Commercial Service Disconnect switch - to fuse or not to fuse???

[Turboburke]

Hi Mike ? hope you are having a great weekend.


OKAY ? I am having a slightly difficult time determining whether or not I am required to utilize a FUSED service disconnect in my installation.


I am starting with what appears to be a 300 amp 120/208 service ( The xfmr 3 pack on the pole is marked ?100 KW? ? if I did my math right that should supply 278 amps at 100% load: 100,000 divided by 208= 480.77 divided by 1.73 = 277.8 amps ). The aerial hits the building with what appears to be something in the neighborhood of 3 aut wire ( free air), passes thru a CT set, and then immediately bugs into what looks like 350 or 500 MCM. These MCM conductors enter the weatherhead and travel down a 4? rigid, thru a longmouth LB and through the wall of the building. INSIDE, the 4? nipple exits the block and travels about a foot and enters a 12? X 12? by 6 foot long trough. Mounted atop this trough is FOUR 200 amp panels; TWO of them are 3 phase and the other TWO are single phase. The 3 AUT copper conductors from each of these main breakers travel down into the trough, where the whole shebang is supertapped together (The incoming MCM?s and all FOUR panel mains). Now I understand that each individual panel?s main supply is protected by it?s own panel main breaker. If a panel overloads, it?s main trips and the supply is protected.


WHAT I want to do is install a 400 amp disconnect on the side of the trough opposite where the MCM? come in, remove the MCM?s from the supertaps, and extend the unprotected MCM?s from the weatherhead all the way into the line side of my disconnect. Then, I want to run parallel 3 AUT?s from the load side of my disconnect into the UTILITY side of a 400 amp XFR switch. Finally, I want to run another set of parallel 3 AUTs from the load side of the XFR switch back to feed the supertaps.


DO I need a FUSED 400 amp disconnect, or not? I just can?t seem to determine this one piece of my puzzle. Thanks so very much for your help!

 

[augie47]

IMO, unless your transfer switch is "service rated" (in which case it would have service OCP), you will need to use a fusible disconnect. 230.91 requires your service OCP be a integral part of or immediately adjacent to the service disconnect.
(It is also possible your AIC would exceed the 10k rating of a non-fuse switch)

 

[Turboburke]

I just went looking around for a fused disconnect and found a NEMA 1 400 amp breaker enclosure and thought "This solves my cunundrum!".
My biggest fear was that if those fuses blow on the hottest summer day, there's no way to restore utility power, figure out what went wrong, and get the genny to shut down. With a main circuit breaker, my worries are over!

Additionally, I want to thank you for basically verifying my thoughts in my head, because if I am interpreting the code right, my new disconnecting means has to protect my source.....and no, the xfr switch is not service rated.

My brain block came from thinking I was just inserting two disconnecting means in series and not disturbing the tap setup other than increasiing it's length - now that I have typed about it, I realize I have to protect that xfr switch AND the prime disconnecting means itself from overcurrent due to a short-circuit somewhere in that goofy trough setup before any of my panels.

The only bummer is that now ALL FOUR of my panels become subs...... grrrr. Time to sort the neutrals......

Thanks so much!

 

[ceb58]

My biggest fear was that if those fuses blow on the hottest summer day, there's no way to restore utility power, figure out what went wrong, and get the genny to shut down

You could try replacing the fuse.

 

[hillbilly1]

If the fuse blows, there's usually a problem, most of the time when a breaker trips, nobody looks for the problem, they just slam the breaker back on. At least with fuses, it will get agravating enough that someone will look for the problem, whether it was just a bad design issue, (too small of service) or a wiring or equipment problem.