Main Bonding Jumper` 1

[txg84]

I was inspecting a 13.8-208/120V pad-mount transformer that serves two separate buildings. The equipment grounding conductors and the grounded conductor(1 for each building) are all bonded together at the transformer. Each building is fed by 3-phase conductors, 1-grounded conductor, and 1-equipment grounding conductor. They are not bonded together at the first disconnecting means. Is this allowed by the NEC? Article 250.30(1) allows this for any SDS, but I cannot find anything on services.

Thanks

 

[alehman]

250.30 does not apply if this is a service. See 250.24 instead.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[txg84]

Is there a reason why the code allows bonding at the transformer or the first disconnecting means for a SDS but not for a service?

 

[rbulsara]

By service, if you mean the service entrance equipment (main service switch or the breaker), it is because by the code definition the service conductors and transformer upstream of the service entrance equipment is owned by the utility company, which do not fall under NEC.

NEC is applicable to owner's premise wiring systems, starting with the service entrance equipment. Code does not permit grounding the neutral (grounded conductor) at more than one point on the premise wiring systems which includes SDS and the service entrance equipment.





Rafiq Bulsara

http://www.srengineersct.com

 

[racobb]

For the utility, neutral and ground are one in the same. It is odd that you have a 5 wire instead of a 4 wire service. Normally the neutral and grounds from the customer's branch circuits are tied together at the customer service entrance equipment and not allowed to be tied past that point.

Is this a new installation? I would expect the AHJ would require the utility neutral and grounds to be tied together at the service entrance, but maybe there is something "special" with these.

Did the customer install the wire to the transformer and just add the extra one?

Alan

 

[txg84]

The transformer feeds two separate buildings. Originally there was a circuit breaker installed inside the LV compartment of the padmount transformer, which fed one of the buildings. At some point the breaker was removed and the conductors were connected directly to the transformer. I'm guessing that's the reason why there is a 5-Wire service.

 

[waross]

Who owns the transformer?
Where are the meters?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[txg84]

The distribution system as well as the building is owned by the government. There is a meter for each building located on the transformer.

Would bonding at the transformer prevent objectionable currents from occurring?

 

[rbulsara]

Who owns the transformer makes the difference. Also location of transfromer makes the difference.

The outdoor transformers supplying a service disconnect are required to have additional bonding of the neutral to the ground, either at the transformer or some where outside the building. See 250.24 as alehman said.

Rafiq Bulsara

http://www.srengineersct.com

 

[rockman7892]

I would not think that you would want the EGC, and the grounded conductor tied together at both the transformer and the service disconnect, for this would create a parallel path for neutral current, and essentially put neutral current on the EGC during normal operation of L-N loads.

I have typically seen 4-wires used for utility services with the grounded conductor bonded to the enclosure and grounding electrode at the transformer and bonded to the ground and enclosure at the disconnect. Is it also allowed to use only 4-wires with this bonding scheme for SDS within a facility?

 

[rbulsara]

rockman:

Outdoor transformers feeding the service equipment is exception to that. In fact, no ECG need be run from the service transformer as the neutral acts as the ECG as well, between the service disconnect and the service transformer. This also avoids a direct parallel path to neutral except for the actual earth. Due to the higher resistance of the path through the earth, not much neutral current passes through the earth. So your observation of 4 wires for utility services is correct.

SDS (separately derived system) is different, where the bonding of neutral is permitted only at one place, either at the transformer or the first disconnect. So you will always run a separate ECG and no parallel path is created or allowed.

Rafiq Bulsara

http://www.srengineersct.com

 

[alehman]

Whether the service disconnect is inside or outside, the service conductors would be 4 wires. There should not be an EGC between the service disconnect and transformer.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rockman7892]

alehman

So with the (4) wires you are saying should be used between disconnect and transformer, this 4th grounded conductor should be bonded to both the transformer enclosure and disconnect enclosure at both ends as well as being bonded to the grounding electrode at the transformer or panel?

 

[alehman]

Yes, the grounded service conductor must be connected to a grounding electrode at the service disconnect and at some point outside of the building (typically at the transformer).

The service disconnect enclosure and transformer must be bonded to the grounding electrodes.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[resqcapt19]

The fact that this is a campus type distribution system and not owned by the utility makes a difference to the NEC. Electrically it may be exactly the same as what the utility installs, but code wise it is different. The customer owned transformers are SDS and the conductors on the primary and secondary side are feeders, not service conductors. The NEC requires an EGC to be run with the building feeder conductors if the grounding electrode is connected to the grounded conductor at the transformer and would require a "supply side bonding jumper" to be run with the feeder conductors if the grounding electrode connection is at the building disconnect.
That being said, most of these customer owned systems are installed exactly like the utility would install them, even though the NEC requires otherwise.

 

[rockman7892]

What if both the transformer and disconnect are located outside? Do we then need two grounding electrodes?

With two grounding electrodes and the grounded conductor bonded to these electrodes at both locations, doesn't this create a prallel path for neutral current to flow on the ground?

 

[EEJaime]

You need to have a grounding electrode and grounding electrode conductor for each buildings' service entrance. You do not need to bring the grounding condoctor from the transformer to the buildings. When your four wire service enters each building's service entrance equipment, the neutral and ground busses must be bonded at that point. These in turn will have a system grounding electrode conductor to your grounding electrode, be that a Ufer ground, ground rods, building steel, etc..., with the appropriate bonding of water, gas metal service piping, etc....

If as a later question asked, you have exterior service entrance equipment adjacent to the transformer, both the transformer star-point grounding electrode conductor and equipment grounding conductor should be tied to the same grounding electrode system as the service to prevent differences in potentials, circulating ground currents, etc....

I think the above agrees with previous responses for the most part. We see this on many campus type distribution systems where the owner is 'the utility' for each building. Are you saying that both of the building's individual meters are located at the transformer?

Regards,
EEJaime