Branch circuit s.c. & ground fault multiple motors

[cppoly]

In Mike Holt's examples, he calculates s.c. and ground fault for multiple motors by taking the largest motor's protection (and takes the next size UP) plus the remaining motor FLCs then rounds down.

To take the next size up for the largest motor, he is using the exception to use the next largest OCPD size. But since this is using the exception, are you are allowed to round down in both cases as another alternative?

 

[Smart $]

As you likely know, compliance can at times be generalized. However, in your case, please consider that a lot of us do not have the "example" you refer to, nor do we know which you are specifically referring to or its details. Also, you reference an exception. The exception to what? Please provide more detail.

 

[cppoly]

Ok, here's some more details.

Part V of this article:
http://www.mikeholt.com/mojonewsarc...s-Motor-Circuits-and-Controllers~20031126.htm

"Part V. Feeder Short-Circuit and Ground-Fault Protection[FONT=Arial, Helvetica, sans-serif]Per [430.62], protect feeder conductors against short circuits and ground faults by a protection device sized:[/FONT]

• [FONT=Arial, Helvetica, sans-serif]Not greater than the largest rating of the branch-circuit short-circuit and ground-fault protective device for any motor[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Plus the sum of the full-load currents of the other motors in the group.[/FONT]

[FONT=Arial, Helvetica, sans-serif]What size feeder protection (inverse-time breakers with 75?C terminals) and conductor do you need for the following two motors (Figure 430-15)?[/FONT]
[FONT=Arial, Helvetica, sans-serif]Motor 1: 20-hp, 460V, 3? = 27A
Motor 2: 10-hp, 460V, 3? = 14A[/FONT]

[FONT=Arial, Helvetica, sans-serif](a) 8 AWG/70A
(b) 8 AWG/80A
(c) 8 AWG/90A
(d) none of these[/FONT]
​

[FONT=Arial, Helvetica, sans-serif]Answer: (b) 8 AWG/80A
(27A x 1.25) + 14A = 48A
[FONT=Arial, Helvetica, sans-serif]8 AWG rated 50A at 75?C [110.14(C) and Table 310.16][/FONT][/FONT]
[FONT=Arial, Helvetica, sans-serif]Feeder Protection [430.63(A)] not greater than largest branch-circuit protection device plus other motor FLC.[/FONT]
[FONT=Arial, Helvetica, sans-serif]Step 1. Determine largest branch-circuit protection device [430-52(C)(1)][/FONT]
[FONT=Arial, Helvetica, sans-serif]20-hp Motor = 27A x 2.5 = 68, next size up = 70A [430.52(C)(1) Ex.]
10-hp Motor = 14A x 2.5 = 35A[/FONT]
[FONT=Arial, Helvetica, sans-serif]Step 2. Size Feeder Protection = 70A + 14A, = 84A, next size down = 80A"

In his example, he rounds UP for the largest motor short circuit and ground fault protection using the exception allowed (430.52(C)(1)

Then adds the remaining FLC and rounds down.

My question is as an alternative, can you round down in both cases?[/FONT]

 

[Smart $]

...

My question is as an alternative, can you round down in both cases?

The manner shown in Mike Holt's example is to determine the largest permissible rating or setting. If you round down, you will simply have a result which is less than that... :huh:

 

[cppoly]

Yes I understand that. Is this how you typically calculate SC and GF protection for multiple motors or do you do it Mike's way?

 

[Smart $]

Yes I understand that. Is this how you typically calculate SC and GF protection for multiple motors or do you do it Mike's way?

Actually, I seldom do motor load calculations in the real world.

As for how I would do it, it would be the same as exhibited in the example for scenarios falling under 430.62, which is GF/SC for a feeder supplying one or more motors... only. Note the (A) subsection title is "Specific Load". When there are non-motor loads on the feeder, 430.62 does not apply directly, but it may indirectly under 430.63 condition (3).

 

[augie47]

Most often I see it done as Mike did.