When to provide neutral on the primary side of a transformer

[Volt-Amps]

Hello Forum,
I'm uncertain as to when I need to pull the neutral on the primary side a transformer?
Switchboard is 480Y/277V, 3-Phase, 4-Wire
feeding single phase transformer 120/240V.
Breaker on the primary side: single phase, 240V breaker
On the primary side of the transformer, do I need to bring (2) hots, (1) neutral and (1) G, OR (2) hots and (1) G?

Does it matter if primary power is coming from a Wye or Delta switchboard?

 

[jim dungar]

Unless some power engineer has a specific set of drawings, you would never pull a primary side neutral to a transformer. This is similar to how you never pull a neutral to a motor, even though the system is 480Y/277.

Yes, there are are exceptions, like for reverse power flow, but they are relatively rare.

 

[LarryFine]

I'm uncertain as to when I need to pull the neutral on the primary side a transformer?

Definitely yes for a wye-wye transformer, such as 480Y/277v to 208Y/120v.

Switchboard is 480Y/277V, 3-Phase, 4-Wire
feeding single phase transformer 120/240V.
Breaker on the primary side: single phase, 240V breaker

Is the 120/240v the primary or the secondary?

Where are you getting 240v to feed the transformer?

On the primary side of the transformer, do I need to bring (2) hots, (1) neutral and (1) G, OR (2) hots and (1) G?

Does it matter if primary power is coming from a Wye or Delta switchboard?

Again, where are you getting 240v?

 

[Volt-Amps]

Definitely yes for a wye-wye transformer, such as 480Y/277v to 208Y/120v.


Is the 120/240v the primary or the secondary?

Where are you getting 240v to feed the transformer?


Again, where are you getting 240v?

480V single phase feeders to transformer
Transformer receiving power from the switchboard is single phase 120/240
Transformer is fed from 2P breaker (single phase, 480V) from the 3-phase switchboard.

 

[Volt-Amps]

The switchboard is 480Y/120V, 3-phase, 4W. But I'm using a 2P, 1-Phase breaker from this switchboard to feed the single phase transformer.
So it's single phase feed from the 3-Phase switchboard to the transformer, and single phase 120/240V secondary output.

 

[LarryFine]

So, you have a unit with a 480v primary and a 120/240v secondary?

Then the primary receives 480v 1ph on just two wires, no neutral.

 

[jap]

No you will not pull a neutral to the line side of the single phase transformer.
Your feed to the line side of the 1ph transformer from the 480v Switchboard 2p breaker will be a 480v Single phase feed (2 hots and and Equipment grounding conductor)

The transformer produces the secondary neutral in the transformer itself for the 120/240v side which will need to be bonded to ground most generally in the transformer itself or in the first means of overcurrent protection.

If bonded at the transformer (which is the most common) you'll have (2) Hots, (1) Neutral and (1) Equipment grounding conductor from the load side of your 1ph transformer to your 1ph 120/240v Panelboard.


JAP.

 

[Volt-Amps]

Thank you, Gentlemen!