Power Circuit Breakers in MCC

[philly]

I have a customer that currently has a 480V LV Switchgear lineup which feeds several MCC's. The SWGR lineup is M-T-M fed from two transformers.

They are looking to replace the LV SWGR as well as one of the MCC's closest to the switchgear and have a desire to have the SWGR and MCC combined into a single structure. It has been suggested to provide an MCC structure with Power circuit breakers for the M-T-M as well as the other feeders that feed downstream MCC's.

Are there any disadvantages to providing power circuit breakers in an MCC structure as opposed to a UL listed LV SWGR structure? A couple of things that come to my mind:

- Breakers may not have draw out capability in an MCC lineup.
-MCC lineup will have 3-cycle rating (as opposed to 30cycle for SWGR) which may not allow adequate coordination with mains
-Arc Flash - Since MCC structure will be considered as a single structure with no barriers the entire MCC would take on elevated AF rating (cant take advantage of AF reduction on Main breaker in separate structure like switchgear)

 

[philly]

I guess there would also be the issue of a UL listing? I’m assuming that in MCC construction these would have to be insulated case breakers but not sure even in this case if MCC would still carry a UL listing?

 

[bwat]

Good questions here. I see the line blurred all the time between swgr and mcc and it usually seems to just be a semantics issue. I know a number of places that have MCCs that are really just used for power distribution (simple feeder breakers or fuses in buckets) without much of any motor starters. However, I’m trying to think if I’ve seen any that far upstream in the plant where there’s a MTM. Looking forward to hearing answers on this.

 

[Jraef]

You hit the main points already. MCC mfrs often use what appear to be PCBs as mains because of sizing, but those will actually be listed under UL489 as an MCCB /ICCB, not UL1066 for PCBs. In an MCC it will be fixed mount. Although some mfrs offer a draw-out mechanism, that is NOT the same as it is in draw-out switchgear, there is no “cell” around the breaker and you would draw it out with the door open; totally different concept.

MCCs are built to UL845, which is an offshoot of UL891 for switchboards, not switchgear (UL1558). So yes, the hold-in timing is different; 3 cycle vs 30 cycle. You can’t just plop a UL1066 breaker into an MCC and get the longer hold time, the structure has to be rated for it too. That’s why when PCBs are added to MCCs, they end up as UL489 rated devices.

And as far as I know, you can only get the PCB style breakers in MCCs as Mains, not feeders. I don’t know every brand however.

 

[philly]

Jaref - Thank You that is very helpful info

To provide a little more context a customer has an existing MTM LV Swgr that is fed from two unit substation transformers. Swgr has feeder breakers which feed several mcc’s including and adjacent one which will be replaced along with switchgear. In order to keep a condensed footprint for new Swgr and MCC it was suggested that MCC is constructed with sections to essentially replace Swgr thus having MTM and MCC feeders in an MCC construction with rest of MCC connected through breaker to contain buckets, etc....

I’ve never seen a Switchboard used in unit substation application let alone an MCC construction. To me it’s always been LV Swgr feeding MCC’s for many of the reasons discussed above. To keep a condensed footprint it is likely best to provide LV Swgr with close coupled MCC or small section of OH bus connection.

For additional consideration the LV Swgr currently has HRG system which I have also never seen in MCC construction.

 

[petersonra]

You can do what you want, keeping in mind both your application and the size of your wallet.

MCCs tend to be more cost effective. But you may end up with no effective way to isolate a feeder circuit as someone mentioned.

I am a fan of using MCCs for small feeders though. Say 200 Amps and under. Above that you start to run into issues with having to make the bus rating and AIC rating on the MCC high enough that you can lose some of the cost advantages.

You might also want to consider adding a 480 V panel board into the mix if cost is a big deal. They can be surprisingly affordable and the feeders won't eat up space in your MCC.