New Bolted Pressure Disconnect Switch & MSB in existing service

[msilva94]

Hello, I'm a young engineer and a bit over my head here. I have a site that's upgrading their antiquated electrical service (1940's~1950's).
The existing feeders according to the utility are 3-4-500AC and they'll be running another set of 3-4-500AC Parallel in a 4" Duct.
I assume this means 3 sets of 3 conductors, 1 ground (4W) 500KCMil Armored Clad. The service is 208Y/120V.
Although they haven't explicitly stated the feeder intent, my understanding is that they will want to land both existing and new feeders together on the disconnect switch and feed out.
My design approach is to replace the antiquated Disconnect switch with a new 4000A Fusible Bolted Pressure Switch.
I will be powering a new 4000A MSB (Main Switchboard) near the service from that new disconnect switch. The MSB will backfeed existing loads on one breaker, and new loads from another breaker(s).
The new MSB will have at least two sections, a 1000A section to feed existing loads and a 3000A to feed new loads - most providers like Square D only allow breaker sizes up to 1200 or 1600 in a box so I realize a singular section of 3000A may not be possible and I may even run into multiple MSB's.
This is one of those "30% sets that's actually 100% bid docs".

My most prevalent questions:
Can I still utilize an existing enclosure with a new pringle switch or will everything need to be ripped out?
The existing system is using bust duct to power the existing distribution boards, is running (3) Sets of (4)400KCMIL #2/0G in 2-1/2"MC to the bust realistic?
Anything seasoned professionals can tell me/warn me here?
I also really want to learn from electricians, I am trying my best to learn how to see things from a contractor's perspective so my designs best represent and support their load.
This is my first post here, prior to today I've been a lurker, so hi!

 

[jim dungar]

Why use a fusible switch and its required protective relaying (arc flash reduction) versus a 4000A breaker?
Switchboard manufacturers can certainly build sections with 3000A devices. The problem is often with the bussing between sections being limited to 3000A, one work around is to place the incoming section between, like in the middle of, distribution sections.

 

[msilva94]

Hello Jim, The Electrical Room is situated underground, with feeders directly connecting to the existing disconnect switch from a large pullbox positioned above it. This is also feeding residential loads. Unfortunately, space constraints prevent the demolition of the switch to replace it with a Main Switch Board (MSB). The MSB will need to be centrally located within the room. The current distribution system comprises open bus and knife switches, occupying the entire length of wall. The adjacent walls house other electrical equipment that the owner does not want removed.
Initially, I proposed a direct feed to the MSB, with one section backfeeding to the existing load and another to accommodate new loads, so if power is lost it can be flipped back on instead of requiring new fuses.
When I talked to my senior, he determined that a fusible switch is necessary to feed the MSB. He mentioned we might need to replace the entire enclosure, although it looked to be in good condition.
I also believe he wants to add additional cost, since we signed up for a 30% package and they're pulling our legs for what he deems 100% docs.
This is much different than my prior experiences, so thank you for your support and knowledge!!

 

[Knightryder12]

Square D can put (2) 2000 amp breakers in one section and only (1) 2500-4000a breaker per section. I designed an apartment complex (6 story) and that was what we designed around.

 

[electrofelon]

I am not sure on the utility's conductor call out , but I would also guess three sets of four (note there would not be a "ground" with service conductors). But I'm not sure what the AC means. My first thought was aluminum conductor.

Also a note on terminology, you are using the word feeder to refer to service conductors. Feeder has a specific NEC definition. Not trying to be picky, it just can be very confusing when people use the incorrect terms.

 

[electrofelon]

Obviously it's hard to offer much advice without having a lot more details, but I also like your idea of just a single switchboard with a main section. I guess I'm not clear what the issue with that is. If it's an unfused conductor length issue, you could encase the service entrance conductors in concrete per 230.6.

Also perhaps consider two smaller switchboards (or a switchboard and a panel board if one can be 1200 amps) with two service disconnects per 230.40 exception #2