Transformer Over Current Protection

[fifty60]

I have a center tapped transformer with a 120-0-120 volt secondary. I have circuits on the 120-0, the 0-120, and the 120 to 120 taps. So, that equates to two 120V circuits and one 240V circuit.

Do I have to size the ampacity for the center tap ground conductors to carry the load of the 120V and 240V circuits? Or, does it only need the ampacity for the two separate 120V circuits?

 

[petersonra]

I have a center tapped transformer with a 120-0-120 volt secondary. I have circuits on the 120-0, the 0-120, and the 120 to 120 taps. So, that equates to two 120V circuits and one 240V circuit.

Do I have to size the ampacity for the center tap ground conductors to carry the load of the 120V and 240V circuits? Or, does it only need the ampacity for the two separate 120V circuits?

it is a neutral. it has to be minimally sized for whatever unbalanced current it might see. There is some minimum size beyond that requirement.

 

[fifty60]

But the only circuits using the neutral are the 120V circuits right? The 240V circuit is not using the neutral at all?

 

[Tony S]

The neutral can’t exceed the line currents. As far as wire sizing, the neutral should equal the line.

 

[petersonra]

The neutral can’t exceed the line currents. As far as wire sizing, the neutral should equal the line.

that is certainly the easiest way to handle it.

 

[Dennis Alwon]

I would think the neutral can be sized as small as the required grounding electrode conductor. No?

 

[Smart $]

I'm a bit baffled here.

Title is transformer overcurrent protection but the body asks about sizing the ground (or is it grounded) conductor. So what are we actually going for?

Let's get some pertinent yet-to-be-divulged parameters:

Transformer rating...
Primary voltage...
240V load(s)...
120V loads...

 

[augie47]

I think it's simply a case of the thread title being misleading,.... the OPs question seems to concern conductor sizing. As petersonra mentioned, its basically a Art 220.61 question

 

[Smart $]

I think it's simply a case of the thread title being misleading,.... the OPs question seems to concern conductor sizing. As petersonra mentioned, its basically a Art 220.61 question

Could be... waiting on fifty60...

 

[Tony S]

The neutral can’t exceed the line currents. As far as wire sizing, the neutral should equal the line.

that is certainly the easiest way to handle it.

It is if as in this case L-N loads could be all be on one leg. Unlikely I know but…….. stranger things have happened.

 

[fifty60]

It is a single phase general purpose transformer. The primary is 456VAC, and the secondary is 120-0-120. I have three circuits---two 120V circuits (Line to grounded center tap, and other line to grounded center tap) and a 240V circuit (line to line). If I were only using the line to line 240V, then I need to ground the center tap but it only needs to be sized to 240.122.

Since I am using the two 120V to grounded center tap circuits, then I have to size the neutral to carry both of the combined line to grounded center tap loads, right? How do I have to add in the contribution from the line to line 240V circuit? Do I have to since the 240V circuit is not using the grounded center tap?

 

[Tony S]

I_n=I₁-I₂

If it comes out as a negative value, for -5A read 5A.

What size is this transformer that the size of the neutral conductor is so important?

 

[Smart $]

It is a single phase general purpose transformer. The primary is 456VAC, and the secondary is 120-0-120. I have three circuits---two 120V circuits (Line to grounded center tap, and other line to grounded center tap) and a 240V circuit (line to line). If I were only using the line to line 240V, then I need to ground the center tap but it only needs to be sized to 240.122.

Since I am using the two 120V to grounded center tap circuits, then I have to size the neutral to carry both of the combined line to grounded center tap loads, right? How do I have to add in the contribution from the line to line 240V circuit? Do I have to since the 240V circuit is not using the grounded center tap?

Actually you have to size it to the largest minimum of three: 1) protected by overcurrent device, 2) largest line-to-neutral load, and 3) minimum grounding conductor.