transformer neutral connection question

[travish]

installed a GE 150kva xformer today delta - delta 480 to 240 mid tap neutral connection on the secondary. Note on the neutral wire says kva of neutral is 0.05% of xformer kva rating. the wire at the neutral tap looked like a #8 awg. only running 3 phase loads so I will not have any current on the nuetral anyway. Here is my question

Hypothetical lets say 4/0 conductors on the secondary feeding a panel 25 ft away. somebody runs a forklift into the conduit feeding the panel. Only 1 phase shorts to ground. Can a #8 wire carry enough fault current to trip the primary or would it burnout like a fuse?

Looks like to me the manufacture would be required to provide a large enough neutral connection to carry the fault current availible from the xxformer

anybody got any thoughts on this?

Travis

 

[brian john]

One there should be secondary protection for feeder protection this should trip if there was a short on the secondary. (The issue with the number 8 still comes into the circuit)
Two the short term current rating of a conductor is much higher than 310-15 ratings, the brains to populate this forum will have the data I am sure.
Three supposedly the manufacture engineered this properly, and UL or some listing agency inspected this unit and tested it.

But you could easily overload (have high temperatures on the conductor) the #8 (I would think) in the long term with loads on "A" and C" phase. But as noted this is a short length of conductor and if insulated I would think this is a very high temperature insulation.

 

[mike_kilroy]

could this be some kind of special purpose transformer? I believe most wye nuetrals are good for full kva ratings as standard. what is the transformer part no? Maybe we canlook it up and find if it is for special use?

 

[augie47]

could this be some kind of special purpose transformer? I believe most wye nuetrals are good for full kva ratings as standard. what is the transformer part no? Maybe we canlook it up and find if it is for special use?

It's probably a 240/120 delta with a center tap. They are often used where you have a heavy 3 phase load and minimal single phase. I once heard them referred to as "pump house transformers" as they were in pump houses with large 3 phase motors and only a light and receptacle on the 120v tap.

 

[jim dungar]

There are some inherent problems with 'closed-delta' arrangements on a common core, with the most common result being that L-N loading must be limited to 5% of FLA. This is one reason that center-tapped delta banks are usually built using individual transformers.

The important thing for fault handling, is the size of the grounding conductor not the grounded ones. Your need a GEC and EGC based the the FLA of the transformer.

 

[don_resqcapt19]

...
The important thing for fault handling, is the size of the grounding conductor not the grounded ones. Your need a GEC and EGC based the the FLA of the transformer.

But in the case of this transformer, isn't the only path back to the source for a ground fault via the small interal grounded conductor in the transformer?
I think the OP's question is if the #8 in the transformer will act as a fuse prior to the OCPD clearing the ground fault.

 

[jim dungar]

But in the case of this transformer, isn't the only path back to the source for a ground fault via the small interal grounded conductor in the transformer?
I think the OP's question is if the #8 in the transformer will act as a fuse prior to the OCPD clearing the ground fault.

A short circuit study needs to be done to determine the amount of Single Line-Ground fault current.
The internal transformer X4 to Lug/Pad conductor will most likely have a different fault capacity than one with normal thermoplastic/thermoset insulation. I am guessing it was evaluated during the listing process, but there is no harm in checking it out.

 

[travish]

Don

You are correct, the only path for a ground fault is back on the small wire. This was my original question. looks like if only 1 phase shorted to ground the #8 possibly would melt before the breaker tripped and then you would basically have a corner grounded delta?

Travis