Can the manufacture supplied bonding strap in a normal 480v delta/120/208v wye transformer be used as the main bonding jumper for the separately derived system? Or must a properly sized bonding jumper be added according to the size of the secondary phase conductors? Thanks
The bonding strap that comes on most transformers is to bond the core frame not to act as a MBJ. You need to install a jumper sized by table 250-66 based on the secondary phase conductors from the equipment grounding lugs to the transformer XO.
Re: main bonding jumper#8840 04/07/0211:27 AM04/07/0211:27 AM
Nick, Doesn't the neutral also have to be connected to the Grounding Electrode System? I have seen quite a few installations that only consisted of a jumper from XO to the transformer housing. Please comment.
Re: main bonding jumper#8841 04/07/0212:06 PM04/07/0212:06 PM
I always run my own MBJ from XO to a ground terminal in the xfmr. On smaller wires 1/0 and less I usually install a ground buss (having removed the paint under the buss) in the transformer to connect all the grounding conductors usually 5 or so. 1)MBJ from XO 2)grounding conductor to service equipment 3)grounding conductor routed with primary circuit to transformer 4)water bond conductor 5)GEC On larger conductors is it ok to use several one or 2 port equipment grounding lugs for the purpose of tying all the grounding conductors together at one point? Or is it necessary to obtain a ground buss, so the continuity of the grounding conductors isn't dependant on the transformer enclosure? thank you
Re: main bonding jumper#8843 04/07/0208:05 PM04/07/0208:05 PM
The neutral & GEC leave the xfmr enclosure, where they are bonded together, and go to the panelboard. The neutral connects to an insulated neutral bus. However the GC only connects to the panel enclosure. There is no connection to the Grounding Electrode System. There are at least 3 setups like this at my site.
Dom, If your GEC is run in your primary conduits, does that mean you have 2 in there? (1 as an EGC for the primary feeders and on 1 larger GEC for the secondary?)
Re: main bonding jumper#8845 04/08/0205:59 PM04/08/0205:59 PM
One EGC routed with the primaries based on the ocpd of the primary circuit. The GEC connects directly to the ground electrode system in its own raceway, and/or just directly to the ground rod installed below the transformer through the concrete pad.
Re: main bonding jumper#8846 04/08/0207:40 PM04/08/0207:40 PM
I've seen the primary equipment ground used as the GEC. I believe that is not permitted by NEC. That could form a long and indefinite path for a completed circuit from a lightning strike or overvoltage to the grounding system.