Parallel Conductors Feeding A Single Buss

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I&E Chris

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Maysville
I have been asked a question in which i am unable to find a clear answer. So I will ask it here.

So if i am feeding a 3 phase 480V switchgear from a pole mounted transformer with parallel conductors is it allowable to land the sets of parallel feeders on seperate breakers? These breakers at one time fed the buss from generators but they are now out of service. i am looking to use these breakers to feed the buss from grid power. The only advantage i can see to doing this would be for future breaker maintence by shedding all the load except critical items and being able to work on one breaker without shutting completely down.
 

rcwilson

Senior Member
Location
Redmond, WA
The only advantage i can see to doing this would be for future breaker maintenance by shedding all the load except critical items and being able to work on one breaker without shutting completely down.

You could not work on one breaker at a time and still have the load energized. Both line and load terminals of the "OFF" breaker will still be energized. If this is a drawout breaker in switchgear, maintenance may be possible on a withdrawn breaker.

While I can't think of a specific code section prohibiting dual feeds; it is not good design. No way to guarantee equal load sharing between the two feeds. Having more than one source to a facility could be an issue. Someone opening Main 1 may get surprised by the voltage still on the service from Main 2.
 

mbrooke

Batteries Included
Location
United States
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Technician
I wouldnt do it, it sounds like an accident waiting to happen. Might be good economy wise but having multiple disconnects for the same buss is a bad idea.
 

I&E Chris

Member
Location
Maysville
These are draw out style breakers and that was my opinion on the situation as well, Too dangerous for the others who come along later not knowing about the dual breaker situation. I was hoping to learn about a code requirement somewhere that I could use to change the decision makers mind.

Does anyone think that 200.8 of the 2011 NEC would apply to this?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Parallel feeds to a single busways are fairly common in automotive installations. I have also seen them where the loading in predominantly rectifier systems. And it is kind of what occurs when multiple generators feed a single switchgear lineup.

However, I probably would not design one simply for the purpose of 'breaker maintenance'.
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
240.8 prohibits breakers in parallel unless a factory assembled device. That prohibits this installation.
240.8 applies to situations where the line sides and the load sides of protective devices are connected in parallel, effectively creating a single device.
In this case per the OP, the secondary sides are separated by lengths of conductors.
 

I&E Chris

Member
Location
Maysville
I have also seen this in applications using multiple generators supplying a single buss (in fact that is the setup we are taking out of service). My main concern is that with that setup your conductors from the generator to breaker are sized for the load the generator is capable of putting on the buss. But in the conversion to purchase power we will be using parallel conductors to refeed the buss. If the decision to use the two breaker setup is made and one of the breakers trip due to mechanical reasons then the entire load will still be running on only one set of the parallel feeder. I have looked thru the code trying to find something to clarify that this is not the thing to do but have not had any luck.
 
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