480delta to 240delta xformer size

[mag100]

We are installing a new extrusion line and I got schematics from the OEM. The first thing that caught my eye is the value of the xformer. 55KVA primary overcurrent device 150amp breaker. THe values they give me are 388 Fla on the 240v side. Their formula is 388/3phase=129.3anps.So 240*129.3*1.73=53.7KVA.I thought 240*388*1.73/1000=161KVA xformer.My question is why are they dividing by 3 when their is no phase to nuetral voltage. The know values I have are 75210watts for total heater band values at 327.14amps. SO 240*327.14*1.73/1000=135.8KVA. With these values a 150KVA be correct? Also is secondary overcurrent protection nessacary for delta delta xfromers since the oem is already not providing?

 

[alehman]

Assuming this actually is a 3-phase, 55kVA transformer:
55kVA/480/sqrt(3) = 66A @ 480, 3ph
55kVA/240/sqrt(3) = 132A @ 240, 3ph

The 150A primary breaker is somewhat on the high side, but if NEC applies, it would be OK if you have a secondary breaker < 1.25 * 132A.

388A*240*sqrt(3) = 161kVA

Are there three 55kVA, 1-phase transformers connected delta-delta?

75210W of heaters at 240V is 181A. I'm not sure where 327.14 comes from.

 

[busbar]

Addressing your questions in reverse, 99NEC 240-3{f} permits primary overcurrent protection to ‘cover’ the secondary for 3-wire ? service, but 99NEC 450-3 seems to mandate a 125% multiplier for ‘primary-only protection.’

55kVA is not a standard transformer size for conventional drytype components or drive-isolation transformers. [It may be a very expensive typo for the vendor.]

 

[mag100]

Let me rephase my question at my total wattage of 75210watts at 240 volts and the ampacity totalled of theses wattages at 327.14 what would be the proper sized xfrormer size 480 to 240 3phase delta delta and is secondary overcurrennt protection nesacarry

 

[jbartos]

Suggestion:
240V x 327.14=78513.6VA
The closest three phase standard transformer rating is 112.5kVA.
480Vdelta/240Vdelta, 112.5kVA transformer
The secondary overcurrent protection is recommended; especially, if the transformer is safety related.

 

[rbulsara]

To add to what jb said:

Your total load is 75,210W or 78514 VA. Depending up on where you are, all you need is a total transformation capacity of more than 78514 VA or 78.5 kVA.

If a three phase transformer is used and the load is balanced, the load per phase will be 78514/3 = 26171 kVA or 327A/3 =109A per phase.

I beleive what you have is three 55 kVA single phase transformers arranged as a 3 phase bank, providing you 165kVA of total transformation.

In theory, you can connect all three single phase transformers to a single phase primary circuit.

 

[rbulsara]

One more item:

150A primary breaker sounds like a very acceptable size for a 480V primary of a 112.5 kVA transformer, which is a standard size (and voltage) in the USA. You may wish to verify your 55 kVA figure.

 

[mag100]

The major question at this point for me is that the &quot;OEM&quot; is stating to me the value of this ONE transformer dry type 480vdelta to 240vdelta is 55KVA. With the value of FLA which they have caculated to be on the 240v side to be 388amps. How can this be 55KVA. I know they took 388/3=129.3amps then * 240v* 1.73/1000=53.68kva. Why divid by three? Where are they getting this formula? KVA 3phase= V*A*1.73/1000 correct?

 

[jbartos]

Suggestion: Please notice that 55kVA transformer is not standard rating; 50kVA and 37.5kVA, 1Ø are.

 

[rbulsara]

mag100:

Consider the follwoing:

1. The OEM may be considering the Load Diversity. The total connected load may be 75kW, but they must have known that the process does not use more than 55KVA at any given time.

2. 55kVA, 240V, 3 phase xftmr will have 55/(1.732*0.24)=132A.

3. 388A may be the total single phase load at 240V. If shared evenly by a 3 phase system the current per phase will be 388/3 =129A. However the 240 is L-L voltage. The voltage per phase is 240/1.732 =139V, regardless of wye or delta connection. So kVA per phase= 139V*129A/1000= 18kVA. The total of 3 phase is 3*18=51kVA and they chose 55kVA as minimum size.

4. Are you sure they are not suggesting 150A secondary main breaker?

 

[mag100]

rbulsara:
At startup and maintance that will have to be performed I know they will turn on all of the heats at once. I know it won't be often but will happen. The 150amp breaker is on the primary side of this transformer and the secondary is not protected. They are using individual CB for every zone.

 

[rbulsara]

mag100:

It's a matter of load diversity. If you feel all loads will be turned on at the same time, provide a larger tranformer, in which case the standard size will be 112.5kVA as noted before.

Good Luck.

 

[jbartos]

Suggestion: Sometimes, it may be necessary to prove the load switching via timing diagrams and interlock logic, to evidence how much load can be switched at a time.