bonding xfmr neutral [xo]

[dwe68]

In the past every company i have worked has always had me bond the neutral on the secondary side {of course delta/wye} of our xfmrs in the xfmr as well as the first means of diconnect. But i can only find that requirement in the first means of disconnect in the code for seperatly derived systems. Is this just a good practice or am i overlooking something.{ PS.No high impedance}

 

[__dan]

If

If

If you bond the secondary neutral more than once, you place a grounding path in parallel with the neutral. There is current flow on the neutral and it is grounded at both ends (at the xfrmr and first disconnect). Bad practice, bond the neutral at one point only to avoid making the ground path into a shunt for neutral current.

 

[augie47]

Take a look at 250.30(A)(1). Exceptions are shown, but normally the bonding and GEC all occur at one location, either at the source or at the 1st means of disconnect.

 

[dwe68]

thanks

thanks

how bout power company they always bond neutral in xfmr and it is both neutral and ground and bond to services ex to pull a ground wire in service conduit is not required and ive heard it called an illegal parrallel to do so

 

[dwe68]

thanks augie47

thanks augie47

some of our sub panel are main lug those would have to be done in xfmr you sound like the guys i was learning from in 86

 

[dwe68]

thanks again ive seen knob and tube wiring scared me sumthin fierce till i saw it was dead

 

[Cold Fusion]

Gus has it right. If it is an owner supplied transformer, then it is a separately derived system, and (2008) 250.30.A.1 applies. Bond the system grounded conductor at one point only - anywhere from the xfm to the first disconnect. If the bond is at the xfm, then one runs separate grounded conductor and grounding conductor to the first disconnect.

I suspect the practice of bonding in two places comes from what I consider one of the dumbest code sections currently in print - 250.24.A.2. If this transformer was a utility owned transformer and is located outside, then one bonds at the transformer and at the first disconnect.

So:
Utility owned, outside, bond in two places.
Customer owned, Inside or out, bond in one place
??? I have no clue as to what code panel 5 is thinking. Or maybe they are not.:roll:

Note to charlie b:
I would even consider this one dumber than the 8.5 foot clearance issue currently under discussion.:roll:

cf

 

[infinity]

I suspect the practice of bonding in two places comes from what I consider one of the dumbest code sections currently in print - 250.24.A.2. If this transformer was a utility owned transformer and is located outside, then one bonds at the transformer and at the first disconnect.


cf

CF,

Care to elaborate as to why this is so bad on the line side of the service disconnect?

 

[dwe68]

thanks cold fusion

thanks cold fusion

i will definately do as engineer and prints call for but i think the power companys are looking at lines being cut as well as the possibillity of unqualified installers and an unbonded neutral is just another hot wire thanks again

 

[dwe68]

u get im infinity

u get im infinity

one of the smartest code men ive ever known and another one an engineer on a usafb has had me bond in both why is it so bad to have two is it better to have only 1 and if it gets loose lookout the voltage gets loose too just a dumb countryboy got a good christian raisein and an eight grade education

 

[dwe68]

still waiting

still waiting

im waiting for an answer and he's weighing electrons

 

[Cold Fusion]

CF,

Care to elaborate as to why this is so bad on the line side of the service disconnect?

I work almost entirely industrial. There is almost always a ground mat that includes the area under the utility owned transformer. The mandated two grounded conductor bonds garrantees a parallel path for any neutral current. If this was a customer owned xfm that would not be okay. But just because the transformer is utility owned it is okay??

I'm thinking the utility transformer should be treated the same as if were an SDS. The code made a similar change for outbuildings in the 2008. They finally recognized that 3W was not good - should be 4w. This is about the same kind of thing.

cf

 

[__dan]

X0

X0

The power company supplies a neutral. In the NEC, the earth is not considered a parallel path for neutral current if there is neutral to earth ground bonding at the utility xfmr and again at the service, which is typical.

For overhead triplex, it hits a porcelain anchor and an insulated neutral conductor. If underground, has to be PVC conduit. The idea stays the same, do not provide a path in parallel for neutral current except as intended and allowed.

 

[augie47]

one of the smartest code men ive ever known and another one an engineer on a usafb has had me bond in both why is it so bad to have two is it better to have only 1 and if it gets loose lookout the voltage gets loose too just a dumb countryboy got a good christian raisein and an eight grade education

A few notes from some info available at MikeHolt.com might help:

Separately Derived Systems ? The neutral-to-case bond for a separately derived system shall not be made at more than one location because doing so results in a parallel path for neutral return current.

Transformers ? If a neutral-to-case bond is made at both the transformer and at the secondary panelboard, then neutral current will flow through metal raceways and grounding and bonding path on its return path to the power supply
Because SDS are pervasive, it's important to know-and correctly implement-SDS requirements. A mistake in the grounding and bonding of an SDS can prevent the clearing of a fault and it can allow dangerous potential to build on metal parts of the electrical system. Your new knowledge of SDS grounding and bonding requirements allows you to prevent both problems.

 

[dwe68]

im catching on

im catching on

sorry i was flippant ill do more research my bonding will be in xfmr for main lug panels and ill question my idols mabye were assuming larger neutral wire will have less resistance main breaker panels ill bond in panel only xfmrs hit bldg steel and ufer and have primary ground, panels steel and ufer as well and bldg has ground ring lightning prot. all bonded should be good

 

[augie47]

while you are questioning them, ask about the "no main" and 408.36

 

[Cold Fusion]

im waiting for an answer and he's weighing electrons

That one was a bit of a waste of bits - but it was fun. Even so, I promise I won't weigh any more electrons.

cf

 

[Cold Fusion]

The power company supplies a neutral. In the NEC, the earth is not considered a parallel path for neutral current if there is neutral to earth ground bonding at the utility xfmr and again at the service, which is typical.

For overhead triplex, it hits a porcelain anchor and an insulated neutral conductor. If underground, has to be PVC conduit. The idea stays the same, do not provide a path in parallel for neutral current except as intended and allowed.

underscore dan -
You are absolutely right. If all anyone did was residential or other installs where the utility supplied transformer was off the customer premisis, then it wouldn't matter a bit.

But, alas, we don't.

To my thinking there is no difference in the science between a utility owned transformer and a customer owned transformer. Yet some how the code makes this distinction.

Take a look at the following two cases:

1. Cusomer owned 69kv/480v grounded Y, 1000kva, service connection is on the primary side. This one gets an NG bond at the xfm, 5 wires from the xfm to the first disconnect. There is no N/G bond at the first disconnect. This is right by 250.30.A.1. Everything downstream from the first disconnect 5 wire.

2. Utility owned 69kv/480v grounded Y, 1000kva, Service connection is on the secondary. Transformer is on cutomer property, same place it would have gone if installed as Case 1 above. This one gets an NG bond at the xfm, 4 wires from the xfm to the first disconnect, another NG bond at the first disconnect. This is right by 250.24.A.2. Everything downstream from the first disconnect 5 wire.

Why are these two different? (There is a reason -I'll tell you later)

Now add that other pesky issue - parallel paths:
Make this an industiral site and put a groundmat under the facility including the transformer area. For the utility owned transformer, following 250.24.A.2, there is a code mandated parallel path for the neutral current.

under Dan - one question:

... The idea stays the same, do not provide a path in parallel for neutral current except as intended and allowed.

When is it a good design practice to have an "intended and allowed" parallel path for neutral? And I am not considering a path solely through the earth as a parallel path. That just doesn't sound like a good idea at all.

I said I'd tell you why the the utility transformer case is different than the SDS case. It's cause, "The code says what it says - moronic or not.":roll::grin:

cf

 

[__dan]

Take a look at the following two cases:

1. Cusomer owned 69kv/480v grounded Y, 1000kva, service connection is on the primary side. This one gets an NG bond at the xfm, 5 wires from the xfm to the first disconnect. There is no N/G bond at the first disconnect. This is right by 250.30.A.1. Everything downstream from the first disconnect 5 wire.

2. Utility owned 69kv/480v grounded Y, 1000kva, Service connection is on the secondary. Transformer is on cutomer property, same place it would have gone if installed as Case 1 above. This one gets an NG bond at the xfm, 4 wires from the xfm to the first disconnect, another NG bond at the first disconnect. This is right by 250.24.A.2. Everything downstream from the first disconnect 5 wire.

Why are these two different? (There is a reason -I'll tell you later)

cf

just 'dan' will be fine

The OK parallel path I was thinking of is when one neutral conductor is not big enough, a second neutral cable.

case #1. is correct. The neutral is an insulated and isolated conductor with a single point of connection to the "earth grounding" and "equipment grounding" systems.

case #2. is correct. Local earth grounding at the xfmr secondary and again at the service. The arrangement places "the earth" in parallel with the neutral as a shunt path for neutral current. Some neutral current will flow in parallel through the earth. The earth is a high resistance path and per NEC the earth is not considered a parallel path. It is in parallel by circuit but not by code. I would say this is the preferred method. Lightning strikes coming in the utility side have a quick path to ground. The premises wiring system uses the earth as a reference point.

" Now add that other pesky issue - parallel paths:
Make this an industiral site and put a groundmat under the facility including the transformer area. For the utility owned transformer, following 250.24.A.2, there is a code mandated parallel path for the neutral current. "

Recommendation: Approach case #2. Local earth grounding at the transformer N, G, using ground rods and connection to local, in the pad or vault, UFER concrete encased steel or copper. Do not create a parallel path on the grounding system that is metal all the way from the N, G, earth bond at the xfrmr and the one at the service. Having a low impedance grounding path in parallel with the neutral will make the grounding path very noisy. Something that may create hard to trace problems that need to be corrected.

Have a known and intended cut between the local transformer pad earth ground and the premises ring / UFER so the only connection between the two is through the earth. Do not connect the transformer to the premises ring ground using a metal conductor.

The requirement that both ground electrodes be connected together, they are, by the neutral. Single point connection at both ends and the earth, a high resistance path, electrically between the two points.

 

[Cold Fusion]

just 'dan' will be fine. ...

Ok, dan is is. (Just so you know - "underscore dan" was suposed to be humorous, not demeaning.)

...The OK parallel path I was thinking of is when one neutral conductor is not big enough, a second neutral cable. ...

Ah, yes, of course. For some fool reason I had thought you were talking about a path other than 310.4 paralleled conductors.

dan -
Just so I am clear - I'm not in disagreement with anything you have said here.

...Recommendation: Approach case #2. Local earth grounding at the transformer N, G, using ground rods and connection to local, in the pad or vault, UFER concrete encased steel or copper. Do not create a parallel path on the grounding system that is metal all the way from the N, G, earth bond at the xfrmr and the one at the service. Having a low impedance grounding path in parallel with the neutral will make the grounding path very noisy. Something that may create hard to trace problems that need to be corrected.

Have a known and intended cut between the local transformer pad earth ground and the premises ring / UFER so the only connection between the two is through the earth. Do not connect the transformer to the premises ring ground using a metal conductor. ...

Yes one could do this. So there I am on the jobsite, talking to the electricians.
EE: That's right , when the ground mat is down, before the gravel pad is placed, go around the substation and cut a gap in the mat. And all of those MC-HL cables being installed - don't terminate the sheaths or the EGCs.

Elec: What particular goof came up with that spec?

EE: It's a code issue guys. We have to screw this up so we meet NEC. If it were our transformer we would not have to do this .

Elec: Okay, we will do it as speced. But don't you think the Inspector is going to choke over the unterminated MC-HL sheaths and grounding conductors.

EE: Yes, I certainly think she will - and rightfully so. I think by meeting code we will look dumber than a box of rocks.

... The requirement that both ground electrodes be connected together, they are, by the neutral. Single point connection at both ends and the earth, a high resistance path, electrically between the two points.

Again you are absolutely right.

But the part not answered yet is: Other than the answer I gave earlier, how come we treat the utility owned transformer different than the customer owned transformer. The electrons won't.

cf