Clarification of bonding neutral / creating parallel neutral paths.

[steveng]

In the photo attached the neutral bars are bonded by strap and the ground screw is bonded to
The case, here is the question, my master says to install a grounding bar to case for equipment ground use.

1. Does this create a parallel path for neutral current back to xfmr?


This is a xfmr fed 208 3ph panel feeding multiple 3ph loads in a machine shop,
Primary voltage is 480 3ph. 75kva

Thanks

 

[augie47]

The answer may depend somewhat on the Code cycle involved as the '11 Code requires a "supply side bonding jumper" but more to the point, are you installing a grounding and grounded conductor from your transformer to your panel or just a grounded conductor ?

 

[MarineTech]

I will attempt to answer in the context of the 2011 NEC. I use the 2008 currently in CA, but learning for 2011 adoption. Please flag any errors I make.

I believe the question relates to the where to locate the System Bonding Jumper for the separately derived system (transformer) in accordance with 250.30(A)(1), and if done at the panelboard, how is the Y neutral and grounding conductor isolated from one another.

If the System Bonding Jumper (screw to case) is at the disconnect panelboard, which includes connection to the grounding electrode conductor, then there is an equipment bonding conductor back to the transformer metal enclosure. Because the secondary Y neutral is not bonded to the enclosure at the transformer output, there is no parallel path for load currents from the panelboard to the transformer output.

The neutral wire carries load current and the secoundary equipment bonding jumper protects for ground faults.

In addition, just to make sure I get this right, the equipment bonding conductor from the secoundary panel connects to the transformer case and to the primary equipment bonding to the delta side OC device panel. All sized to 250.66 based on the ungrounded conductor size.

Just to check some math for 75kVA Delta to Y Polyphase (3 phase), 480V to 120/208V:

Assume 75 degree C terminals. No non-linear loads (3rd harmonic).

Primary OCD = 125A, with 2AWG ungrounded conductors.
Secondary OCD = 300A, with 350 kcmil ungrounded conductors.

 

[iwire]

In the photo attached the neutral bars are bonded by strap and the ground screw is bonded to
The case, here is the question, my master says to install a grounding bar to case for equipment ground use.

1. Does this create a parallel path for neutral current back to xfmr?


This is a xfmr fed 208 3ph panel feeding multiple 3ph loads in a machine shop,
Primary voltage is 480 3ph. 75kva

Thanks

So we are not taking about a service but a separately derived system? Is that correct?

If it is ....

Doing what the you are being told could either be required or a violation. To determine that would require knowing if the bonding has already been done at the transformer.

The NEC allows doing it at the transformer, or at the first disconnecting means, or both under some specific conditions.

 

[steveng]

So we are not taking about a service but a separately derived system? Is that correct?

If it is ....

Doing what the you are being told could either be required or a violation. To determine that would require knowing if the bonding has already been done at the transformer.

The NEC allows doing it at the transformer, or at the first disconnecting means, or both under some specific conditions.

1. Yes, this is a separately derived system . 480 3ph primary from existing service panel. School

2. Yes the xo terminal at the 75 kva sec 208y is bonded to the case, egc, bldg steel.

3. Yes there is an egc from the xfmr to the panel.

Thanks

 

[don_resqcapt19]

1. Yes, this is a separately derived system . 480 3ph primary from existing service panel. School

2. Yes the xo terminal at the 75 kva sec 208y is bonded to the case, egc, bldg steel.

3. Yes there is an egc from the xfmr to the panel.

Thanks

If XO is bonded at the transformer, it cannot be bonded in another location. You need a supply side bonding jumper, not an EGC between the transformer and the panel

 

[steveng]

If XO is bonded at the transformer, it cannot be bonded in another location. You need a supply side bonding jumper, not an EGC between the transformer and the panel

Ok, thanks,

The term. (supply side bonding jumper) is a wire that runs from the ground lug at the xfmr
To the grounding lug at the panel, correct?

 

[augie47]

To a degree, what you were told was correct, but, the next unmentioned step is to remove the green bond screw so you don't have a parallel path.

 

[steveng]

To a degree, what you were told was correct, but, the next unmentioned step is to remove the green bond screw so you don't have a parallel path.

Ok, thanks Augie

Now the hard part, tell my boss he was wrong :happyno:

 

[steveng]

Thanks

Thanks

I will attempt to answer in the context of the 2011 NEC. I use the 2008 currently in CA, but learning for 2011 adoption. Please flag any errors I make.

I believe the question relates to the where to locate the System Bonding Jumper for the separately derived system (transformer) in accordance with 250.30(A)(1), and if done at the panelboard, how is the Y neutral and grounding conductor isolated from one another.

If the System Bonding Jumper (screw to case) is at the disconnect panelboard, which includes connection to the grounding electrode conductor, then there is an equipment bonding conductor back to the transformer metal enclosure. Because the secondary Y neutral is not bonded to the enclosure at the transformer output, there is no parallel path for load currents from the panelboard to the transformer output.

The neutral wire carries load current and the secoundary equipment bonding jumper protects for ground faults.

In addition, just to make sure I get this right, the equipment bonding conductor from the secoundary panel connects to the transformer case and to the primary equipment bonding to the delta side OC device panel. All sized to 250.66 based on the ungrounded conductor size.

Just to check some math for 75kVA Delta to Y Polyphase (3 phase), 480V to 120/208V:

Assume 75 degree C terminals. No non-linear loads (3rd harmonic).

Primary OCD = 125A, with 2AWG ungrounded conductors.
Secondary OCD = 300A, with 350 kcmil ungrounded conductors.

Thanks for making it clear!!!

 

[steveng]

A link to the video explaining this system bonding jumper and parallel paths.

A link to the video explaining this system bonding jumper and parallel paths.

Can you point me to the video link that explains the system bonding jumper and creating parallel
Paths for neutral current?

 

[steveng]

So we are not taking about a service but a separately derived system? Is that correct?

If it is ....

Doing what the you are being told could either be required or a violation. To determine that would require knowing if the bonding has already been done at the transformer.

The NEC allows doing it at the transformer, or at the first disconnecting means, or both under some specific conditions.

Can you cite the code reference to this specific conditions?

The nec allows the xo term at the xfmr to be used or the first disconnecting means, Why not just permit only xo bond to case and limit any further confusion?

Thanks

 

[Dennis Alwon]

Can you point me to the video link that explains the system bonding jumper and creating parallel
Paths for neutral current?

Here is one by Mike Holt about secondaries of a transformer- I have not seen it but it may help

 

[kwired]

Can you cite the code reference to this specific conditions?

The nec allows the xo term at the xfmr to be used or the first disconnecting means, Why not just permit only xo bond to case and limit any further confusion?

Thanks

250.30(A)(1) exception #2:



Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor. If a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.



There would have to be no conductive paths between the transformer and the first disconnecting means to use this exception.

 

[steveng]

250.30(A)(1) exception #2:



Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor. If a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.



There would have to be no conductive paths between the transformer and the first disconnecting means to use this exception.

Would this exception be the use of PVC or other nonconductive raceway?

 

[kwired]

Would this exception be the use of PVC or other nonconductive raceway?

Yes, but you also have to consider other paths that are not specifically electrical equipment, like steel framed building/equipment structures.