FULL LOAD ON A SWITCHGEAR

[mopar]

I WORK FOR A COMPANY AS A PLANT ENGINEER THAT CURRENTLY HAS A 1200 TON PRESS AND THEY ARE LOOKING FOR INSTALLING ANOTHER 1000 TON PRESS. THE BUS IS ON A 2000 AMP SWITCHGEAR AND THERE IS CURRENTLY A PARTS WASHER THAT PULLS 200 AMPS, A 600 TON PRESS THAT PULLS ABOUT 100 AMPS AND THE 1200 TON PULLS 600 AMPS. ON TOP OF THESE ARE OVERHEAD CRANES, AND PLANT LIGHTING. IF THE CODE IS TO RATE THE LOADS AT 125% OF PEAK CURRENT, WON'T THE 2000 AMP SWITCHGEAR BE INSUFFICIENT? THE MAINTENANCE DIRECTOR IS UNDER THE OPINION THAT WE JUST NEED TO TAKE THE LOAD OF THE MACHINERY WHEN IT IS RUNNING TO RATE THE SERVICE, THEREFORE THE 2000 AMP THAT WE HAVE NOW IS SUFFICIENT. I DISAGREE AND I WONDER WHO IS RIGHT?

additionally our current buss plug going to this area is 1200 amps feeding a 1200 amp i line panel
the current machinery running off this panel have machine ratings of 400amps, 400amps, 100amps,5@ 20amps, 1@ 30 and 50 amps doesn't section 430 say
that the feeder circuit has to rated at machine rating and not running draw.

[ June 06, 2003, 09:30 AM: Message edited by: mopar ]

 

[bennie]

Re: FULL LOAD ON A SWITCHGEAR

You can load the switchgear to 100% with a calculated load.

 

[stuartw]

Re: FULL LOAD ON A SWITCHGEAR

I work in a medium size chemical plant and whenever there are plans to increase the loading on a transformer/switchgear/MCC/etc we take actual loading information. We have a recording kva/kw meter (Voltage, amps, KW, KVA, and PF). Depending on how much load we want to add, we will take anywhere from a 1 day to 1 week survey. During that time, we make sure the plant is running at the maximum rate for some part of the recording. We will then analyze the information to verify the switchgear has sufficient room to allow the expansion, looking at current and KVA. We typically use max amps for new load, even though it may run less. The swithgear could be loaded to its listed rating.
Quite often there are other things to think about in addition to just adding the new load. Voltage drop on starting can be an issue with adding large motors to existing switchgear. The taps on the transformer may need to be adjusted to get optimum voltage during normal operations due to the additional loading. Coordination of group breakers to mains may need to be examined.

Hope thats useful to you.

 

[charlie tuna]

Re: FULL LOAD ON A SWITCHGEAR

that all sounds good but you really need to look at connected load.
example: office building emergency switchboard is being expanded to service some additional elevators being installed in a parking garage expansion. data loggers were installed for two weeks. the results were used to justify the new loads to be added to the existing switchboard. everything was fine for two years, then during a fire system test the main breaker tripped due to a blown fuse caused by "overload"... nobody thought about the smoke evacuation fans!!!! we do data logging installations and many times are called for this type of idea by engineering firms. the results of "normal loads" can be useful but then non-operating loads must also be considered....

 

[roger]

Re: FULL LOAD ON A SWITCHGEAR

Charlie, this doesn't hold water. The elevators could (by design) have already recalled or have been "load shed" by the time the smoke evac took place. There seems to be more than the connected load being a design issue here.

Roger

[ June 09, 2003, 05:56 AM: Message edited by: roger ]

 

[charlie tuna]

Re: FULL LOAD ON A SWITCHGEAR

reduced load due to elevator recall cannot be considered in total load calculations. what would happen during fire system testing. the elevators get bypassed and wam! the main breaker trips. smoke fans carry a large current draw. before the codes changed preventing adding loads to the emegency generator, buildings would add computer loads to the emergency generator by looking at the full load current drawn by the generator. then they would have a fire alarm and loose the entire emergency system due to the generator's output breaker tripping!!

 

[roger]

Re: FULL LOAD ON A SWITCHGEAR

Charlie,

reduced load due to elevator recall cannot be considered in total load calculations.

why, and where is this written? In design and engineering, loads that are selectively shed or sequentially stepped can certainly be considered in the design and sizing of the system. This is done in sizing services per article 220 considering A/C and heating loads.

what would happen during fire system testing. the elevators get bypassed and wam! the main breaker trips.

I don't understand, if the elevator load is bypassed, and the main trips, there would be something other than the additional elevator loads causing the problem. Maybe these fan motors are starting to hard and need a slower ramp, or the fan motors should start sequentially.


Roger

[ June 09, 2003, 12:42 PM: Message edited by: roger ]

 

[charlie tuna]

Re: FULL LOAD ON A SWITCHGEAR

roger,

normal sequence for fire would be for the elevators to go into recall or report to the floor selected for recall and remain there with their doors open until used by fire personnel using the fire over-ride switch or until the fire system is reset and they then return to normal operation.

many buildings fire test during normal building operating hours by "bypassing" the fire signal to the elevators which allows the normal elevator operations during the test. maybe the first signal is used to assure elevator recall and then it is bypassed otherwise you would have the lobby full of people trying to get up in the building.

i have been involved with actual high rise office building fires and the elevators are used by fire personnel to move equipment. the smoke evacuation fans are not sequenced for load considerations in any buildings we have worked in.. they come on full bore and if they are vfd- the go to the fire mode speed and go directly to maximum speed.

connected load must be part of the calculation, not just load recordings over a period of time.