Article 640.7(B)...

[chevyx92]

States that the system must be grounded in accordance with 647.6.
647.6(B) says the grounding bus must be connected to the grounded conductor on the line side of the separately derived system's disconnecting means.

Situation: I will have a 208V WYE Primary TO 120V Delta Secondary transformer. I was planning on "Floating" the neutral connection on the primary side since there is no need for it with a delta secondary. If I do this, how do I comply with 647.6(B) so I can properly ground the grounding bus?

 

[LarryFine]

647.6(B) says the grounding bus must be connected to the grounded conductor on the line side of the separately derived system's disconnecting means.

That's on the secondary of the transformer. The primary isn't separately derived.

I was planning on "Floating" the neutral connection on the primary side since there is no need for it with a delta secondary.

You must, regardless of the secondary. Connect the primary as if it was a Delta.

 

[chevyx92]

That's on the secondary of the transformer. The primary isn't separately derived.

I understand the primary isn't separately derived. So tell me how am I suppose to ground the grounding bus to the grounded conductor per 647.6(B)? The secondary has no "Neutral Connection Point", its a Delta secondary.

 

[Volta]

. . . Situation: I will have a 208V WYE Primary TO 120V Delta Secondary transformer. I was planning on "Floating" the neutral connection on the primary side since there is no need for it with a delta secondary. If I do this, how do I comply with 647.6(B) so I can properly ground the grounding bus?

I'm confused. How will you use a delta secondary for multiple 120 volt center-tapped legs?

Or is this going to provide only one grounded 60/120 phase?

Section 647.5 requires three-phase to have a wye secondary.

 

[chevyx92]

I'm confused. How will you use a delta secondary for multiple 120 volt center-tapped legs?

Or is this going to provide only one grounded 60/120 phase?

Section 647.5 requires three-phase to have a wye secondary.

It is a 208v WYE to 120V Delta 3 phase Transformer, and yes that means 60v to ground and 120v phase to phase voltage. The transformer in question will serve only 3 phase loads. I dont see where 647.5 plays in this? Where/what is 6-phase "Wye" system?

 

[chris kennedy]

What type of load/s is this secondary supplying?

 

[chevyx92]

What type of load/s is this secondary supplying?

3 phase electronic loads. Government equipment but dont know exactly what. I think radar stuff. I only know what they are willing to tell me. You know how that goes with the government.

 

[Volta]

It is a 208v WYE to 120V Delta 3 phase Transformer, and yes that means 60v to ground and 120v phase to phase voltage. The transformer in question will serve only 3 phase loads. I dont see where 647.5 plays in this? Where/what is 6-phase "Wye" system?

Well a system operating at 120 volts L-L and 60 volts to ground for sensitive electronic equipment is subject to Article 647.

If 3-phase, then 647.5. Multiple center-tapped legs on multiple secondaries.

 

[chevyx92]

Well a system operating at 120 volts L-L and 60 volts to ground for sensitive electronic equipment is subject to Article 647.

If 3-phase, then 647.5. Multiple center-tapped legs on multiple secondaries.

Multiple center tapped legs?

 

[Volta]

Multiple center tapped legs?

Three separate windings on the secondary, each fed from a different primary combination, and each secondary grounded at the mid-point. Not an actual wye, because the ends aren't connected to one single point.
But still three phase angles and rotation with six phases.

Never seen one, myself.

 

[chris kennedy]

Would you have the make and model# of this tranny handy?

 

[chevyx92]

Would you have the make and model# of this tranny handy?

No I dont. I will try to get it tomorrow.

 

[chevyx92]

Would you have the make and model# of this tranny handy?

Here is the exisiting transformer and setup, I will disconnected it today and will be relocating it and wiring it back up. I still dont see how you're suppose to comply with 647.6(B).

 

[cadpoint]

Clear as mud, need to know bases... :roll:

I think Volta touched on it correctly!
Note bottom left of page 418...

Here's what I found, note this one response to a query on Article 640.7(B)
Illustrated guide to the National Electrical Code, by Charles R. Miller.

Total search engine results; Here

 

[Volta]

No I dont. I will try to get it tomorrow.

3-21-10

Here is the exisiting transformer and setup, I will disconnected it today and will be relocating it and wiring it back up. I still dont see how you're suppose to comply with 647.6(B).

4-7-10

Typical contractor schedule. :grin:
<jk>

You can't. At least without damaging it. :roll: If the H0 taps were cut to separate the coils, and the midpoints of the H coils were connected together, and grounded, and you fed the X side with 120 volts, you would effectively have three or six phases (depending on your view of a recent looong thread ), and comply with 647.5 at the expense of ruining the customer's equipment and 110.3(B).

But that would provide three 60/120v midpoint grounded sources.

Edit: I see that you said 647.6(B) while I was thinking 647.5. It gets messy.

 

[chevyx92]

3-21-10

Edit: I see that you said 647.6(B) while I was thinking 647.5. It gets messy.

I agree. They obviously make this transformer. So...........I honestly dont know how to comply with 647.6(B). The best thing I can do would be to run a GEC from building steel to transformer frame, from transformer frame run a ground with the secondary conductors and land it on the ground bar.

 

[Volta]

As it's wired now, is the primary on the H coils or the X side?

 

[chevyx92]

As it's wired now, is the primary on the H coils or the X side?

Its on the H coils.

 

[Volta]

Ok, I started in this thread with confusion, and I may have been contagious. :roll:

1. If your secondary is 120 volt delta, with no mid-piont taps, then 60 volts do not exist on the secondary. Right?

If that's true, then you do not need to comply with 640.7(B).

2. The delta secondary needs to be grounded per 250.20(B)(1).

The corner phase can be grounded. It would be a white or gray conductor [200.6(D) if needed], connected to the closest part of the GES. It would still be a phase conductor, but grounded.

 

[chevyx92]

Ok, I started in this thread with confusion, and I may have been contagious. :roll:

1. If your secondary is 120 volt delta, with no mid-piont taps, then 60 volts do not exist on the secondary. Right?

If that's true, then you do not need to comply with 640.7(B).

2. The delta secondary needs to be grounded per 250.20(B)(1).

The corner phase can be grounded. It would be a white or gray conductor [200.6(D) if needed], connected to the closest part of the GES. It would still be a phase conductor, but grounded.

Yes 60 volts do exist. From phase to ground there is 60 Volts, phase to phase it measures 120 Volts.