|Motor Calculations, Part 2: Feeders
|What is the correct way to size motor
feeders and related overcurrent protection?
Mike Holt for EC&M
In Part 1, we looked at how to size motor overloads. We also
looked at how to size circuit short-circuit and ground-fault protection for
motor branch circuits. The key lesson learned there is motor overload protection
requires separate calculations from short-circuit and ground-fault protection.
Understanding this fact clears up a common source of confusion and a common
point of error. But, another source of confusion arises when it comes to sizing
short-circuit and ground-fault protection for a feeder that supplies more than
one motor. Let's look again at branch calculations, then resolve the feeder
issues so your calculations will always be correct.
conductors and protection devices
Per 430.6(A), branch-circuit conductors
to a single motor must have an ampacity of not less than 125% of the motor FLC
as listed in Tables 430.147 through 430.150. To illustrate this, let's size the
branch-circuit conductors (THHN) and short-circuit ground-fault protection
device for a 3-hp, 115V, 1Ø motor. The motor nameplate FLC is 31A, and we're
using dual-element fuses for short-circuit and ground-fault protection. See
The FLC current per Table 430.148 is 34A.
34A x 125% =
Per Table 310.16 (60°C terminals), the conductor must be a 6 AWG THHN
You size the branch-circuit short-circuit and ground-fault
protection devices by using multiplication factors based on the type of motor
and the type of protection device per the motor FLC listed in Table 430.52. When
the protection device values determined from Table 430.52 do not correspond with
the standard rating of overcurrent protection devices listed in 240.6(A), you
must use the next higher overcurrent protection device. To illustrate this,
let's use the same motor as in the previous example.
multiply 34A x 175%
You need a 60A dual-element fuse.
To explore this
example further, see Example No. D8 in Annex D. Once you've sized the motor
overloads, branch circuit conductors, and branch circuit protective devices, you
are ready to move on to the next step.
Motor feeder conductor
From 430.24, we see conductors that supply several motors
must have an ampacity that is not less than:
125% of the highest-rated
motor FLC [430.17], plus
the sum of the FLCs of the other motors (on the
same phase), as determined by [430.6(A)], plus
the ampacity required to
supply the other loads on that feeder.
Let's look at Figure 7-12 and solve
the following problem:
For what ampacity must you size the feeder
conductor, if it is supplying the following two motors? One is 71/2-hp, 230V
(40A), 1Ø; the other is 5-hp, 230V (28A), 1Ø. The terminals are rated for
(a) 50A (b) 60A (c) 70A (d) 80A
o Answer: (d)
How did we arrive at this answer? Let's walk through the
The largest motor is 40A.
40A x 1.25 + 28A = 78A.
Rounding up to the next ten gives us 80A.
What size conductor would give
us this ampacity?
(a) 2 AWG (b) 4 AWG (c) 6 AWG (d) 8 AWG
see how to select the correct answer. By looking at Table 310.16, we find a 6
AWG at 75ºC provides 65A of ampacity, so it is too small. However, a 4 AWG
conductor provides 85A of ampacity, which will accommodate the 80A we need.
Therefore, you need to size this feeder conductor at 4 AWG.
Next, we have
to answer another question. What size overcurrent protection device (OCPD) must
we provide for a given feeder? Using a slightly more complex example, try sizing
the feeder conductor (THHN) and protection device (inverse-time breakers 75°C
terminal rating) for the following motors (Figure 7-17):
120V, 1Ø motors
Three 5-hp, 208V, 1Ø motors
One wound-rotor 15-hp, 208V,
Did you get the right answers? Let's walk through this and see. Our
references are 240.6(A), 430.52(C)(1), Tables 430.148, and Table 430.52. We'll
start by determining the ampacities required for each size of motor, then walk
through each step until we arrive at the correct OCPD size.
1-hp: FLC is
16A. 16A x 250% = 40A
5-hp: FLC is 30.8A. 30.8A x 250% = 77A. Next size up =
15-hp: FLC is 46.2A. 46.2A x 150% (wound-rotor) = 69A: Next size up =
Now, let's look at the feeder conductor. Conductors that supply several
motors must have an ampacity of not less than 125% of the highest-rated motor
FLC [430.17], plus the sum of the other motor FLCs [430.6(A)]. See Figure
Continuing on with this example, add up all the ampacities,
multiplying the highest rated motor by 125%. Thus, (46.2A x 1.25) + 30.8A +
30.8A + 16A = 136A. From Table 310.16, we see we need 1/0 AWG THHN because at
150A it is the smallest conductor that accommodates the 136A of ampacity we're
working with. When sizing the feeder conductor, be sure to include only the
motors that are on the same phase. For that reason, we used only four motors in
You must provide the feeder with a protective device
having a rating or setting not greater than the largest rating or setting of the
branch-circuit short-circuit and ground-fault protective device (plus the sum of
the full-load currents of the other motors of the group) [430.62(A)]. Remember,
motor feeder conductors must have protection against the overcurrent resulting
from short circuits and ground-faults but not those resulting from motor
Let's illustrate this with a sample motor feeder protection
calculation. What size feeder protection (inverse-time breaker) do you need for
the following two motors? They are a 5-hp, 230V, 1Ø motor and a 3-hp, 230V, 1Ø
motor. Refer to Figure 7-13.
(a) 30A breaker (b) 40A breaker (c) 50A
breaker (d) 80A breaker
o Answer: (d) 80A breaker.
through the solution.
1. Get the motor FLC from Table 430.148.
5-hp motor FLC is 28A.
A 3-hp motor FLC is 17A.
2. Size the
branch-circuit protection per the requirements of 430.52(C)(1), Table 430.52,
5-hp: 28A x 2.5 = 70A.
3-hp: 17A x 2.5 = 42.5A. The
next size up is 45A.
3. Size the feeder conductor per 430.24(A).
largest motor is 28A.
(28A x 1.25) + 17A = 52A.
Table 310.16 shows 6 AWG
rated 55A at 60ºC as the smallest conductor with sufficient ampacity.
Size the feeder protection per 430.62.
It must not be greater than the
70A protection of the branch circuit plus the 17A of the other motor (which is
the total of all loads on that feeder).
70A + 17A = 87A.
The next size
down is 80A, so that is the breaker you choose. Here is where many people get
confused and say something must be wrong. How can you be safe if you are
selecting the next size down instead of the next size up? Remember, you have
already accounted for all the loads and the NEC requires that you not exceed the
protection of the branch circuit. Again, keep in mind that you are not
calculating for motor overload protection, here. Motor calculations are
different from other calculations. With motor feeders, you are calculating for
protection from short circuits and ground faults, only-not overload.
seemed like a lot of steps, but by proceeding methodically you will arrive at
the correct answer. See Example D8 in Annex D of the NEC, for more detail. Also,
see Figure 7-19.
When sizing the feeder protection, be sure to include
only the motors that are on the same phase. For that reason, we used only four
motors in this calculation.
Putting it all together
calculations get confusing when you forget there's a division of responsibility
in the protective devices. To get your motor calculations right, you must
separately calculate the motor overload protection (typically located near the
motor), the branch circuit protection (from short circuits and ground faults),
and the feeder circuit protection (from short circuits and ground faults).
Remember that overload protection is at the motor, only.
Any time you
find yourself confused, just refer to NEC Figure 430.1. It allows you to clearly
see the division of responsibility between different forms of protection in
motor circuits. Example D8 illustrates this with actual numbers. Keeping this
division of responsibility in mind will allow you to make correct motor
calculations every time.