480 to 120/208 Step down Bonding

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Fordean

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
On a 30 KVA transformer. Bonding of the Xo terminal to casing is correct ( neutral/ground bond). Question is on the MLO (208 side) panel Being served. Would you bond the Neutral and ground (Box) the Same as you would bond Neutral and ground at a Main Breaker service. I would say no, ( But I think it should be bonded both spots in trans and In panel) Since separate system. if it already bonded at Trans. The Neutral and ground (At Panel) should be separate? Or bonded a second time in MLO. What would be downsides to this. Their are a ground wire and neutral from transformer to panel.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
250.30(A)(1) System Bonding Jumper. An unspliced system bonding jumper shall comply with 250.28(A) through (D). This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices, in accordance with 250.30(A)(1)(a) or (b). The system bonding jumper shall remain within the enclosure where it originates. If the source is located outside the building or structure supplied, a system bonding jumper shall be installed at the grounding electrode connection in compliance with 250.30(C).
 

Fordean

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
250.30(A)(1) System Bonding Jumper. An unspliced system bonding jumper shall comply with 250.28(A) through (D). This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices, in accordance with 250.30(A)(1)(a) or (b). The system bonding jumper shall remain within the enclosure where it originates. If the source is located outside the building or structure supplied, a system bonding jumper shall be installed at the grounding electrode connection in compliance with 250.30(C).

For easy understanding. Would you bond at Transformer X0 or the MLO next to it, Or both. The Lug kits on Transformers are only 1 barrel on the Kit. I don't know why they don't provide a 2 barrel. Unless they expect you to bond at MLO panel.
 

kwired

Electron manager
Location
NE Nebraska
For easy understanding. Would you bond at Transformer X0 or the MLO next to it, Or both. The Lug kits on Transformers are only 1 barrel on the Kit. I don't know why they don't provide a 2 barrel. Unless they expect you to bond at MLO panel.
They give you a choice - one or the other but not both locations. You must run an EGC between the source and first disconnect as well, if you choose to bond at the first disconnect this EGC will bond the transformer case to the separately derived system.

If you bond the grounded neutral at both locations then you potentially give the neutral multiple paths to take between the two points as anything connected to the EGC becomes a possible neutral path.

You mention this is a MLO panel being supplied, you may want to look at that some more as well as your transformer secondary conductors likely need to land at a single overcurrent device
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
They give you a choice - one or the other but not both locations. You must run an EGC between the source and first disconnect as well, if you choose to bond at the first disconnect this EGC will bond the transformer case to the separately derived system.

Not an EGC in this instance, but a Supply Side Bonding Jumper per 250.30(A)(2).
 

Fordean

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
Bonding

Bonding

They give you a choice - one or the other but not both locations. You must run an EGC between the source and first disconnect as well, if you choose to bond at the first disconnect this EGC will bond the transformer case to the separately derived system.

If you bond the grounded neutral at both locations then you potentially give the neutral multiple paths to take between the two points as anything connected to the EGC becomes a possible neutral path.

You mention this is a MLO panel being supplied, you may want to look at that some more as well as your transformer secondary conductors likely need to land at a single overcurrent device

Attached is photo of service. Only Bonding off Neutral and ground at the Trans. MLO is separated.
View attachment Service Bonding .pdf
 

kwired

Electron manager
Location
NE Nebraska
Attached is photo of service. Only Bonding off Neutral and ground at the Trans. MLO is separated.
View attachment 10247
That is one of your two choices.

The other choice is to install the bonding screw (most likely a screw) in the panel and bond the transformer case with a "supply side bonding jumper" coming back from the panel.

If you bonded it at both locations in addition to the neutral conductor you would have parallel neutral paths via the equipment, raceways, etc.
 

iwire

Moderator
Staff member
Location
Massachusetts
For easy understanding. Would you bond at Transformer X0 or the MLO next to it, Or both. The Lug kits on Transformers are only 1 barrel on the Kit. I don't know why they don't provide a 2 barrel. Unless they expect you to bond at MLO panel.

My preference is at the transformer.

I don't buy 'lug kits' as they never have what I want. I order the lugs individually. I usually end up using two - 3 barrel lugs. One of them on XO and one of them on the enclosure.

At the 3 barrel lug on XO will be neutral to panel, the GEC and the bonding jumper to the enclosure.

At the 3 barrel lug on the enclosure will be the system bonding jumper from XO, the supply side EGC and the outgoing bonding jumper to the panel.
 

iwire

Moderator
Staff member
Location
Massachusetts
On a 30 KVA transformer. Bonding of the Xo terminal to casing is correct ( neutral/ground bond). Question is on the MLO (208 side) panel Being served.

The transformer cannot supply an MLO panel unless you install an over current device between the transformer and the MLO.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
My preference is at the transformer.

I don't buy 'lug kits' as they never have what I want. I order the lugs individually. I usually end up using two - 3 barrel lugs. One of them on XO and one of them on the enclosure.

At the 3 barrel lug on XO will be neutral to panel, the GEC and the bonding jumper to the enclosure.

At the 3 barrel lug on the enclosure will be the system bonding jumper from XO, the supply side EGC and the outgoing bonding jumper to the panel.

Although I like the idea, the 2014 NEC will no longer allow your use of a 3 barrel lug.

450.10 Grounding.
(A) Dry-Type Transformer Enclosures. Where separate
equipment grounding conductors and supply-side bonding
jumpers are installed, a terminal bar for all grounding and
bonding conductor connections shall be secured inside the
transformer enclosure. The terminal bar shall be bonded to
the enclosure in accordance with 250.12 and shall not be
installed on or over any vented portion of the enclosure.
 

Fordean

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
You sound pretty confident in your response, do you have a code section to back that up? You might want to look at 240.21(C).

Actually Iwire I think may be correct. States for single phase is acceptable. I will read up on the order points it directs you to. I checked my Material LIst. And I asked for a MCB not MLO. I had this done 5 months ago and forgot. Vendor messed up.

Thanks Guys. I was always under impression of that. 3 phase can have on either side. I will view Stallcups again. I think I may mixed up single phase.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
IMO what I am doing directly complies with that section.

IMO it should say "terminal bar or functionally equivalent terminal". Some would argue that they're the same, some would argue that they're different. :)

I'm too lazy to look up the ROP to figure out why we needed this code change.
 

kwired

Electron manager
Location
NE Nebraska
IMO it should say "terminal bar or functionally equivalent terminal". Some would argue that they're the same, some would argue that they're different. :)

I'm too lazy to look up the ROP to figure out why we needed this code change.
Do they define what is a lug and what is a terminal bar? Without a definition they are the same and they are different.
 

iwire

Moderator
Staff member
Location
Massachusetts
IMO it should say "terminal bar or functionally equivalent terminal". Some would argue that they're the same, some would argue that they're different. :)

Of course you are right but I would go for it.

I'm too lazy to look up the ROP to figure out why we needed this code change.


I laughed when I read that because that was exactly how I felt about it last night.

Today I found the ROP.

9-144 Log #3446 NEC-P09 Final Action: Accept
(450.10(A) (New) )
________________________________________________________________
TCC Action: The Correlating Committee understands that the Exception
is also new text.

Submitter: Michael J. Johnston, National Electrical Contractors Association

Recommendation: Add new text to read as follows:
(A) Dry-Type Transformer Enclosures. Where separate equipment grounding
conductors and supply-side bonding jumpers are installed, a terminal bar for all
grounding and bonding conductor connections shall be secured inside the
transformer enclosure. The terminal bar shall be bonded to the enclosure in
accordance with 250.12 and shall not be installed on or over any vented portion
of the enclosure.

Exception. Where a dry-type transformers is equipped with wire-type
connections (leads), the grounding and bonding connections shall be permitted
to be connected together using any of the methods in 250.8 and shall be bonded
to the enclosure, if of metal.
Include the current text of this section as Subdivision (B) and new title as
follows:

(B) Other Metal Parts. Insert current text of this section unchanged.

Substantiation: The connections of grounding and bonding conductors in drytype
transformer enclosures continue to be problematic and inconsistently made
without clear requirements in the NEC. It is very common to see grounding
and bonding conductors connected to the enclosure at or over the venting
openings in the bottom of the transformer, resulting in less than effective
connections. I don?t believe these enclosures have been evaluated as grounding
and bonding equipment and should not be depended upon to serve as effective
ground fault current paths. The requirement for a terminal bar eliminates the
inconsistencies and provides needed direction for installers.


Panel Meeting Action: Accept

Number Eligible to Vote: 12

Ballot Results: Affirmative: 12

After reading that I would still expect the inspector to accept a 3 barrel lug for 3 conductors. :)
 

iwire

Moderator
Staff member
Location
Massachusetts
Here is one I did in the past where the only usable location was over the vent holes. I removed all the paint under the L-shaped bus bar I had bolted to the enclosure.

25012.jpg


856a2bb6.jpg
 

kwired

Electron manager
Location
NE Nebraska
Of course you are right but I would go for it.




I laughed when I read that because that was exactly how I felt about it last night.

Today I found the ROP.



After reading that I would still expect the inspector to accept a 3 barrel lug for 3 conductors. :)

Same here, seems the 3 barrel lug meets the intent of the ROP as long as no more then 3 conductors are needed. Otherwise you could have only a 3 hole terminal bar and more then 3 conductors and still have the same problem they were trying to avoid.
 
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