Transformer separately derived or not and cable protection

[hhsting]

I have 277V single phase primary to 120V secondary single phase Acme T279742S transformer.

1. Is this transformer separately derived system or non separately derived system? See below snapshot. Engineer says he has to bring neutral and phase primary to get 277V single phase and secondary he has to bring phase and neutral get 120V single phase. The two neutrals are connected since this is not separately derived. I dont see how

2. Would the transformer require secondary conductor overcurrent protection? If not then what protects secondary conductors

 

[hhsting]

I have 277V single phase primary to 120V secondary single phase Acme T279742S transformer.

1. Is this transformer separately derived system or non separately derived system? See below snapshot. Engineer says he has to bring neutral and phase primary to get 277V single phase and secondary he has to bring phase and neutral get 120V single phase. The two neutrals are connected since this is not separately derived. I dont see how

2. Would the transformer require secondary conductor overcurrent protection? If not then what protects secondary conductors

Anyone have any opinions?


Sent from my iPhone using Tapatalk

 

[infinity]

Looking at the wiring diagram the primary and secondary neutrals are not connected together.

 

[hhsting]

Looking at the wiring diagram the primary and secondary neutrals are not connected together.

So H1 and H2 one would be phase and another neutral primary? This would make this separately derived system???

Also does the transformer require secondary overcurrent protection or not for protection of secondary conductors?

 

[GeorgeB]

H1 neutral from the 480Y, H5 a hot from the 480Y.
If you really only want 120, not 120-0-120, X1-X3 to 120V ground, X2-X4 to 120V hot. The chart covers it perfectly.

 

[wwhitney]

Looking at the wiring diagram the primary and secondary neutrals are not connected together.

That's the transformer diagram itself, right? So there's no factory connection that would force it not to be an SDS. But the field installation could be done either as an SDS or not as an SDS.

If I understand correctly, both the OP's questions depend on how the transformer is going to be wired in the field, and that info is missing from the attachment. If the secondary of the transformer is configured as a 2-wire 120V system, then 240.21(C)(1) would let the primary conductor protection be used for secondary protection. And if the primary side (277V) neutral is connected to one of the secondary side conductors, then it wouldn't be an SDS.

Cheers, Wayne

 

[infinity]

And if the primary side (277V) neutral is connected to one of the secondary side conductors, then it wouldn't be an SDS.

The primary and secondary neutrals are not connected. Primary 277 volt is H1&H5. There is no connection of the primary neutral to the secondary.

 

[augie47]

With separate windings (no common connection between primary and secondary) it meets the definition of a SDS.
Secondary protection requirement would depend on the size primary protection and how you connect to the secondary...
120-0 or 240-120

 

[wwhitney]

The primary and secondary neutrals are not connected. Primary 277 volt is H1&H5. There is no connection of the primary neutral to the secondary.

Again, that's the transformer spec sheet that tells you various ways that it can be installed.

But I'm not aware that there's anything that prohibits you from installing it as a non-SDS by connecting the primary side neutral to one of the secondary conductors. That would save you running one conductor in this case, as you wouldn't need a GEC for the transformer secondary, and you already have a primary neutral since the primary is two-wire 277V.

With separate windings (no common connection between primary and secondary) it meets the definition of a SDS.

Depending on how it is installed. For example, it could be used as a boost transformer to get ~400V (obviously not happening here), and then it wouldn't be an SDS.

Cheers, Wayne

 

[hhsting]

Again, that's the transformer spec sheet that tells you various ways that it can be installed.

But I'm not aware that there's anything that prohibits you from installing it as a non-SDS by connecting the primary side neutral to one of the secondary conductors. That would save you running one conductor in this case, as you wouldn't need a GEC for the transformer secondary, and you already have a primary neutral since the primary is two-wire 277V.


Depending on how it is installed. For example, it could be used as a boost transformer to get ~400V (obviously not happening here), and then it wouldn't be an SDS.

Cheers, Wayne

I want to get from primary 277V to 120V secondary. So if primary neutral is connected to load meaning like you are saying connecting primary neutral to secondary and not H1 then what gets connected to H1? Only one phase conductor is connected to H5??

 

[augie47]

Don't complicate the matter. Just connect as the instructions show. 277 to H1 & H5, 120 to X1 & X4 Connect X1 to X3 and X2 to X4

 

[wwhitney]

Just connect as the instructions show. 277 to H1 & H5, 120 to X1 & X4 Connect X1 to X3 and X2 to X4

[To the OP] Right, that's what you need to do to generate 120V 2-wire from 277V 2-wire with that transformer at its rated capacity.

The question of SDS or not depends on what further connection you make. One of the two secondary wires, X1/X3 or X2/X4, needs to be grounded per 250.20(B)(1). Let's say we choose X1.

Then there are two possible way to ground X1. One way is to run a new GEC from the GES to the transformer or its secondary disconnect, and connect the GEC to X1. That will create an SDS.

The other way to ground X1 is to note that one of the 277V circuit conductors is already grounded, it is the 480Y/277V neutral. Suppose that neutral is connected to H1, with the ungrounded conductor connected to H5. Then the installer can connect H1 to X1 to ground X1. No GEC is required at the transformer, and the system is not an SDS.

So if you are reviewing plans, and the only details on the transformer provided are what you posted, without any further details about how one of the 120V secondary conductors is going to be grounded, the submitter hasn't specified how they plan to comply with 250.20(B)(1). Is there a multi-line diagram submitted that shows the grounding details?

Cheers, Wayne

 

[infinity]

Don't complicate the matter. Just connect as the instructions show. 277 to H1 & H5, 120 to X1 & X4 Connect X1 to X3 and X2 to X4

I agree, the engineer specified the wires to be pulled. There's is no need to go off on a tangent.

 

[wwhitney]

The other way to ground X1 is to note that one of the 277V circuit conductors is already grounded, it is the 480Y/277V neutral. Suppose that neutral is connected to H1, with the ungrounded conductor connected to H5. Then the installer can connect H1 to X1 to ground X1. No GEC is required at the transformer, and the system is not an SDS.

Rereading the OP, the above is what the engineer has specified:

Engineer says he has to bring neutral and phase primary to get 277V single phase and secondary he has to bring phase and neutral get 120V single phase. The two neutrals are connected since this is not separately derived.

Cheers, Wayne

 

[hhsting]

Rereading the OP, the above is what the engineer has specified:


Cheers, Wayne

Thats correct engineer has note saying that neutral is tied to the secondary so it is wired as non separately derived. I see how based on your explanation. Thanks

How would secondary protection of conductors and transformer work in that case?

 

[jim dungar]

Thats correct engineer has note saying that neutral is tied to the secondary so it is wired as non separately derived. I see how based on your explanation. Thanks

How would secondary protection of conductors and transformer work in that case?

Protection doesn't change.

 

[infinity]

Thats correct engineer has note saying that neutral is tied to the secondary so it is wired as non separately derived. I see how based on your explanation.

The two are field connected together? Given the nameplate info you provided I don't think that many electricians would wire the transformer that way.

 

[wwhitney]

The two are field connected together? Given the nameplate info you provided I don't think that many electricians would wire the transformer that way.

What do you see on the nameplate that would preclude that?

Admittedly SDS is more common, but as I've been saying, the non-SDS configuration is allowed. Anytime you use an isolation transformer as a buck or boost transformer, you are doing something similar, namely adding primary/secondary connections not shown on the nameplate.

Cheers, Wayne