Disconnector/Breaker for Auto Transfer Switch

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timm333

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Minneapolis, MN
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Electrical Design Engineer
We have a situation where an old ATS (auto transfer switch) is to be re-used, however as this ATS is old so it is not service rated. So we will bring the 480 V service conductors directly into the MCC. From MCC we will run one cable (Cable-1) to ATS, and run another cable (Cable-2) from ATS back to MCC, so that generator can be used if needed.

As per NEC, do we need breaker in MCC for Cable-1 (cable from MCC to ATS), or will only the disconnetor be enough for Cable-1?
Also there is no existing breaker/disconnector in ATS. Should we put new breaker (or disconnector) in ATS for Cable-2 (cable from ATS to MCC)?

Thanks
 
Cable 1 has to protected per its rated ampacity, what is it connected to in the MCC?
Cable 2 would be protected by the gen OCPD or the cable 1 OCPD
 
I think for cable-1 (MCC to ATS) we will have to put a breaker on horizontal bus of MCC (just like main breaker), is it correct?

Cable-2 (ATS to MCC) would also be connected to the horizontal bus of MCC. Should there be a disconnector in the MCC for cable-2, or can cable-2 be connected directly to the horizontal bus by using main-lugs?
 
Cable 1 would have a feeder breaker sized to the ampacity of the cable.
Does cable 2 feed the same MMC as cable 1?
Cable 2 is the load side of the ats. If the ats is in a different room/building then iI think it should have a breaker where it feeds the buss, breaker woukd be backfed, ok if not marked line/load.
 
The ATS is outdoor and MCC is indoor. Even if there is no breaker in ATS, I think still it would be Ok. Because when ATS is connected to Cable-1, then Cable-2 would be protected by the feeder breaker of Cable-1. And when ATS is connected to the generator, then Cable-2 would be protected by the breaker of the generator.

Yes Cable-2 feeds the same MCC. Is it possible to have two horizontal buses in the same MCC?

The first horizontal bus will not have any loads, it will only have the "service entrance cable" and "cable-1". The second horizontal bus would have "cable-2" as main incoming cable (terminated by main lugs); and all the loads (outgoing feeders to motors) will be on this second horizontal bus.
 
A single line diagram would be helpful, even on a napkin...

The important issue is the Service Entrance. Is there a separate SUSE device ahead of the MCC, or is it the Main in the MCC? If it's the Main in the MCC, you would have to split off the Bus from the breaker, then have the breaker feed the ATS utility side, the generator feeds the ATS generator side, and the COMMON of the ATS would have to come back to Lugs on the MCC. If your MCC is not set up that way, it's a big project to try to change it in the field.
 
Assuming that it is the way Jared imagines.......I’d be afraid of someone wanting to work on MCC....kills main breaker......but now MCC is live from generator and there is no way to kill the MCC short of going to the generator and opening it’s breaker.

When I lock out the main breaker I’d like to gear to be safe to work on.
 
The single-line is attached. Service rated breaker is inside MCC, it will not connect to any bus of MCC, it will just have incoming cable coming from transformer and out going cable (Cable-1) going to ATS. I think it would be better to put disconnector on Cable-2 (as shown in one line.)

What is difference between service rated breaker and non-service rated breaker, is the body of service rated breaker connected to ground?
 

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As far as I know there isn't anything special about the breaker. It is really about the enclosure.

I'm curious how this main beaker in the MCC currently feeds the MCC bus. Everyone I have seen is bolted to the bus. I'd much rather see you add a service disconnection means ahead of the MCC as opposed to modifying the MCC.
 
my opinion.

if the cb in the mcc or generator is not sized to protect cable 2, then cable 2 is a tap and the install might well meet one of the tap rules.
 
Russs57 said:
As far as I know there isn't anything special about the breaker. It is really about the enclosure.

I'm curious how this main beaker in the MCC currently feeds the MCC bus. Everyone I have seen is bolted to the bus. I'd much rather see you add a service disconnection means ahead of the MCC as opposed to modifying the MCC.
Click to expand...
you can get cbs in an mcc that are not connected to the bus.
 
Yes it is possible to get breaker in MCC which is not connected to the bus. In this case we are using the breaker in MCC as the service entrance. So I think four conductors (3 Phases + Ground) are not enough for Cable-1 and Cable-2. And actually Cable-1 and Cable-2 should have five conductors (3 Phases + Neutral + Ground), is it correct?
 
FROM YOUR DIAGRAM;

Assuming the MCC is in use now and that Main CB is the SUSE point for your utility service, what you show there is how it SHOULD be done. Whether or not you CAN do it is a separate issue. Here are your potential problems;

1. You don't say the sizes involved here, but if that Main CB is 600A or more, it is likely directly coupled to the main horizontal bus bars. If so, when you remove that bus connection to change to cable connections, you might not have enough wire bending space available for that, because being a bus coupling, that was not required.

2. You have casually added a new non-fused main disconnect in there for where cable 2 returns back to the MCC. Again, depending on sizes, that is not likely going to fit in the existing MCC without adding a new section. If you were going to do that, just add a section that is a stand-alone SUSE breaker ahead of the MCC and leave the existing stuff alone. By the way, non-fused disconnects are typically not rated for the same level of SCCR (withstand) as breakers or fused disconnects. Often they are just 10kA, so you may end up with a breaker or fused disconnect anyway, again meaning it might be easier to just add the stand-alone SUSE breaker.

3. (Side issue). If there are no neutral connected loads in the MCC, the 4th wire to the MCC might be pointless. As the SUSE point now, you likely have it because the SUSE label would require a neutral connection and bonding point in the SUSE labeling. If you are replacing the Main CB (as suggested above), just get the neutral landing pad there and forget the 4th wire. Again, IF nothing in the MCC is actually using the neutral. Motors never need it, but if you have a 480/277V panel in there with 277V 1 pole feeders for lighting, then you do.
 
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