Motor Calculations, Part 2: Feeders
What is the correct way to size motor feeders and related overcurrent protection?
Mike Holt for EC&M Magazine
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.
Branch-circuit 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 Figure 7-16.
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.
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 calculations
From 430.24, we see conductors that supply several motors must have an ampacity that is not less than:
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 75ºC.
(a) 50A (b) 60A (c) 70A (d) 80A
o Answer: (d) 80A.
How did we arrive at this answer? Let's walk through the solution.
What size conductor would give us this ampacity?
(a) 2 AWG (b) 4 AWG (c) 6 AWG (d) 8 AWG
Let's 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):
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.
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 7-18.
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 this calculation.
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 overload.
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.
Let's walk through the solution.
1. Get the motor FLC from Table 430.148.
2. Size the branch-circuit protection per the requirements of 430.52(C)(1), Table 430.52, and 240.6(A)]
3. Size the feeder conductor per 430.24(A).
4. Size the feeder protection per 430.62.
That 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
Motor 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.
Copyright © 2002 Mike Holt Enterprises,Inc.