Fused Disconnects in lieu of MCB panel

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CelectricB

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MEP Designer
Hi all,

Lately, we've had difficulty getting a hold of commercial sized MCBs, so this is the solution we've come up with. We'd have each disconnect being fed from loadside lugs from the 600A outdoor disconnect according to tap rules. Anybody have something better they've been doing or see something non-compliant in our new design? See diagram below,

Kind regards,

1660766180642.png
 
Looks good as long as the wire length does not exceed 25' for any of the interior fused disconnects. Also has the advantage of reducing the incident energy for the interior MLO panels.
 
What does the potential and current transformer designation mean? Is that for utility metering?

What is the "M" on the outdoor disconnect mean?
 
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Go by your original design which is a much better plan and change panel DP to a MLO.

WIth the 600 amp fused service disconnect mounted to the building you don't need that 600 amp Main Breaker in Panel DP.

JAP>
 
I agree with Jap. If getting the 600 amp main CB is the issue just eliminate it. The new design gets complicated with tap rules as Don mentioned.
 
Can you get rid of the outdoor 600 amp fused disconnect and panel dp, and just put the three fused disconnects shown in the first diagram outside?
 
Yeah agree with others there is in needed equipment. For the original plan, you could ditch the MCB or the outside disconnect (unless you need the outside disconnect per utility or local rule). In fact seems like you could ditch the entire distribution panel in the first graphic and use the tap rules to come into MB panels or MLO with fused discos (assuming you can meet the tap rules).

For the second graphic I would use MB panels instead of discos and MLO. For one thing, 225A discos are going to have to be 400A, where 225A MB panels are readily available. Or you could possibly try to use a separate enclosed circuit breaker, for example the Siemens QR frame goes up to 250A. Another issue with fuses might be series ratings. I can't speak off the top of my head for other brands, but with Siemens for example a standard 10k branch doesn't series rate with a rk5 fuse. Yes you can get a class T fuse which will, but you need a separate fuse holder kit for each disconnect which is not cheap. Whereas a 10k branch will series rate with a 250A QR frame up to 100k. Again this is just siemens, maybe you can find a better series rating with fuses on another brand, just be aware.
 
CelectricB said:
Hi all,

Lately, we've had difficulty getting a hold of commercial sized MCBs, so this is the solution we've come up with. We'd have each disconnect being fed from loadside lugs from the 600A outdoor disconnect according to tap rules. Anybody have something better they've been doing or see something non-compliant in our new design? See diagram below,

Kind regards,

View attachment 2561814
Click to expand...
If the problem is availability of MCCB's you might want to double check availability of fuses as well. I know some are hard to come by recently.
 
It's different every time we talk to someone what today's supply chain problem. We heard any molded case breaker over 200A is difficult to get.

Feels weird having different sizes of wire coming off the same disco.
 
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