3000A Bus Load

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mull982

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I've been posting a lot recently (first time building a new plant) but wanted to thank everyone for their responses and sharing of knowledge. I've said it before and I'll say it again, this site is great!

Our plant has a main 4.16kV 3000A bus with an incoming 5kV breaker set to 3000A. Off of this main bus there are (5) 1200A feeder breakers feeding (5) 1200A 4.16kV buses and a feeder breaker feeing a 6500hp 5kV motor. Ignoring the motor breaker for a second when I add up the (5) 1200A busses which are fed from this main bus I get 6000A which is twice the rating of the main bus. Is this correct? How can 6000A worth of buswork be fed from only a 3000A main bus? I could see if the (5) 1200A breakers that are feeding these (5) busses were set to some lower value but the relay settings shows to set all of these breakers to 1200A.

Is there something that I am missing here? What section of NEC would reference this?
 
Load diversity, coordination, and selectivity.
Think of as your 200Amp main house panel with 42 20 amp feeder breakers (42x20 amp > 800 Amps! (if you look at it that way))
 
key word might be "load diversity"
on a much simplier scale, your 200 amp home panel has 600 amps worth of branch breakers installed.
Hopefully, there was an engineered load calculation on the facility and the main sized accordingly. The branch buss appears to be designed to carry
attached or future loads but tassuming that all loads will not be on simutaneously.
Actually, from an outside point of view, sounds like pretty good engineering.
 
That is the difference between 'connected' load and 'demand' load. Like augie said, if you add up the 20 amp breakers in your house you'll get a much bigger number than the rating of the panel, but that doesn't mean anything. The assumption is that at any given time, only a certain percentage of the equipment will be on.
 
augie47 said:
key word might be "load diversity"
on a much simplier scale, your 200 amp home panel has 600 amps worth of branch breakers installed.
Hopefully, there was an engineered load calculation on the facility and the main sized accordingly. The branch buss appears to be designed to carry
attached or future loads but tassuming that all loads will not be on simutaneously.
Actually, from an outside point of view, sounds like pretty good engineering.
Click to expand...

I understand the house panel analogy thats a good way of looking at it. So to understand correctly as long as the main bus is sized to handle the total connected load then it is ok to have branch feeders similar to branch breakers that add up to more than the main bus? Using the house panel example although you have 600A worth of breakers installed can your total connected load be over 200A, or must it be under? Same with my plant, the toal calculated connected load is 2894A which is under 3000A but must this connected load stay under 3000A even though we have room to add load onto the 1200A buses?

drbond24 said:
The assumption is that at any given time, only a certain percentage of the equipment will be on.
Click to expand...

I'm assuming that the "assumption" that you are talking about referes to the divirsity factor? How are these divirsity factors calculated, or is there a method or rule of thumb for selecting these dirvirsity factors?
 
mull982 said:
I've been posting a lot recently (first time building a new plant) but wanted to thank everyone for their responses and sharing of knowledge. I've said it before and I'll say it again, this site is great!

Our plant has a main 4.16kV 3000A bus with an incoming 5kV breaker set to 3000A. Off of this main bus there are (5) 1200A feeder breakers feeding (5) 1200A 4.16kV buses and a feeder breaker feeing a 6500hp 5kV motor. Ignoring the motor breaker for a second when I add up the (5) 1200A busses which are fed from this main bus I get 6000A which is twice the rating of the main bus. Is this correct? How can 6000A worth of buswork be fed from only a 3000A main bus? I could see if the (5) 1200A breakers that are feeding these (5) busses were set to some lower value but the relay settings shows to set all of these breakers to 1200A.

Is there something that I am missing here? What section of NEC would reference this?
Click to expand...


With MV metalclad switchgear, a 1200A bus or breaker is the smallest that is available, regardless of what is needed, and it seems that is what you are trying to add up. Jsut because you have a 1200A bus doesn't mean you are going to use it.

Load flow, short circuit and motor starting calculations should have been done by a power system engineer (hopefully anyway) and the relays will set to trip the breakers as a coordinated system.

The fact that if the relays are all being set at 1200A continuous then you might want to make sure 1200A worth of cable was actually installed, as it may or may not be able to handle the full 1200A at each location. Depends on the designers/owners preference.

I would make sure that relay setting sheets and time current coordination (TCC) curves are provided and that the factory default settings are not inadvertantly being used.
 
At the medium voltage level, the "diversity" is usually very well known. MV systems are much more closely scrutinized during the design phase.

It sounds like you truly would like to understand this issue. To do so, ask the engineer if he would get you a copy of the load flow study that was done. It will show the load at each bus, under normal operating conditions as well as any contingencies planned for. Keep in mind there may multiple scenarios and contingencies analyzed. The design should have been based on worst case project design requirements set, or agreed to by the Owner.
 
"the toal calculated connected load is 2894A which is under 3000A but must this connected load stay under 3000A even though we have room to add load onto the 1200A buses?"

It is possible for the connected load to exceed the 3000A rating of the incoming bus for the same reasons as stated above.

Connected load = all possible electrical loads connected to the electrical system

demand load = all electrical loads required to operate the facility and all ancillary items.

Diversity load allows for a factor that not all required loads will be running at 100% of nameplate values

Example
If there is a critical operation and there are two motors for same function (one running and the other in standby). If each has a connected load (nameplate full load amps) of 100 amps. The connected load would be 200 amps. However the operation only requires one motor to be on at a time. The diversity load would be 100 amps. If the motors are sized to operate around 75% of capacity to allow for future growth of process (upgrading impellers etc). The demand load would only be about 75amps. All of the electrical loads are included by engineer (should be anyway) in load study and load flow calculations which are done to properly size the electrical distribution system for facility

If you are still not comfortable, have a meeting with the engineer to walk you through the plant electrical loading and equipment sizing for your particular facility.
 
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