NEC Article 670.4(A) and 670.4(C) Industrial Machinery Supply Conductors and Supply OCPD

[Rotato]

I am trying to figure out the size of the supply conductors and OCPD for an existing industrial control panel. (From Bus Plug to Machine Disconnect) However, when I write it down on paper. I am noticing a significant discrepancy. So naturally I pump the brakes and I am assuming I am doing this incorrectly.

The machine I am looking at. I have:
1.) VFD supplying a 15HP 460V motor. Table 430.250 gives me a FLA of 21A
2.) 12KW 480V 3ph Resistive Heating Element (12,000W / 480 / 1.732) = 14.43A
3.) 5kVA transformer 1ph (5,000 / 480) = 10.4A

NEC 670.4(A) 125% FLC rating of resistance heating loads + 125% FLC of Highest Rated Motor + 100% FLC of all other:
Motor (21*1.25) + Heater (14.43*1.25) + Transformer(10.4)
26.25+18.04+10.4 = 54.69A Size of Supply Conductor 6AWG (60*C)

NEC 670.4(C) Largest Rated or Rating of Branch Circuit Short Circuit Protective and Ground Fault Device + 125% of FLC of all resistance heating loads + 100% of all other
VFD(45A) + Heater (14.43*1.25) + Transformer (10.4)
45 + 18.04 + 10.04 = 73.08A Size of Supply OCPD

So as you can see. I am misunderstanding what the code book is getting at with this, because I wouldn't think I would be justified putting 70A Fuses on 6AWG (55A) conductors. It just seems wrong. Why wouldn't the code be written to just 670.4(A) and leave it at that. Then you would size the OCPD to the supply conductors and that's it. Why does the code go on to include 670.4(C)? I am guessing that I am misunderstand when do we use 670.4(C). Or do we use them both?
Thanks in advance

 

[Rotato]

Wow, just checked to see if anyone replied to this. Seen I had a 161 views and no comments.. So thinking on it over time, I think I have come to the following conclusions:

#1 Always size the conductor to the load.
#2 Always size the protection to the conductor
#3 670.4(A) is minimum supply load sizing for an industrial machine (following both #1 & #2)
#4 670.4(C) is maximum supply load sizing for an industrial machine (following both #1 & #2)

Does anyone have any caveats to this conclusion, or am I way off base here?

I would think that as long as you follow conclusion #1 and conclusion #2, then there shouldn't be a problem as long as the load is based off of somewhere between 670.4(A), and 670.4(C).

 

[EngineeredElectric]

Wow, just checked to see if anyone replied to this. Seen I had a 161 views and no comments.. So thinking on it over time, I think I have come to the following conclusions:

#1 Always size the conductor to the load.
#2 Always size the protection to the conductor
#3 670.4(A) is minimum supply load sizing for an industrial machine (following both #1 & #2)
#4 670.4(C) is maximum supply load sizing for an industrial machine (following both #1 & #2)

Does anyone have any caveats to this conclusion, or am I way off base here?

I would think that as long as you follow conclusion #1 and conclusion #2, then there shouldn't be a problem as long as the load is based off of somewhere between 670.4(A), and 670.4(C).

Unfortunately you are way off base, but in an understandable way.

I think your misunderstanding stems from thinking that table 310.16 strictly defines the maximum allowed OCPD (Over Current Protection Device) for a conductor, however this is not true. You need to think about things differently. Conductor ampacity and conductor over current protection are actually two totally independent concepts in the NEC. Strictly speaking, the OCPD you choose actually has nothing to do with the size of the conductor you choose. That is why 670 is written the way it is, with one section for conductor size and one section for overcurrent protection.

I know. Hang in there.

I know this seems weird, because so often the OCPD you choose ends up matching the conductor ampacity, however this is a result of a general rule, not an absolute one. That general rule is 240.4. I'd recommend reading 240 parts 1 and 2 in it's entirety very carefully, then come back with further questions. After you understand 240 parts 1 and 2, check out all the refences in table 240.4(G), and ask questions until you understand how all the applications in T240.4(G) diverge from the general rule of 240.4. Then when you understand all of those, check out the other applications in the tables found in the sections referenced by T240.4(G) (Particularly T430.5 where you might find just find our friend 670 come up again).

If you can do all of that you will be far ahead of the game.