Motor Calculations, Part 1: Motors and branch conductors
Overcurrent and short-circuit protection are not the same for motors.
Mike Holt for EC&M Magazine
When providing overcurrent protection for most circuits, we use a circuit breaker that combines overcurrent protection with short-circuit and ground fault protection. This is not usually the case for motors. With rare exceptions, we accomplish motor overcurrent protection by separating the overload protection devices from the short-circuit and ground-fault protection devices (see Figure 7-4).
Motor overload protection devices (i.e., heaters) protect the motor, the motor control equipment, and the branch-circuit conductors from motor overload (and the resultant excessive heating) [430.31]. They do not provide protection against short-circuits or ground-fault currents (see Figure 7-5). That is the job of the branch and feeder breakers, which do not provide motor overload protection. This arrangement makes motor calculations different from calculations used for other types of loads. Let's look at how to apply Article 430, starting at the motor.
The motor overload devices are often integrated into the motor starter. But, you can use a separate overload device such as a dual-element fuse (usually located near the motor starter, not the supply breaker). If you use fuses, you must provide one for each ungrounded conductor [430.36 and 430.55]. Thus, a three-phase motor requires three fuses. Keep in mind that these devices are at the load end of the branch circuit and they do not provide short-circuit and ground-fault protection.
Motors rated more than 1-hp (without integral thermal protection) and motors 1-hp or less (automatically started) [430.32(C)], must have an overload device sized per the motor nameplate current rating [430.6(A)]. You must size the overload devices no larger than the requirements of 430.32. Motors with a nameplate service factor (S.F.) rating of 1.15 or more must have the overload protection device sized no more than 125 percent of the motor nameplate current rating.
Let's look at Figure 7-7 and solve the following problem. Suppose you use a dual-element fuse for overload protection. What size fuse do you need for a 5-hp, 230V, 1Ø motor (service factor 1.16), if the motor nameplate current rating is 28A?
(a) 25A (b) 30A (c) 35A (d) 40A
Answer: (c) 35A
We size the overload protection per the motor nameplate current rating [430.6(A), 430.32(A)(1), and 430.55]. Thus, 28A x 1.25 = 35A [240.6(A)]
You also have to consider another factor: nameplate temperature rise. For motors with a nameplate temperature rise rating not over 40ºC, size the overload protection device no more than 125 percent of the motor nameplate current rating.
Let's look at Figure 7-8 and solve the following problem. Again, you are using a dual-element fuse for the overload protection. What size fuse do you need for a 50-hp, 460V, 3Ø motor that has a temperature rise of 39ºC and motor nameplate current rating of 60A (FLA)?
(a) 40A (b) 50A (c) 60A (d) 70A
Answer: (d) 70A
We size the overload protection per the motor nameplate current rating, not the motor Full Load Current (FLC) rating. Thus, 60A x 1.25 = 75A, 70A [240.6(A) and 430.32(A)(1)].
Motors that do not have a service factor rating of 1.15 and up, or a temperature rise rating of 40ºC and less, must have the overload protection device sized at not more than 115 percent of the motor nameplate ampere rating [430.37].
Sizing branch circuit conductors
Branch-circuit conductors to a single motor must have an ampacity of not less than 125 percent of the motor's FLC as listed in Tables 430.147 through 430.150 [430.6(A)].
You must select the conductor size from Table 310.16 according to the terminal temperature rating (60º or 75ºC) of the equipment [110.14(C)]. See Figure 7-1. Let's review this, with a sample calculation. What size THHN conductor do you need for a 2-hp, 230V, 1Ø motor (see Figure 7-2)?
(a) 14 AWG (b) 12 AWG
(c) 10 AWG (d) 8 AWG
o Answer: (a) 14 AWG
Let's walk through the solution:
The minimum size conductor the NEC permits for building wiring is 14 AWG [310.5]. However, local codes and many industrial facilities have requirements that 12 AWG be used as the smallest branch-circuit wire. So, in this example you might need to use 12 AWG instead of 14 AWG.
Branch-circuit protection for short-circuits and ground-faults
Branch-circuit short-circuit and ground-fault protection devices protect the motor, the motor control apparatus and the conductors against short circuits or ground faults. They do not protect against an overload [430.51]. See Figure 7-6.
The short-circuit and ground-fault protection device required for motor circuits is not the type required for personnel [210.8], feeders [215.9 and 240.13], services [230.95] or temporary wiring for receptacles [527.6].
Per [430.52(C)], you must size the motor branch-circuit short-circuit and ground-fault protection (except torque motors) so they are no greater than the percentages listed in Table 430.52.
When the short-circuit and ground-fault protection device value determined from Table 430.52 does not correspond with the standard rating or setting of overcurrent protection devices as listed in 240.6(A), use the next higher protection device size [430.52(C)(1) Ex. 1]. See Figure 7-9.
Did that statement stop you? Does it strike you as wrong? That's a common reaction, but remember, motors are different. The motor overload protection devices (e.g., heaters or fuses) protect the motor and other items from overload. The short-circuit and ground-fault protection does not need to perform this function, thus an oversizing won't compromise protection. An undersizing will prevent the motor from starting.
What's an easy way to determine what percentage from Table 430.52 you should use to size the motor branch-circuit short-circuit ground-fault protection device? It's basically a two-step process:
Step 1: Locate the motor type on Table 430.52.
Step 2: Select the percentage from Table 430.52 according to the type of protection device such as nontime delay (one-time) fuse, dual-element fuse or inverse-time circuit breaker. Don't forget, you may need to use the next higher protection device size.
Let's see if you have this concept down, with a short quiz. Use Table 430.52 to look up the numbers. Of the following statements, which one is true?
(a) The branch-circuit short-circuit protection (nontime delay fuse) for a 3-hp, 115V, 1Ø motor shall not exceed 110A.
(b) The branch-circuit short-circuit protection (dual-element fuse) for a 5-hp, 230V, 1Ø, motor shall not exceed 50A.
(c) The branch-circuit short-circuit protection (inverse-time breaker) for a 25-hp, 460V, 3Ø synchronous motor shall not exceed 70A.
(d) All of these
Let's walk through each of these. We'll be referring to 430.53(C)(1) Ex. 1 and Table 430.52.
(a) Table 430.148. 34A x 3.00 = 102A. The next size up is 110A. So, this is true.
(b) Table 430.148. 28A x 1.75 = 49A. The next size up is 50A. So, this is also true.
(c) Table 430.150. 26A x 2.50 = 65A. The next size up is 70A. All three are true, so d is the correct answer.
Let's review three very important principles:
1. You must size the conductors at 125% of the motor FLC [430.22(A)].
2. You must size the overloads from 115% to 125% of the motor nameplate current rating [430.32(A)(1)].
3. You must size the short-circuit ground-fault protection device from 150% to 300% of the motor FLC [Table 430.52].
If you put all three of these together, you can see there is no relationship between the branch-circuit conductor ampacity (125%) and the short-circuit ground-fault protection device (150% up to 300%). See Figure 7-10.
Now, here's a final quiz to test your knowledge. Are any of the following statements is true for a 1-hp, 120V motor, nameplate current rating of 14A? Refer to Figure 7-11.
(a) The branch-circuit conductors can be 14 AWG THHN.
(b) Overload protection is from 16.1A.
(c) Short-circuit and ground-fault protection is permitted to be a 40A circuit breaker.
(d) All of these are true
Walking through each of these, we see:
(a) Conductor Size [430.22(A)]: 16A x 1.25 = 20A; Table 310.16 requires 14 AWG at 60ºC.
(b) Overload Protection [430.32(A)(1)]: 14A (nameplate) x 1.15 = 16.1A
(c) Short-Circuit and Ground-Fault Protection [430.52(C)(1): 16A x 2.50 = 40A circuit breaker.
The 16A overload protection device protects the 14 AWG conductors from overcurrent, while the 40A short-circuit protection device protects them from short circuits. This example explains the sometimes confusing fact that, when doing motor calculations, you are calculating overcurrent and short-circuit protection separately.
Motor calculations have long been a source of confusion and a point of error for many people. Now that you understand what makes these calculations different, you should be able to do your motor calculations correctly every time. In Part 2, we'll look at one more aspect of this: sizing motor feeders.
Copyright © 2002 Mike Holt Enterprises,Inc.