Service Bonding with Outside Transformer

Status
Not open for further replies.

ROKETRIK

Member
We have an existing 3-phase 208v service with an on premises outdoor transformer. X0 at the transformer is bonded to ground (a counterpoise) as required by 250.24(a)(2). There was no main bonding jumper installed at the service disconnect, a 300 amp distribution panel. The service lateral is run in RGC and the grounding conductor run in it and connected to X0 as well. Neither the ground bar or the neutral bar in the service are connected to the counterpoise.

One side of the discussion maintains that no bonding jumper is needed at the service disconnect because of the connection at the transformer and because installing one would put the grounding conductor in parallel with the grounded conductor thereby putting objectionable current on the grounding conductor. The other side maintains there should be a bonding jumper installed in the panel per 250.24 (1) and it should connect to the counterpoise.

As I look at the illustration in the handbook, Exhibit 250.7, I notice that this example shows the service lateral as underground cable which would not have a separate grounding conductor and so there is no direct connection between ground at the service and ground at the transformer and I wonder if this is the situation that the Code committee had in mind and if that should make a difference in this case.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Is this a service or a SDS? I have never heard of a case where the utility would connect the EGC to XO in their transformer.
 

ROKETRIK

Member
SDS/Service

SDS/Service

I understand where you're coming from, this is on the space center so things are a little different out here. FPL supplies power to the center and a contractor (URS) maintains the high voltage feeders and the transformers feeding the services and acts as a utility. In this particular case we get power via a 13.8 KV transformer and we, as the user, have access to the low voltage side of the vault. The facility in question is what provides navigational radar type information to planes or the space shuttle on landing. My question comes about because URS had maintenance to perform on their HV feeders and put us on a portable generator, making the connections in the vault. This is in addition to a permanent on-site back-up generator (3-pole ATS, not a separately derived system). Long story-short the permanent generator was wired wrong, the neutral was connected to the grounding conductor and nobody had landed the grounding conductor at the transformer to the neutral from the temporary generator so when they shut down the temporary generator and the permanent one fired up the neutral current had no path home. In trying to sort out this mess I found that the main bonding jumper was made at the transformer, not in the service, and now I am trying to convince my NASA bosses that it should be in both places. I have come to think that we really ought to drive a ground rod for the service and put in a bonding jumper and then remove the grounding conductor coming out of the service from X0, leaving X0 connected to the counterpoise. Got a head ache yet?
 

pete m.

Senior Member
Location
Ohio
If I understand this correctly it would seem to me that an EGC routed with the service conductors (lateral) is superfluous and most likely would constitute a violation of 310.4 since the EGC would be in parallel with the neutral conductor.

Since the service lateral is contained in RMC the bonding jumper should be removed at the transformer end and installed at the service equipment end (I suppose since the transformers are customer owned and not utility the bonding jumper could be at either end but should not be at both).

The XO at the transformer should be connected to a grounding electrode at the transformer and then a separate grounding electrode system should be connected to the service equipment neutral conductor via of a grounding electrode conductor. The neutral conductor should be bonded at the service equipment.

This method will mitigate objectionable current on the RMC.

Just a thought...

Pete
 

SG-1

Senior Member
The EGC is bonded at the transformer to the GEC, X0, the grounded conductor, neutral & the transformer enclosure. The EGC is bonded to the ground bar in the service disconnect.

The grounded conductor, neutral, is bonded to the neutral bar in the service disconnect.

The Service Disconect has no main bonding jumper.

How can the EGC be in parallel with the neutral ?

Are they landed together in the service disconnect ?
 

Smart $

Esteemed Member
Location
Ohio
The EGC is bonded at the transformer to the GEC, X0, the grounded conductor, neutral & the transformer enclosure. The EGC is bonded to the ground bar in the service disconnect.

The grounded conductor, neutral, is bonded to the neutral bar in the service disconnect.

The Service Disconect has no main bonding jumper.

How can the EGC be in parallel with the neutral ?

Are they landed together in the service disconnect ?
No MBJ at the service-disconnect enclosure is a violation of 250.24(B). IMO, one cannot use 250.6 to mitigate objectionable current by removing a required MBJ at service disconnect enclosure and adding a non-required bonding of the GSC to GES at the xfmr.
 

Smart $

Esteemed Member
Location
Ohio
If I understand this correctly it would seem to me that an EGC routed with the service conductors (lateral) is superfluous and most likely would constitute a violation of 310.4 since the EGC would be in parallel with the neutral conductor.

Since the service lateral is contained in RMC the bonding jumper should be removed at the transformer end and installed at the service equipment end (I suppose since the transformers are customer owned and not utility the bonding jumper could be at either end but should not be at both).

The XO at the transformer should be connected to a grounding electrode at the transformer and then a separate grounding electrode system should be connected to the service equipment neutral conductor via of a grounding electrode conductor. The neutral conductor should be bonded at the service equipment.

This method will mitigate objectionable current on the RMC.

Just a thought...

Pete

That is my thought as well although you stated it more succintly than I managed.
The NEC cannot prevent the utility (or its contractor) from bonding X0 to the local GE and the xfmr enclosure. In this case, 250.6 should be invoked to isolate the SSBJ (2011 NEC terminology) connection at the xfmr.

EDIT to add: For clarification, the above statement is regarding a utility owned xfmr. FWIW, this would not be a service if it were a consumer-owned xfmr.
 
Last edited:

SG-1

Senior Member
No MBJ at the service-disconnect enclosure is a violation of 250.24(B). IMO, one cannot use 250.6 to mitigate objectionable current by removing a required MBJ at service disconnect enclosure and adding a non-required bonding of the GSC to GES at the xfmr.

No argument there, I was only trying to figure out how the EGC was in parallel with the neutral with no main bonding jumper in the disconnect.
 

erickench

Senior Member
Location
Brooklyn, NY
There were some changes in the 2011 NEC to address this. It seem's that the SBJ must still be installed where the GEC is connected. But the new changes specifically state that the SBJ must bond the neutral to whichever enclosure the GEC is installed. Take a look at NEC 250.30(A)(1) in the 2011 NEC. Also, if the separately derived system and the disconnect are located in separate enclosures then a supply-side bonding jumper must be run between them as per NEC 250.30(A)(2) in the 2011 NEC.
 

Smart $

Esteemed Member
Location
Ohio
"X0 at the transformer is bonded to ground (a counterpoise)


What is a counterpoise?:confused:
.
Counterpoise

(v. t.) To act against with equal weight; to equal in weight; to balance the weight of; to counterbalance.

(v. t.) To act against with equal power; to balance.

(n.) The relation of two weights or forces which balance each other; equilibrium; equiponderance.

(n.) An equal power or force acting in opposition; a force sufficient to balance another force.

(n.) A weight sufficient to balance another, as in the opposite scale of a balance; an equal weight.

Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The NEC cannot prevent the utility (or its contractor) from bonding X0 to the local GE and the xfmr enclosure. In this case, 250.6 should be invoked to isolate the SSBJ (2011 NEC terminology) connection at the xfmr.
...
The code specifically requires the service bonding at two locations when the utility supplies a grounded system.
This is one of those applications where the rules for a service and a SDS are much different. Multiple points of bonding are required for services and prohibited for SDS, even though they are electrically identical systems.
 

Speedskater

Senior Member
Location
Cleveland, Ohio
Occupation
retired broadcast, audio and industrial R&D engineering
Counterpoise

Counterpoise

Counterpoise - part of a radio antenna.

A system of wires or other conductors, elevated above and insulated from ground, forming a lower system of conductors of an antenna.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician

Counterpoise

(v. t.) To act against with equal weight; to equal in weight; to balance the weight of; to counterbalance.

(v. t.) To act against with equal power; to balance.

(n.) The relation of two weights or forces which balance each other; equilibrium; equiponderance.

(n.) An equal power or force acting in opposition; a force sufficient to balance another force.

(n.) A weight sufficient to balance another, as in the opposite scale of a balance; an equal weight.

Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter.



That's about as clear as mud.:) I was thinking of some type of grounding electrode.:confused:
 

Smart $

Esteemed Member
Location
Ohio
The code specifically requires the service bonding at two locations when the utility supplies a grounded system.
This is one of those applications where the rules for a service and a SDS are much different. Multiple points of bonding are required for services and prohibited for SDS, even though they are electrically identical systems.
Correct... per 250.24(A)(2)... provided the service point is at the transformer's secondary terminals. If the service point is somewhere load-side of the secondary terminals, the bonding of the secondary terminals is outside the purview of the NEC. If the service point is at the primary terminals, or line-side of such, the issue also becomes moot for the consumer owns the transformer, thus making the secondary not a service.

What I am suggesting as the topic of discussion is how to run and connect grounding conductors and metallic raceways... so as not to create a path for objectable current (i.e. grounded-conductor current on grounding conductors and non-current-carrying metallic parts, such as enclosures) which CAN be avoided without compromising personnel safety, property, and the grounding system itself... when the utility bonds the transformer enclosure also.
 

Smart $

Esteemed Member
Location
Ohio
...That's about as clear as mud.:) I was thinking of some type of grounding electrode.:confused:
It has more to do with purpose, I believe. I have heard the term used a few times in the same regard. I don't know who, if anyone is teaching this as appropriate terminology, but I consider it ill-conceived, and will not use it myself.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
It has more to do with purpose, I believe. I have heard the term used a few times in the same regard. I don't know who, if anyone is teaching this as appropriate terminology, but I consider it ill-conceived, and will not use it myself.

Thanks Smart $, I have never heard the term. When the OP said he bonded XO to it I thought it must be some type of grounding electrode.
 
Status
Not open for further replies.
Top