Transformer Rating

Status
Not open for further replies.
K

ken44

Senior Member
Location
Austin, TX
I have a 150KVA transformer that is located inside a Power Distribution Unit (PDU), made by Liebert, the PDU has 4-42 circuit panels and each panel is fed by 250MCM (255 Amps, 75 Degree Column), the name plate rating is 277/480Volts Input/186Amps, the Output is 120/208Volts/417Amps. I have a few questions concerning this that have arisen over the years but I have'nt concluded that anyone to this point has actually given me a satisfactory answer.

1. Is the 417 Amps output per phase or total? I thought is was total based on 150KVA/360=417Amps

2. Even if the answer is 417 Amps per phase, thats only 1251 Amps total and with 168 circuit breakers available and if each one were only 20 Amps this would be a potential load of 3360 Amps or 2688 at 80% which still seems to be way off.

Load Calculation: The customer needs 100 each of NEMA L6-20 circuits (208V/20A/2Pole), 78 each of NEMA L6-30 circuits (208V/30A/2Pole) and 5 each of Russellstol 208V/50A/3Pole circuits and they need redundant feeds, this requires 380 circuit breaker spaces but with redundant feeds (190 circuits from 2 power sources) it gets a little more complicated because I only have capacity for 168 spaces in each PDU so I understand that they will either need another PDU or two or will have to give up some of their rendundancy. I would also appreciate any insight that someone may have about the load calculation.
 
ken44 said:
I have a 150KVA transformer that is located inside a Power Distribution Unit (PDU), made by Liebert, the PDU has 4-42 circuit panels and each panel is fed by 250MCM (255 Amps, 75 Degree Column), the name plate rating is 277/480Volts Input/186Amps, the Output is 120/208Volts/417Amps. I have a few questions concerning this that have arisen over the years but I have'nt concluded that anyone to this point has actually given me a satisfactory answer.

1. Is the 417 Amps output per phase or total? I thought is was total based on 150KVA/360=417Amps
Click to expand...

Assuming the secondary is wye (star) connected, 417A is the rated maximum current for each leg of the wye.
 
ken44 said:
Load Calculation: The customer needs 100 each of NEMA L6-20 circuits (208V/20A/2Pole), 78 each of NEMA L6-30 circuits (208V/30A/2Pole) and 5 each of Russellstol 208V/50A/3Pole circuits and they need redundant feeds, this requires 380 circuit breaker spaces but with redundant feeds (190 circuits from 2 power sources) it gets a little more complicated because I only have capacity for 168 spaces in each PDU so I understand that they will either need another PDU or two or will have to give up some of their rendundancy. I would also appreciate any insight that someone may have about the load calculation.
Click to expand...

You need to get the required loads for each circuit to do a load calculation.
Typically these circuits don't get loaded to 80%.
Are the number of circuits that you list for one side of the redundency or split between the two sources?

PDU-1
50, L6-20
39, L6-30
3, 3p 50

PDU-2
50, L6-20
39, L6-30
2, 3p 50
 
Two things come to mind: First, I would confirm there is some type of overcurrent protection on the transformer. If it's a factory PDU, there is probably manufacturer OCP of some type and, as a factory assembly, it is only required to meet the listing requirement and not the NEC.
Secondly, Load diversity seems to be the key here. As you have both single phase and three phase loads you will need to carefully watch how you balance them and not overload one phase of the transformer.
Looking at the number of the circuts and the ampere of each is to some degree not important. That number normally far exceeds the capacity of the system (add up the ampere ratings of the individual breakers on your 200 amp home load center). You will need to deternine the possible or probable load based on how many of the circuits will be used at one given time.
If the PDU has OCP the exact number isn't too critical as it will protect itself, but obviously it needs to be addressed. As far as spaces needed, if your calculatins show that, due to load diversity there will be no overlaod, then you could feed sup panels from the PDU and gain breaker spots.
 
Transformer Rating

TKB-The way you show it is correct.
Kesober-I am not absolutely sure is the secondary is wye, it is 208/120 Volts and I know that most Dry-Type transformers that we use are Delta-Wye unless we need 120/240 then we use Delta-Delta Transformers. However, I would appreciate further explanation as I am intrigued by the possibilities of your answer.
 
Transformer Rating

Besoeker, sorry I screwed your name up so bad!
Gus, This PDU is fed through a building UPS and thats where the OCPD is and I am not sure what the size of the CB is but can check into that, however it would take some time as it is across town.
 
ken44 said:
TKB-The way you show it is correct.
Kesober-I am not absolutely sure is the secondary is wye, it is 208/120 Volts and I know that most Dry-Type transformers that we use are Delta-Wye unless we need 120/240 then we use Delta-Delta Transformers. However, I would appreciate further explanation as I am intrigued by the possibilities of your answer.
Click to expand...
I would think it most likely that it is wye connected. That would give the 208V line to line and 120V line to neutral.
Two ways to get to the line current:
150kVA at 208V is a line current of 417A
IL = 150,000/(208*sqrt(3)) = 416.3A
It doesn't matter whether the transformer is wye or delta, 417A is the line current.

If the transformer is wye connected you can consider it as three 120V single phase circuits:
IL = 150e3/(120*3) = 416.7A*
In wye, the phase current and the line current are, of course, the same.

*the slight discrepancy is because 280V LL doesn't give exactly 120V LN.
 
Status
Not open for further replies.
Top