New 3phas Transformer Effects on Feeders

Status
Not open for further replies.
E

eeee

Senior Member
I am thinking about adding a new 225KVA 3 phase, 208/120volt wye-wye connected transformer to a facility that presently has a 300KVA, 208/120 volt three phase wye-wye connected transformer on it. The loads on the building are less than before and will stay that way. After I complete my calculations and determine the size of cable required from the utility pole drop down location to the transformer and complete my calculations for the service drop from the new 225KVA 3 phase transformer to the MDP, I expect not to have to worry about resizing the electrical feeders from the MDP to the various subpanels since they have already been sized and this is not related to the size of the transformer, but only the loads that existed in the building at the time they were sized.

I am almost positive about this statement. Each segment of a distribution system is sized just for it's relative loads, not for what is upstream on different segments of a distribution system.

I know that making this statement is bold, but I believe it to be true.
 
Re: New 3phas Transformer Effects on Feeders

Originally posted by eeee:I am almost positive about this statement. Each segment of a distribution system is sized just for it's relative loads, not for what is upstream on different segments of a distribution system.
Click to expand...
Your statement is close to being true. It is true, if the only concerns were ampacity and voltage drop. But adding another transformer will result in an increase in the value of fault current that is available at each panel in the distribution system. I think it most likely that this will not cause any panel to have a higher "available fault current" than it is rated to handle. But you will have to perform a calculation to prove that that is the case.
 
Re: New 3phas Transformer Effects on Feeders

My description does not state the case accurately. I am replacing the existing transformer with a new transformer rated lower than the existing. I am replacing the 300KVA, wye-wye connected, 208/120 volt 3 phase transformer with a 225KVA wye-wye connected, 208/120 volt 3 phase transformer. This particular segment of my network is wye-wye connected. Other segments of my distsribution network for other buildings are wye-delta connected (but that is not of my concern for this job).

I will calculate the new ampacities for the cable running from the utility pole drop to the new transformer and from the new transformer to the MDP.

I plan to leave the feeder cables from the MDP to the various subpanels in place knowing that they were previously sized in the past for their loads.

I do know that the loads in the building will become less than they are now by about 100 amps for the long term. This means that the original loads calculated for the present feeder cables will be less. I don't want to replace all these feeder cables from the MDP to the various subpanels now if I don't have to. There are no reports of problems now with them now. They will become somewhat oversized feeder cables when the amp demand loads drop after the facility looses the manufacturing function that is moving from this location.
 
Re: New 3phas Transformer Effects on Feeders

IF you leaving all your voltages and configurations the same, why replace the transformers at all?
 
Re: New 3phas Transformer Effects on Feeders

Answer to question 1: The present transformers are aerially distributed, but ground mounted and that is a NEC code violation. Live exposed parts (i.e. the stingers, etc.) must be mounted 9 feet in the air above the head. It must be an underground run.

Answer to question 2: The present configuration has only 3 wires being used as a service drop as I look at it. I think the records are wrong. I have a load study and a survey study done by our electicians that say it is a wye-wye. The 3 wires going to the building means it is a wye-delta based on my knowledge knowing a delta has no ground.
 
Status
Not open for further replies.
Top