Main Bonding Jumper`
Main Bonding Jumper`
(OP)
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
Thanks






RE: Main Bonding Jumper`
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Main Bonding Jumper`
RE: Main Bonding Jumper`
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
RE: Main Bonding Jumper`
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
RE: Main Bonding Jumper`
RE: Main Bonding Jumper`
Where are the meters?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Main Bonding Jumper`
Would bonding at the transformer prevent objectionable currents from occurring?
RE: Main Bonding Jumper`
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
RE: Main Bonding Jumper`
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?
RE: Main Bonding Jumper`
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
RE: Main Bonding Jumper`
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Main Bonding Jumper`
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?
RE: Main Bonding Jumper`
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.
RE: Main Bonding Jumper`
That being said, most of these customer owned systems are installed exactly like the utility would install them, even though the NEC requires otherwise.
RE: Main Bonding Jumper`
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?
RE: Main Bonding Jumper`
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