NEC 670.4(C) Calculation Question

[ntan]

I am in the process of designing an electrical control panel for industrial machinery and am following NEC 2017 670.4(C) to size the disconnect OCPD for the panel. The loads in the panel are as follows:
Motors:

HP	AMPS
3	4.8
1.5	3

Other Loads:

AMPS	Breaker
1.35	6
3.2	4
8	10
8	10
8	10

When I calculate the OCPD according to the article, I get 36.35 amps FLA (sum of the loads: 4.8 + 3 + 1.35 + 3.2 + 8 + 8 + 8) and a maximum overcurrent protective device rating of 38.35A (sum of the loads plus the largest OCPD: 4.8 + 3 + 1.35 + 3.2 + 8 + 8 + 10). This means the largest breaker I can use for the panel is 35 amps since I must use the next smaller size of an available breaker; however, this is below the FLA of the panel. Am I interpreting the NEC article 670.4(C) calculation correctly? Can anyone provide insight how to handle this situation? Thank you all for your help.

 

[ntan]

I have never figured this out. Does anyone out there have any recommendations? Thanks in advance.

 

[charlie b]

I am not completely sure I understand this situation either. I have never worked with industrial machinery. But it seems to me that you are (incorrectly) calculating an OCPD setting twice, using two different methods. That is not how I read the article. I think you start by calculating the minimum ampacity for the circuit supplying the panel. That looks like your first calculation (for which you got an answer of 36.35). But you left off the extra 25% of the largest motor. When you add another 1.2 amps, the minimum circuit ampacity becomes 37.55 amps. I would select a #8 AWG, with its ampacity of 50.

Next you determine the maximum OCPD rating. I think you did that correctly. So you wind up with a 35 amp breaker supplying a conductor with a minimum ampacity of 37.55 amps (and an actual ampacity of 50 amps), for a load that could exceed the breaker's 35 amp rating. I see the conflict here. I am inclined to want to take advantage of the discussion in 240.4(B), and go to the next higher standard rating of 40 amps. But I don't see anything in 670.4(C) that could point me in that direction.

You are using breakers that are not on the list provided by 240.6, the smallest of which is 15 amps. I don't know your source for breakers rated 4, 6, and 10 amps. I also don't know why you are assigning the larger size breaker (6 amps) to the smaller load (1.35 amps). You could get around the problem by using 10 amp breakers for all the "other loads," or some combination of breakers that gives you an answer to your second calculation that is 40 or above. That is sort of bypassing the problem, but it could be a reasonable solution.

 

[kingpb]

NEC 670 applies to the sizing of the OCPD and supply conductors to an industrial machine based on the machine panel ratings. It is not intended for determining the machine panel ratings.

Perhaps start with NFPA 79, Electrical Standard for Industrial Machinery.

 

[petersonra]

from UL508a

32 Overcurrent Protection of Feeder
32.3 Sizing of overcurrent protection
32.3.1 The size of the overcurrent protection shall not exceed:
a) The rating of the largest branch circuit protective device in the circuit plus the full-load
currents of all other motors or other loads in the group; or
b) The ampacity of the conductors or bus bars on the load side of the overcurrent device.

This is the most appropriate way to figure the rating of the main OCPD.

The procedure in 670 of the NEC is for the incoming feeder not anything inside the control panel.

In your case the total FLC is 31.55. Add the rating of the largest OCPD (probably 15A for the motors) and you come up with 46.55 A. the next sized breaker up is 50A. Use 50 A.

eta: fixed the math

 

[petersonra]

Incidentally, what is the benefit to making the OCPD on the feeder coming to the machine be so small? What difference does it make if you made it 100A and just upsized the conductors? It seems like an odd requirement.

It reminds me of the multiple motor OCPD sizing requirements but there it is stated as a maximum rating for a minimum ampacity. I wonder if someone left out something when they copied it over to this section.