grounding delta/delta xfmr

Hello all,

Electrician here that may need a transformer wiring refresher on a common problem!

I have a machine that requires 240v 3 phase 50a and I chose to supply through a 480x240/120 center tap delta/delta 30kva xfmr since my building has 480 or 208 available.
there is no terminal XO, just X4 for the 120

I have no reference to ground from any phase on the secondary at the machine disconnect. Do I need to provide? Not sure if I have to corner ground??

I don't think there is a delta/wye xfmr at these voltages like the common 480x120/208

Thanks!

Ole Electric
 

kwired

Electron manager
Location
NE Nebraska
It is common to call that 120 volt neutral lead in that setup X4 instead of X0, it is not exactly neutral to all three phases, it is 120 volts between two corners but 208 volts to the oposite corner, AKA a high leg system.

I don't know for certain if you have to ground X4 per NEC, but theory wise if you don't need to supply 120 volts to anything you could corner ground it and leave X4 float.
 
ahh yes, now I remember. I used to see these feeding a high leg panels providing 120/240 with limited amperage on the 120
I bonded X4 to equipment ground for feeding the straight 240V machine

I am scratching my head though about a bunch of branch circuits feeding 15kva delta/delta 480/240 transformers wired to machines with no X4 or X0
do I need reference to ground from X1,X2 or X3 for NEC or safety? Main distribution on old factories had it. Some had ground fault lights.
 

kwired

Electron manager
Location
NE Nebraska
ahh yes, now I remember. I used to see these feeding a high leg panels providing 120/240 with limited amperage on the 120
I bonded X4 to equipment ground for feeding the straight 240V machine

I am scratching my head though about a bunch of branch circuits feeding 15kva delta/delta 480/240 transformers wired to machines with no X4 or X0
do I need reference to ground from X1,X2 or X3 for NEC or safety? Main distribution on old factories had it. Some had ground fault lights.
If you are going to utilize X4 for any 120 volt load you must ground X4. If you are going to corner ground it any leg is fine. If you are going to use it as an ungrounded system you must have ground fault indication equipment. You can not directly supply any 120 volt loads if corner grounding or using it as ungrounded system.

Major reason to go ungrounded is for a process that needs an orderly shutdown, you indicate a fault and have time to properly shut the process down, where with a grounded system a ground fault opens overcurrent protection immediately and doesn't allow for any orderly shut down. I'd stay away from ungrounded unless you have such need for orderly shutdown. If you do have ungrounded and ignore first fault indications - when you have a second fault it will abruptly shut things down anyway.
 
Sounds right on distribution transformer wiring

so, back to the little 15 kva on the branch circuit..

a 15kva 480/240 6 lead potted delta/delta transformer mounted on each machine requires more than bonding (equipment ground) at the 480 cord drop?

not sure if I want to corner ground X2 inside the machine

there is no reference to ground from secondary currently of course
 

rlundsrud

Senior Member
Location
chicago, il, USA
One other thing, if you corner ground one of the phases, you would need to use two pole breakers for your three phase equipment. You don't use OCP on the corner grounded phase. I have only seen this on systems that don't have X4, typically delta 480 systems.
 

kwired

Electron manager
Location
NE Nebraska
One other thing, if you corner ground one of the phases, you would need to use two pole breakers for your three phase equipment. You don't use OCP on the corner grounded phase. I have only seen this on systems that don't have X4, typically delta 480 systems.
You can use three pole breakers as they would switch grounded conductor simultaneously with the ungrounded conductors, you can not use fuses in the grounded conductor though.

Also for 240 volt system you have to use straight 240 volt breakers - which are not as easy to find in two pole miniature type breakers but three pole are rated 240 in all the product lines I am familiar with anyway.
 
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