Selecting Tie Breaker Size

mull982

Senior Member
I wanted to hear the philosopy of others when it comes to selecting the size of a tie-breaker in LV equipment (Switchgear, MCC', etc..)

I have heard some arguments that you don't need to size the tie for the same size as the mains or bus since the ties will only see half of the total load on the equipment when its being operated through the tie. So for instance on Switchgear that has an 1600A Mains and Bus you might only find an 800A tie breaker.

I have heard others say that sizing the Tie breaker the same as the mains and bus is a good idea because it provides spare breakers that are sized the same as the mains if you ever needed to replace mains for some reason. Plus I heard that the incremental cost for a tie breaker sized the same as the mains is not that much more than if it is sized at at only half of the mains and therefore may make economical sense.

Was curious to hear the opinions and experiences of others.
 

wbdvt

Senior Member
Since there is a Tie breaker that usually means the busses operated with the Tie Breaker open. Typically in the applications that I see each transformer supplying the bus (like Txf A feeds Bus A and Txf B feeds Bus B with Tie Bkr open) are rated to carry the full load of the facility. Thus in the event of a transformer failure the facility can operate at full capacity. In situations like that the Tie Breaker should be the same size.

The caveat on that becomes that the trip settings of the Tie Breaker should be less than the Main Breaker settings. This is because with one txf out of service and the busses tied together, you want the Tie Breaker to trip before the Main breaker to prevent a total loss of power. For example, Bus B txf is out of service and facility has Bus A tied to Bus B via the Tie Breaker. A fault occurs on Bus B or a feeder breaker on Bus B fails to open, you want the Tie Breaker to open before Bus A Main Breaker so that Bus B still stays in Service.
 

GoldDigger

Moderator
Staff member
Any plan to use a lower rated breaker for the tie would depend on the assurance that at all times the building load is reasonably evenly divided between the two buses.
It seems reasonable to size the tie breaker by the larger of the two two calculated bus loads. But there are other factors to consider.
 

jim dungar

Moderator
Staff member
The caveat on that becomes that the trip settings of the Tie Breaker should be less than the Main Breaker settings. This is because with one txf out of service and the busses tied together, you want the Tie Breaker to trip before the Main breaker to prevent a total loss of power.
This assumes that the facility can continue operation with only 1/2 of its feeders operating.

I have seen installations where the original purpose of M-T-M has been ignored or abandoned (a M-M would have been more appropriate) resulting in some critical loads (e.g. boiler feed pumps) being fed from from only one side of the switchgear.
 

ron

Senior Member
I have heard some arguments that you don't need to size the tie for the same size as the mains or bus since the ties will only see half of the total load on the equipment when its being operated through the tie. So for instance on Switchgear that has an 1600A Mains and Bus you might only find an 800A tie breaker.

I have heard others say that sizing the Tie breaker the same as the mains and bus is a good idea because it provides spare breakers that are sized the same as the mains if you ever needed to replace mains for some reason. Plus I heard that the incremental cost for a tie breaker sized the same as the mains is not that much more than if it is sized at at only half of the mains and therefore may make economical sense.
I subscribe to both of those concepts and a third, and so the results depend on the client and application.

I design many 2N critical facility systems, where each of the two mains need to be able to feed the whole load, and each of the two sides of the tie, need to carry the whole load (redundant feeder to downstream loads with dual inputs). So the consideration is that one service is down (let's say 'A') and the downstream load equipment on the other side ('B') is down. Then the 'B' main has to serve the entire load fed through the tie to the feeders on the 'A' side. So in this case, it requires the tie to be full sized.
 

mull982

Senior Member
I design many 2N critical facility systems, where each of the two mains need to be able to feed the whole load, and each of the two sides of the tie, need to carry the whole load (redundant feeder to downstream loads with dual inputs). So the consideration is that one service is down (let's say 'A') and the downstream load equipment on the other side ('B') is down. Then the 'B' main has to serve the entire load fed through the tie to the feeders on the 'A' side. So in this case, it requires the tie to be full sized.
Interesting scenario. Let me make sure I understand correctly.

Lets say our upstream equipment is double ended switchgear with a M-T-M and the downstream equipment is two MCC's fed individually from each side of the upstream switchgear but also have a tie breaker between them.

In the scenario that you mentioned the upstream switchgear would have its "A" service down and downstream the MCC "B" would have an issue with either the feeder breaker, feeder cable, or main breaker on the MCC that prevented the MCC from being supplied from the upstream "B" Switchgear (MCC bus was fine in this case)

In this scenario the tie breaker between the MCC's would be closed and therefore all the facility load would come through Switchgear "B" main across the switchgear tie and down the A side feeder in order to feed both the MCC's through the Tie breaker? Am I understanding this scenario correctly?
 

mull982

Senior Member
Any plan to use a lower rated breaker for the tie would depend on the assurance that at all times the building load is reasonably evenly divided between the two buses.
It seems reasonable to size the tie breaker by the larger of the two two calculated bus loads. But there are other factors to consider.
I guess in this case you would only need to size the tie breaker for the larger of the load calculations for each bus?

For instance if the "A" side bus had a load calc of 1000A and the "B" side bus only has a load calc of 800A then the Tie breaker technically only needs to be sized at 1000A since the max load it will ever see through it is 1000A when the B size is feeding over to the A side? Main breakers in this case would likely be 2000A to support the combined loads on both "A" and "B".

I can see in a case like this where the incremental cost of increasing tie from 1000A to 2000A to match mains may be justified.
 

ron

Senior Member
Interesting scenario. Let me make sure I understand correctly.

Lets say our upstream equipment is double ended switchgear with a M-T-M and the downstream equipment is two MCC's fed individually from each side of the upstream switchgear but also have a tie breaker between them.

In the scenario that you mentioned the upstream switchgear would have its "A" service down and downstream the MCC "B" would have an issue with either the feeder breaker, feeder cable, or main breaker on the MCC that prevented the MCC from being supplied from the upstream "B" Switchgear (MCC bus was fine in this case)

In this scenario the tie breaker between the MCC's would be closed and therefore all the facility load would come through Switchgear "B" main across the switchgear tie and down the A side feeder in order to feed both the MCC's through the Tie breaker? Am I understanding this scenario correctly?
yes, although my specific scenario is with downstream UPS equipment feeding IT servers that have dual inputs, but the MCC example is similar.
 

Tony S

Senior Member
It’s OK designing things that are just up to the mark. Over time things change, loads get added that you never gave a 2[SUP]nd[/SUP] thought to at the time. You may save a bit of money at the time based on a fact that we all know, you’ll never have to run with a bus-coupler closed.

Having being in the unfortunate position of a transformer failing all I had to do was close the bus-coupler. The failed transformer was the heaviest loaded on the plant so a fair amount of current would need to pass through an undersized bus-coupler.

The coupler was a bit hot after a week.

It’s a false economy.
 

Tony S

Senior Member
True, it is the customers money and as the engineer you can advise them how to spend it wisely. Like many things a bus-coupler is seen as a waste of money, until the day it’s needed. When it is needed whoever specified the undersized switch will be the one that gets the blame.

It’s a false economy.
 
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