Unsure of what is allowed/ not allowed....

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Dweeber

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Let me be honest, I have tried to be a study of the NEC (2008), but it vexes me.

Recently, I attempted to follow the mind(s) of an electrical contractor and an engineer, but have failed. Please humor my description of what I see.

There is a MCC breaker feeding 2 sets of 3 phase 480v 350 MCM conductors to a trough. The MCC breaker is set for 300 amp trip rating. The conductors, which I can tell, are rated (ampacity) for more than the breaker. And as far as I can tell, no conductors have been "upsized" for any electrical reason. So far so good?

Here's what is from the trough. there are 3 fused disconnects, the first disco has a set of phase conductors and a grounding conductor, the second disconnect is fed from the second set of MCC phase conductors, with ground, but has an extra set of lugs on the line side, so the third disconnect is fed from the second disco line side, lug to lug, with 350 MCM. Kind of like an MLO panel. So, are the disco "feeds" a feeder? I think so. With that thought in mind, the load(s) following the fused disconnects are branch circuits.

Also, when the NEC describes that all conductors must be in the same raceway, how is that determined? I see two complete sets of conductors, one set feeding disconnect one and two, even though they originate ona single breaker, and are common to the trough, but there is a phase ABC and ground for each downstream disconnect. Mind there is a combination of rigid and IMC for raceways.


So, if I am reading this right, there are no tap conductors, seeing how the MCC breaker is set lower than the ampacity of the conductors. There are no concerns for separating a set of the 3 phase conductors from the trough, seeing as how that circuit's conductors are in a single raceway and finally, once the disconnect fuses enter the fray,the conductors from that point are protected by the fuses (ocpd).

I am seeking enlightment to affirm or negate my limited knowledge. Forgive my poor explanation of what I attempt to describe.

What seems to be at issue is the idea that the MCC to trough conductors are or are not taps.

Feel free to question me or my poor description, thanks in advance,


D.
 
Since all condutors involved have an ampacity greater than their OCP I don't see a problem. Bit unusual arrangement but seemingly Code compliant.
 
Augie,

Firstmost, thanks for a reply. Secondly. I am glad someone perhaps understood what I typed.

I am new to scouring the NEC for details. I am gettin used to the idea that I need to read more. I also think there are no " taps" in what I described. I am also of the thinking that having two sets of 3 phase conductors originating from a single breaker, but feeding 2 disconnects would be unconventional, but otherwise NEC compliant.

The electrician not involved in this said ".....waste of good copper". Perhaps he is right in more than one way.

D.
 
JimW,


I am apologetic for my poor description. Perhaps I may answer a direct question?

Thanks, D.
 
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Appears compliant

Appears compliant

Even if the sub-feed from the 2nd Disc. Line Side to the 3rd Disc. Line Side lugs was smaller (yet still protected by the fuses in the 3rd Disc.), it would comply.

If all phase wires are in the same raceway (he did mention RGC and IMC) to the trough, then they made need to be derated to 80% (.8 X 350A = 280A).
This is a case where one set would be able to feed all 3 disconnects and not require derating, so the electrician who said "waste of good copper" was spot on.

However, we never know the history of odd arrangements: maybe this once had 2 feeder CB's (both 250A Trips?), and someone desperately needed one of them for another feeder, so they changed the 400A Frame trip from 250 to 300A and added dual hole lugs (if the CB didn't have them to start).

If this is a new install, then "waste of good copper" is an understatement.
 
Wow,

After re-reading my writing, I really am embarassed. What I had hoped to say really did not come across correctly..,

One of the questions I had was whether or not a paralled set of 3 phase conductors (6), originating from an MCC breaker, running in the same ferrous conduit, being then being broken into two seperate disconnects, perhaps violated the NEC by not having the circuit(s) entirely contained within the raceway, one for disconnect one, one for disconnect tw#2. I looked for the NEC definition for a "circuit" and failed to find one.

The question of having the paralled sets (abc and g) seperated at the trough and prior to the disconnect is one of my questions,

Maybe this will help?

Again, thanks for any assistance in getting me a better understanding.

D.
 
Lee,

Thanks for your response. The man I briefly spoke to was knowledgeable, the waste of copper was his statement, to his credit.

The installation is existing, and the disconnects are now feeding a significantly lower electrical load, thus the "extra" MCC feeders.

Again, thank you for your input, best regards,

D.
 
Could the waste be due to that is what was on hand,"paid for"; "SCRAP" rather than buy new?
 
Dweeber said:
Wow,

After re-reading my writing, I really am embarassed. What I had hoped to say really did not come across correctly..,

One of the questions I had was whether or not a paralled set of 3 phase conductors (6), originating from an MCC breaker, running in the same ferrous conduit, being then being broken into two seperate disconnects, perhaps violated the NEC by not having the circuit(s) entirely contained within the raceway, one for disconnect one, one for disconnect tw#2. I looked for the NEC definition for a "circuit" and failed to find one.

The question of having the paralled sets (abc and g) seperated at the trough and prior to the disconnect is one of my questions,

Maybe this will help?

Again, thanks for any assistance in getting me a better understanding.

D.
Click to expand...
The conductors are not paralleled if they are not connected together at both ends. I don't see any code issues here as the conductors are protected at their ampacity.
 
It's not unusual for people to design things based on what they think might be going to happen some time down the road to reduce the pain of making changes when they get there. My experience has been that whatever you think you might need down the road will be wrong and you will need to change it anyway.

I don't see anything wrong with making the conductors larger than the bare minimum they have to be. might be a waste of copper, might be someone is trying to accommodate something down the road. Might be a mistake.

But as long as it is code compliant, I would not get real worked up about it.
 
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