|Motor Calculations, Part 1: Motors and branch conductors
short-circuit protection are not the same for motors.
Mike Holt for
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).
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
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.
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
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
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
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
(a) 40A (b) 50A (c) 60A (d) 70A
Answer: (d) 70A
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
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
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
Answer: (a) 14 AWG
Let's walk through the solution:
sized no less than 125% of motor FLC
Table 430.148 shows the FLC of 2-hp,
230V 1Ø as 12A.
12A x 1.25 = 15A
Per Table 310.16, you need to use 14
AWG THHN rated 20A at 60ºC
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
Branch-circuit protection for short-circuits and
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
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.
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
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:
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.
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?
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
(c) The branch-circuit short-circuit protection (inverse-time
breaker) for a 25-hp, 460V, 3Ø synchronous motor shall not exceed
(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.
430.148. 28A x 1.75 = 49A. The next size up is 50A. So, this is also
(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
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
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.
Short-circuit and ground-fault protection is permitted to be a 40A circuit
(d) All of these are true
Walking through each of these,
(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
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