Thee phase autotransformer question

[ggunn]

Say you have an autotransformer connecting a three phase 480/277 PV inverter to a 208/120 service. The X's and H's share a neutral; is the 480 side still separately derived as it would be with an isolation transformer?

 

[don_resqcapt19]

No. You have a circuit conductor that is common to both sides so it is not SDS.

Separately Derived System.
An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.

 

[ggunn]

No. You have a circuit conductor that is common to both sides so it is not SDS.

Thanks; that's what I thought. Next question: Do I need a ground rod at the transformer (I think yes) and if so, does that electrode need to be bonded to the building ground (again I think yes)?

 

[electrofelon]

Thanks; that's what I thought. Next question: Do I need a ground rod at the transformer (I think yes) and if so, does that electrode need to be bonded to the building ground (again I think yes)?

Not being an sds, I would say no. What is your reasoning for needing one?

 

[iwire]

Not being an sds, I would say no.

I agree.

 

[ggunn]

Not being an sds, I would say no. What is your reasoning for needing one?

Actually, it's another engineer's design I am reviewing, and he has the ground rod there but not bonded to the building ground. I think I should tell him to either remove the ground rod or bond it to building ground.

 

[ggunn]

A mod removed my question from the PV forum about the same project for being a duplicate, but actually it wasn't. Here is the question:

I am connecting a 480/277 inverter to a 208/120 service through an autotransformer, which is new for me.

I have done this with a 208 delta to 480/277 wye transformer, and it is obvious in that in that case the neutral on the 480 side must be bonded to ground, but an autotransformer has a neutral common to the primary and secondary sides which is bonded to ground at the service. It seems to me that I should not bond neutral to ground at the transformer in this case. What say you?

It was asked and answered in this thread that this is not a SDS, but does it follow from that that the neutral at the transformer should not be bonded to ground?

 

[electrofelon]

I am connecting a 480/277 inverter to a 208/120 service through an autotransformer, which is new for me.

I have done this with a 208 delta to 480/277 wye transformer...

Yes interesting, never thought of doing it with autotransformers

 

[iwire]

A mod removed my question from the PV forum about the same project for being a duplicate, but actually it wasn't. Here is the question:

What jerks those mods are! :rant:

Of course when you title them almost identically.

It was asked and answered in this thread that this is not a SDS, but does it follow from that that the neutral at the transformer should not be bonded to ground?

No, you cannot bond it again, it is already bonded.

 

[ggunn]

What jerks those mods are! :rant:

Of course when you title them almost identically.



No, you cannot bond it again, it is already bonded.

Thanks, that's what I thought, although your answer was actually a yes.

I have no quarrel with the mods; I can see where to a cursory glance it seemed like a duplicate.

 

[pv_n00b]

Thanks; that's what I thought. Next question: Do I need a ground rod at the transformer (I think yes) and if so, does that electrode need to be bonded to the building ground (again I think yes)?

Technically there is nothing wrong with having the additional grounding electrode at the transformer, as long as it is considered an auxiliary grounding electrode it does not need to be bonded to the premises grounding electrode system. But why you would want the grounding electrode is another question. And it should only be connected to the equipment ground.