transformers 6

[stevemechanical]

can someone plese check my calculation:
given: (3) 100 kva transformers.

question: what is the amperage output per transformer?

[(100 kva) * (.85 power factor)]/[(230 volts) * (sqrt(3)]=213 amps

if this is correct, does this mean i have 213 amps capacity
per each line?

 

[dpc]

It depends what you mean by "capacity". If it is rated at 230V, the 100kVA transformer can produce 250 amps - it doesn't care(or know)about the power factor. If your load has a power factor of 0.85, then the 100 kVA transformer could support a load of 85 kW.

 

[stevemechanical]

thank you dpc

 

[jcpitts]

Are you saying your secondary voltage from your transformer is 230 volts?

If you are designing a new service and are looking at sizing an appropriate transformer you would typically start with adding up your components loading then pick the right transformer size.

for a 3 phase trasformer setup with a 230 V secondary and a 220 A full load current an 88Kva transformer would be the size. Some might oversize the transformer for future loads

For 230V secondary and 250 amps full load current a 100KVA rated transformer appears to be appropriate

go to

http://www.elec-saver.com/freetool.htm

disclaimer: dont take my word for it though.

 

[rbulsara]

Do not forget to size (ampacity) the conductors for 125% of the continuous load. or 1.25*251A =313A. A requirement n NEC, your code may be little different. Nonetheless it is good engineering too.

 

[remmeal]

you should also consider the connection of the transformer. each of the three transformer may be connected as a single phase 230V.

 

[jbartos]

Suggestion: The transformer current I is:
I=kVA/(sqr3 x kV)=100kVA/(sqrt3 x .230kV)=251Amps

 

[alehman]

Are these (3) 100kVA 1-phase transformers?
If so,
100kVA / 230V = 435 amps

If (3) 100kVA 1-phase transformers are connected in a 3-phase bank, you effectively have one 300kVA 3-phase transformer.
300kVA / 230V / sqrt(3) = 753 amps

 

[jbartos]

Comment on the previous posting: Although the calculation is pretty obvious to an electrical specialist, it may be posing a problem to a novice, namely:
100kVA/0.230kV=435Amps
300kVA/(0.230kV x sqrt3)=753Amps
i.e. the plain crunching numbers would not do.

 

[rbulsara]

I re-read the original question and the last response (question) posted by alehman is very valid.

(3)100kVA single phase transformers will constitue a 300kVA 3 phase bank.

Stevedantonio should clarify.

 

[stevemechanical]

thank you all for your responses.
the set up is 3 separate high voltage lines coming into a transformer vault, therefore each transformer is connected to a single phase.
From the transformers, 4 sets of (4) 500mcm feeders go to a 1600 amp disconnect and ct cabinet.

 

[jbartos]

Comment on the previous posting:
The 1600A Disconnect appears to be too high. Which size of fuses is inside?
kVAsec=sqrt3 x .230 x 1600=637kVA
for three single transformers connected in 230V delta on the secondary
kVAsec=sqrt3 x sqrt3 x .230 x 1600=1104kVA
for three single transformers connected in sqrt3 x 230V = 398V wye on the secondary
It looks like the single transformer may have the 120V secondary.
kVAsec=sqrt3 x .120 x 1600=332kVA
for three single transformers connected in 120V delta on the secondary
kVAsec=sqrt3 x sqrt3 x .120 x 1600=576kVA
This result would not be correct since the transformer could be overloaded.
for three single transformers connected in sqrt3 x 120V = 208V wye on the secondary

 

[alehman]

I should qualify my previous posting. Assuming the transformers are connected as a 3-phase bank, and the secondaries are connected in a delta configuration, then my previous posting is correct. 753 amps is the rated output current of a 3-phase delta bank. (I assumed delta connection because that is most common for 230 volt, 3-phase systems).

If the secondaries are connected in a wye configuration then the rated output current of the 3-phase bank would be the same as for one of the three transformers.

300kVA / 0.230kV / 3 = 435 amps


As noted in the previous post, if the secondary overcurrent protection (OCP) is 1600 amps that may be too high. According the U.S. National Electrical Code, in unsupervised locations you are required to have secondary OCP set at not more than 125% of the rated secondary current of the 3-phase bank.

Are these transformers owned by a utility company? Is so, it may not be subject to code requirements. In the U.S., utilities commonly undersize transformers compared to the secondary disconnect size.

 

[jbartos]

Comment on the previous posting: The Utilities tend to size their equipment for profits. This is especially attractive, if the NEC does not have to be followed. One big advantage of Utilities is the knowledge of various load demands, derating factors, diversity factors, etc. Also, there are suitable "test" points available where to measure current and from that the load.