Three phase trasnformer- single phase loads

[Poolside]

I came across a situation with a residence that has a 600a/3p/480v service. A 2 pole-40amp breaker feeds a 75kVa transformer. The transformer has a 3 phase-480v delta primary and a 120/208Y secondary. The existing wire connections are:

H1---- 480 A phase
H2---- jumpered to H1 with #10 wire
H3---- 480v B phase

X1---- 120 secondary
X2---- 120 secondary
X0---- grounded for neutral
X3---- no connection

The secondary is run with 4/0 Al XHHW. No secondary OCPD installed.
Due to recent additional loads, the 480v/40a primary breaker trips with a load projected to be about 50 amps.

What is the effect of the transformer wiring connections on the load rating of the transformer. A 75 kVa transformer should be able to handle a larger load than this. I would like to install a larger feeder for the primary, and perhaps install secondary OCPD to get the needed power output, but I'm not certain, though of the wiring scheme that is currently in place.

Any suggestions would be welcome.

Thanks,
Greg

 

[frenchelectrican]

I came across a situation with a residence that has a 600a/3p/480v service. A 2 pole-40amp breaker feeds a 75kVa transformer. The transformer has a 3 phase-480v delta primary and a 120/208Y secondary. The existing wire connections are:

H1---- 480 A phase
H2---- jumpered to H1 with #10 wire
H3---- 480v B phase

X1---- 120 secondary
X2---- 120 secondary
X0---- grounded for neutral
X3---- no connection

The secondary is run with 4/0 Al XHHW. No secondary OCPD installed.
Due to recent additional loads, the 480v/40a primary breaker trips with a load projected to be about 50 amps.

What is the effect of the transformer wiring connections on the load rating of the transformer. A 75 kVa transformer should be able to handle a larger load than this. I would like to install a larger feeder for the primary, and perhaps install secondary OCPD to get the needed power output, but I'm not certain, though of the wiring scheme that is currently in place.

Any suggestions would be welcome.

Thanks,
Greg

Huh?? IMO i don't feel safe with this kind of connection there

what size load center it have on secondary side ?

Normally to use full 3? on the transformer the primary breaker i used is 90 amp while secondary breaker is 200 amp that will able to load it up about 70 or so KW.

not very often i heard someone actally get 3? 480v in resdentail place unless megasized one.

but why use the three phase transformer instead of single phase transfomer ?/ [ that part i just never get it in my head i know there is 1? transfomer simair to this size avabile in market.]

i will let other guys chime in as well see what they have to say with this set up

Merci, Marc

 

[Poolside]

Huh?? IMO i don't feel safe with this kind of connection there

Me either.

what size load center it have on secondary side ?

They (I don't know who) ran the 4/0 to a pull box that is tapped to feed a 100a sub-panel for a garage/workshop. The 4/0 feeder continues to another 200a panel. That panel then subfeeds via breaker an additional 125 amp panel (exterior lighting and power), and (2) 60 amp sub-panels for the swimming pool equipment.

Normally to use full 3? on the transformer the primary breaker i used is 90 amp while secondary breaker is 200 amp that will able to load it up about 70 or so KW.

not very often i heard someone actally get 3? 480v in resdentail place unless megasized one.

It's a large property. I believe it was supposed to be a working orange grove at one point in time. The house is under 5000 sq. ft. but the property is probably 10 acres with some outbuildings now.

but why use the three phase transformer instead of single phase transfomer ?/ [ that part i just never get it in my head i know there is 1? transfomer simair to this size avabile in market.]

That's what I don't know, either. They had three phase- they should have kept with it throughout the property. It would have been much better in distribution.

I didn't have time to shoot any pictures, but I'd like to. What a hack job this is. They want a price from me to fix the problem with the tripping breaker, but I think it may be a tad more than just the primary feed. Like I said, I don't know what the effect of the transformer's rating is the way it is wired now. I think I could re-feed it with a larger three phase primary, and install a fused switch on the secondary to keep it at 125 amps. That would probably fix the overload. Then it's just everything else...

- Greg

 

[jrannis]

I came across a situation with a residence that has a 600a/3p/480v service. A 2 pole-40amp breaker feeds a 75kVa transformer. The transformer has a 3 phase-480v delta primary and a 120/208Y secondary. The existing wire connections are:

H1---- 480 A phase
H2---- jumpered to H1 with #10 wire
H3---- 480v B phase

X1---- 120 secondary
X2---- 120 secondary
X0---- grounded for neutral
X3---- no connection

The secondary is run with 4/0 Al XHHW. No secondary OCPD installed.
Due to recent additional loads, the 480v/40a primary breaker trips with a load projected to be about 50 amps.

What is the effect of the transformer wiring connections on the load rating of the transformer. A 75 kVa transformer should be able to handle a larger load than this. I would like to install a larger feeder for the primary, and perhaps install secondary OCPD to get the needed power output, but I'm not certain, though of the wiring scheme that is currently in place.

Any suggestions would be welcome.

Thanks,
Greg

Is the other leg avalible for the primary side of the transformer?

Correct me if Im wrong guys but, I think you can only load that transformer configuration to 57% of its capacity.

 

[jghrist]

If this is a standard US transformer with the low voltage lagging the high voltage by 30?, then shorting H1 to H2 would give you 120 volts on X1 and X3, not X1 and X2. Must be a non-standard transformer.

The voltage would be 120 volts to neutral on either secondary phase and 240 volts between phases. The current in the primary would be 1/4 the current in the secondary. If the load is balanced between the phases, the transformer should be able to serve 50 kVA, 25 kVA on each used winding.

Sounds like this should work theoretically, but try explaining it to the AHJ.

 

[Poolside]

Is the other leg avalible for the primary side of the transformer?

Correct me if Im wrong guys but, I think you can only load that transformer configuration to 57% of its capacity.

The third leg of the primary would be available- I'll have to change the feeder breaker to a 3-pole, but that should be easy enough.

But I'm thinking that would change the secondary line-to-line voltage to 208volts.

Where did you come up with the 57% number?

 

[Poolside]

If this is a standard US transformer with the low voltage lagging the high voltage by 30?, then shorting H1 to H2 would give you 120 volts on X1 and X3, not X1 and X2. Must be a non-standard transformer.

The voltage would be 120 volts to neutral on either secondary phase and 240 volts between phases. The current in the primary would be 1/4 the current in the secondary. If the load is balanced between the phases, the transformer should be able to serve 50 kVA, 25 kVA on each used winding.

Sounds like this should work theoretically, but try explaining it to the AHJ.

Actually, it may be X1 and X3 and not X1-X2 since it is a standard GE transformer.

If I leave the connections as is, I should then treat this as a 50kVa transformer and fuse it accordingly with OCPD on the primary and the secondary?

 

[frenchelectrican]

That number it came up it the same idea with open delta transformer bank as far you know with close delta set up right ?? ok just take one leg off the capaicty reduced to 57% of rated transformer so like example 75 KVA transformer in close delta set up,

But open one of the delta leg it really reduce the performace so 75KVA close delta become 42.75 KVA transformer.

basically it the same idea with 3 individual transformer cans take one out it become open delta or open wye as well.


Merci, Marc