 10.6 amps x 175% (T430.152) = 18.55 next higher size 20 amp
Branch circuit OCP 2hp, 1 phase motor = 13.2 amps x 175% (T430.152) = 23.10 next higher size = 25 amp

For feeder overcurrent protection you add the branch circuit OCP for the largest motor 13.2 amps = 25 amp plus the FLC of the other motor (s).

25 amps (largest FLC motor) plus 10.6 amps (FLC of the small motor) = 35.6 amps.  Go to 240.6 to find standard fuse sizes.  35.6 is not a standard fuse size.  You are not allowed to go up to the next higher size for feeder overcurrent protection.  You must go down.

Maximum feeder overcurrent protection = 35 amp.

I recommend returning to the On-Line Page practice these calculations using the Three Minute Drills and Practice Exams