800 Amp 35kA Distribution Block

[NeilWicks]

Context:
I need an 800 (or 760) Amp Distribution Block for a panel to be rated at 35kA
The panel will be fed from an 800 Amp Circuit Breaker in the MDB about 10 feet away with 2 x 500
Total load in the panel will be 560 Amp with the largest load being 77 Amp
Need a little spare capacity above the 560 Amp of at least 100 Amp for some future additions

Problem:
Can not find a suitable Distribution Block.
All the Distribution Blocks I can find are only rated above their nominal 5 or 10kA when used in conjunction with 600 Amp Type J Fuses or a specified 600 Amp Circuit Breaker or similar, but nothing greater than 600 Amp

Question:
Any recommendations for a solution ?
And would a configuration as shown in the image work ?
And if it would work, would it still work without linking the two Distribution Blocks ?

 

[ron]

The image didn't come through, but what do you mean by distribution block? Can you just use a panelboard rated at 800A, 35kA SCCR?

 

[NeilWicks]

The image didn't come through, but what do you mean by distribution block? Can you just use a panelboard rated at 800A, 35kA SCCR?

The panel will have a number of MSP + Contactors & VFDs etc. with a PLC controlling approximately 50 individual loads
So the 800 Amp in the MDP would feed these devices via a distribution block in this panel.

 

[JoeStillman]

Why don't people use MCC's for applications like this?

 

[jim dungar]

Why don't people use MCC's for applications like this?

Too much floor space and lack of flexibility in design and layout.
A motor control panel can easily be customized with factory installed and tested interwiring. Although a MCC can be sent to an integrator for customization, but then we go back to my first point.

 

[ron]

I guess if you wanted to do it with a bunch of 10' taps on a common feeder, you can run the conductors in a wireway and splice along the run and pickoff power with C-type compression taps. You would have to be careful to balance the load from the paralleled source feeder conductors.
Then you wouldn't have to worry about SCCR of blocks.

 

[jim dungar]

Have you considered putting an 800A breaker in your control panel and then using a Power Distribution Lug for the breaker?

 

[NeilWicks]

Have you considered putting an 800A breaker in your control panel and then using a Power Distribution Lug for the breaker?

Yes, this is what I normally do to achieve a high SCCR for a smaller number of starters, but in this case there isn't a power distribution lug with enough ways, need at least 10.
It seems odd that a power distribution lug is considered part of the breaker, but add a short length of correctly sized wire, and place on the end almost the same piece of copper with holes in it, and now it's a distribution block at 5 or 10kA.
It's the rule, but I don't understand it. IMHO the distribution block is simply a square over sized wire !
Thanks very much for the response.

 

[NeilWicks]

I guess if you wanted to do it with a bunch of 10' taps on a common feeder, you can run the conductors in a wireway and splice along the run and pickoff power with C-type compression taps. You would have to be careful to balance the load from the paralleled source feeder conductors.
Then you wouldn't have to worry about SCCR of blocks.

Ah, So I guess I'm essentially using a Busbar System then ?
Either I make by own with the 10' Taps or use one from an enclosure or switchgear manufacturer.
Thanks very much for the response.

 

[jim dungar]

It's the rule, but I don't understand it. IMHO the distribution block is simply a square over sized wire !

Actually it is likely about the physical strength of the distribution lugs to tolerate the movement of conductors during a short circuit test. It is possible the insulated base of the distribution block could shatter allowing the uninsulated portions to make contact with other conductive surfaces.

 

[Jraef]

Too much floor space and lack of flexibility in design and layout.
A motor control panel can easily be customized with factory installed and tested interwiring. Although a MCC can be sent to an integrator for customization, but then we go back to my first point.

I agree, but also, a panel like that means that to work on ONE unit inside, you must power down the entire panel. So that concept works fine for a machine or system where, if one part fails the entire system must be shut down anyway. But if not, it’s a problem. I’ve seen that way too many times, forcing electricians to work hot. With the recent increased emphasis on NFPA-70E, this is becoming harder to implement. With an MCC, you can shut down one unit without having to shut down everything else. In some MCC designs, you can now remove the stabs from the bus before opening the door, so pulling a bucket out is no longer hot work either.

 

[Jraef]

The only way someone MIGHT list a stand alone distribution block for a higher SCCR in series with a circuit breaker, is if the breaker manufacturer is the one who MAKES the stand alone distribution block too, because it will require series testing, which is extremely expensive. But most of the people who make standalone distribution blocks brand label them to the other manufacturers who make breakers. So they are not going to spend that money just to help out that one breaker mfr. Conversely, the breaker mfr. already has to test their BREAKER which the breaker mounted distribution terminals, so why would they test the brand-labeled stand alone block too?

I have looked by the way, extensively, and have never fount a stand-alone dist. block that is series listed with a breaker. The breaker mounted distribution terminals is the only way to go.

 

[NeilWicks]

Actually it is likely about the physical strength of the distribution lugs to tolerate the movement of conductors during a short circuit test. It is possible the insulated base of the distribution block could shatter allowing the uninsulated portions to make contact with other conductive surfaces.

Ok, That makes a little sense and explains the higher rating of what must be almost the same lump of copper when mounted to a breaker which itself has been rated to some SCCR above 5 or 10.
Thanks again.

 

[NeilWicks]

The only way someone MIGHT list a stand alone distribution block for a higher SCCR in series with a circuit breaker, is if the breaker manufacturer is the one who MAKES the stand alone distribution block too, because it will require series testing, which is extremely expensive. But most of the people who make standalone distribution blocks brand label them to the other manufacturers who make breakers. So they are not going to spend that money just to help out that one breaker mfr. Conversely, the breaker mfr. already has to test their BREAKER which the breaker mounted distribution terminals, so why would they test the brand-labeled stand alone block too?

I have looked by the way, extensively, and have never fount a stand-alone dist. block that is series listed with a breaker. The breaker mounted distribution terminals is the only way to go.

Schneider do have very specific Breaker / Distribution Block pairs with higher SCCR's but I think they max out at 400 Amps or less.
Thanks.

 

[NeilWicks]

I agree, but also, a panel like that means that to work on ONE unit inside, you must power down the entire panel. So that concept works fine for a machine or system where, if one part fails the entire system must be shut down anyway. But if not, it’s a problem. I’ve seen that way too many times, forcing electricians to work hot. With the recent increased emphasis on NFPA-70E, this is becoming harder to implement. With an MCC, you can shut down one unit without having to shut down everything else. In some MCC designs, you can now remove the stabs from the bus before opening the door, so pulling a bucket out is no longer hot work either.

This panel is for one of those "all or nothing" equipment lines.
Maybe a return to Industrial Revolution Belt and Lineshafts would solve my problem with just one big motor :- )