[donf]

Ok,

This is from a class I am taking. I just need verification on two sections and help on one.

Five motors, all are 480V, 3 phase, THHN (75c) – Design “B” motors:

A = 10HP – B= 20HP – C = 25HP – D = 30HP - E = 40HP.

The “FLC” (derived from Table 430.250, 460 volt col.) are:
A = 14A – B = 27A – C = 34 A – D = 40A – E = 52A.

Branch Circuit Protection Breakers are: A = 20A, B = 35A, C = 40A, D = 50A, E = 60A.

Branch Circuit Conductors are: A = 14 AWG, B = 10 AWG, C = 8 AWG, D = 8 AWG, E = 6 AWG.

Feeder Conductors Size: (180 Amps) – 3/0 AWG (?)

Feeder Protection Breaker: 250 Amp (?)

Question: The name plate, “FLAs” were not given so, the motor OCPD breaker / fuse cannot be determined. Correct?

[tkrussell]

First off, you need to work on defintions. Your title is Motor Protection Breaker. Do you see that in the one line flow chart Figure 430.1 shown in Section 430.2 Defintions?

Your working on sizing the motor feeder conductor size and OCPD, (Overcurrent Protection Device), AKA Fuse or Breaker. and the branch circuit conductors and branch circuit OCPD.


The calculated feeder amps of 180 and # 3/0 Cu THHN connected to a 250 Amp circuit breaker is correct.

A few of the branch circuit wire size and CB sizes are off.

Branch circuit conductors need to be 125% of amps listed in Table 430.250.

Branch circuit OCPD will be as per Table 430.52.

Best you follow Example D8 in Annex D.



Again, below I think defintions is the issue,

[QUOTE=Question: The name plate, “FLAs” were not given so, the motor OCPD breaker / fuse cannot be determined. Correct?[/QUOTE]

Above you will work on the Short Circuit and Overcurrent Protection or OCPD.

For these you must use the Tables.

For Overload protection, the nameplate FLC or FLA is needed. These will not be standard fuses or circuit breakers. These will be overload relays with overload and, heaters, or a Motor Protector Circuit Breaker.

[donf]

I used the wrong table, thanks.

Branch Circuit OCPDs should have read:

Per T430.52 - "Design B 250%" X FLCs as determined by T.430.250

A = 35A, B = 70A, C = 90A, D = 100A, E = 130A

[tkrussell]

A through D all correct size OCPD as inverse time CB.

E is technically correct as FLC 52 Amps at 250 % = 130 amps, however, wrong answer.

See Section 240.6 for the correct answer.

Do you fully understand the OCPD and Overload issue?

[donf]

If you mean do I understand that the nameplate amperage is to be used to determine the overload condition for the motor itself, then yes I do. If you mean something else, then I do not.

I thought I checked all of the breakers against 240.6.

"E" should have been 150 amps. No excuses, I obviously did not take the time to look at 240.6.

TK- Thanks for your help.

[tkrussell]

BTW, I forgot to say nice job figuring out the feeder amps, wire and CB size.

I hope your using the Handbook. The notes and diagrams help understand how to apply some requirements.

[donf]

TK,

Actually, I do use the 2008 Handbook.

I also use the normal 2008 code book as well as the text.

Since the course is supposed to teach you how to interpret the code book, I don't often look to the text for answers, I use the code books.