Isolation Transformer Suggestion

[FaradayFF]

Hello all,
I have a project where there is an existing feeder from an AC panel to a site some distance away. Incoming power to this facility is 208V, 3 phase, 3 wire with equipment ground wire. Currently, they are deriving 120VAC locally by using one phase of incoming power and local ground connection, which I understand is not up to code because you can not use ground as neutral wire. We will need to provide 120VAC to supply 120VAC loads at this facility, since we are doing major rehab and refurbishment work, and so we considered replacing the feeder as part of the scope. Instead of replacing the existing feeder with 208V, 3 phase, 4 wire, can we install a 208V to 208/120V isolation transformer instead. The secondary winding of this isolation transformer would be rated 208/120V and we would be able to supply 3-phase and 1-phase loads as a separately derived system. Going with an isolation transformer, we would not have to replace the feeder to bring it up to code.
Do you see any issues with this approach? It sounds like a cost savor, considering we would need 30kVA or so transformer. Would an isolation transformer be recommended for this project? Would this be treated as a separately derived system?
Thoughts?
Thanks,
EE

 

[Jraef]

If the transformer (and associated appurtenances) will cost less than pulling new conductors, then it's a fine approach.

It COULD be a "grounding autotransformer' to do this, but 1) good luck finding one readily available, because 2) there are rules in 450.5 that make it "unpopular" compared to just using a commercial off the shelf isolation transformer.

Yes, it will be an SDS if you use the isolation transformer, but not if you use the grounding autotransformer.

 

[winnie]

Using a transformer to create the desired 208/120V locally is a fine approach.

The transformer would be a 'normal' one with separate primary and secondary coils. This is technically an isolation transformer, but often that term is used to describe particular shielded transformers. Just be careful that you don't specify something more expensive than you need.

Quick question: how long is the run and what voltages are available at the supply building?

If you have 480V available at the supply end, and have to install a transformer anyway, then you might want to change the feeder to 480V to reduce voltage drop.

-Jon

 

[don_resqcapt19]

Hello all,
I have a project where there is an existing feeder from an AC panel to a site some distance away. Incoming power to this facility is 208V, 3 phase, 3 wire with equipment ground wire. Currently, they are deriving 120VAC locally by using one phase of incoming power and local ground connection, which I understand is not up to code because you can not use ground as neutral wire. We will need to provide 120VAC to supply 120VAC loads at this facility, since we are doing major rehab and refurbishment work, and so we considered replacing the feeder as part of the scope. Instead of replacing the existing feeder with 208V, 3 phase, 4 wire, can we install a 208V to 208/120V isolation transformer instead. The secondary winding of this isolation transformer would be rated 208/120V and we would be able to supply 3-phase and 1-phase loads as a separately derived system. Going with an isolation transformer, we would not have to replace the feeder to bring it up to code.
Do you see any issues with this approach? It sounds like a cost savor, considering we would need 30kVA or so transformer. Would an isolation transformer be recommended for this project? Would this be treated as a separately derived system?
Thoughts?
Thanks,
EE

Unless the service is supplied from an ungrounded system, that is a code violation. The grounded conductor of a service supplied from a grounded system is required to be run to the service equipment.

 

[electrofelon]

Unless the service is supplied from an ungrounded system, that is a code violation. The grounded conductor of a service supplied from a grounded system is required to be run to the service equipment.

My interpretation was that was describing the conductors serving the second building

 

[electrofelon]

Perhaps using 250.32(B)(1) exception #1 is an option. Some will consider that a stretch since although the conductors are existing, they were not used that way and you would be "changing" the EGC to a grounded conductor. However such an installation is perfectly safe IMO.

 

[winnie]

Unless the service is supplied from an ungrounded system, that is a code violation. The grounded conductor of a service supplied from a grounded system is required to be run to the service equipment.

IMHO @don_resqcapt19 is correct if this is a _service_.

I was interpreting the description as a _feeder_ from another building, rather than a 'service'.

Further question for @FaradayFF : are you sure that this is a feeder that only has phase conductors and EGC, and not a feeder with neutral, where the neutral is also used as the bonding conductor? In other words, was the original design what @electrofelon is suggesting in post 6?

-Jon

 

[don_resqcapt19]

My interpretation was that was describing the conductors serving the second building

The OP did say feeder, but I was reading this:
"Incoming power to this facility is 208V, 3 phase, 3 wire with equipment ground wire"
as referring to the facility connection to the utility.