Transformer Connected Load

[mull982]

Hello

I am trying to add another load to an existing 480V 1200A Bus MCC. The MCC is fed from an 480V 750kVA transformer capable of supplying 900A on the LV side to the MCC. I have added up my connected loads to the MCC and noticed that the total connected load is somewhere around 1050A. However when I monitor the ammeter on the tranformer LV side, we are only drawing about 300A max during normal operation. There is a 1200 A breaker feeding the MCC from the transformer rated for 1200A but dialed down to 900A.

I was wondering if there was some sort of divirsity factor that allows for a connected load to be more that what a transformer is rated for. Does anyone know anything or is there somewhere in the code that explains the connected load and divirstiy factor? Would my connected MCC load have to be lower than the 1200A bus rating regardless? I am looking to find out how to calculate the divirstiy factor (if avaliable) or what the max connected load I can have on the transformer is.

Seperate but quick question: Is a magnetic instantaneous molded case breaker capable of interrupting a ground fault. Does the breaker trip on the magnitude of the gnd fault flowing through the faulted feeder in the breaker.

Thanks

Mull982

 

[bsh]

Transformer connected load

Transformer connected load

Unfortunately the NEC does not discuss diversity. You may be able to get some information about this application from the owner. He may assume a certain amount of diversity for his system. Since your connected load is 1050 amps but the actual load is 300 amps this gives you a pretty good idea that the actual load is generally a small fraction of the connected load MOST of the time. The safest thing is to always assume that your actual load and connected load are equal. This is also the most expensive way to do things.
I would not load the bus more than it is rated for and I would use the connected load for that value. The transformer life will be shortened if it is overloaded.
A ground faut breaker measures the current in and out of a circuit and trips when the level is exceeds the grount fault setting of the breaker. If the current is high enough through the instantaneous trip breaker it will trip. It doesn't care if the current is feeding a short circuit or ground fault - it just senses current flow. If you need to provide GF protection then this will be in addition to the OC and short circuit features of the breaker.

 

[coulter]

mull -
Take a look at 220.60 (220.21 in 2002). Lots of times industrial installations have installed spares.

Also look at 220.87 on using actual demand for existing loads.

carl

 

[zog]

What type of transformer?

As far as your breaker question is concerned in theory it depends on the type of breaker and trip device used and if it is in a delta or wye system.

 

[coulter]

... Is a magnetic instantaneous molded case breaker capable of interrupting a ground fault. Does the breaker trip on the magnitude of the gnd fault flowing through the faulted feeder in the breaker....

Mull -
Kind of interesting the way you phrased the questions. About the only place you can use a mag-only CB is as part of a listed combination controller. Is that your application?

CBs don't care if the current going through them is line-fault, ground-fault, or just plain load current. The trip operates on the phase currents and trip equation. In the case of a "normal" mag-only molded case, the equation is just magnitude peak current > setting.

1. The interupt rating of a CB has to be greater than the available short circuit current. For example one would not want to put a 25KA rated CB in a system with 40kA available.

2. I don't think I understand your second Q. The short answer is, "Yes."

 

[mull982]

The transformer is a 750kVA 4160V - 480V Delta - Y transformer. It is a dry type transformer and has an impedance of 5.55%. The nameplate says that the LV side of the unit is rated for 900A.