It's my understanding that with all sub panels the ground and neutral remain floating or isolated from one another. The only bonding connection should exist at the main panel. Reason being, anything common between the house / garage / barn or whatnot could become energized should a neutral become open creating a shock hazard.
However, I am working on a farm location where there is a 100 amp overhead feed coming from the customers 200 amp service at their house, going to a 60 amp panel in a barn, and just before the barn their is a 70 amp rated sub panel mounted to a utility pole feeding receptacles. Their are no common conductive paths (by conductive paths I am referring to water / gas pipes or the like) between the house or the barn, and its quite a distance away. So given that there are no water or gas pipes that could become energized, is it a requirement to bond the neutral to the ground at the sub panel?
At the utility pole: I'm wiring a 240v receptacle for a customer to feed a job trailer at the utility pole and noticed a couple things with the existing wiring to the receptacles:
First off, none of the receptacles had a ground wire. The only means of grounding they had were through the conduit fittings attached to the painted sub panel-box. I thought this was something not to be done, unless either the receptacles were marked as ungrounded or the ground prong was plugged.
Secondly, there are no ground rods around the utility pole. The only ground connection appears to be coming from an extra overhead conductor they ran from the barn panel to the pole mounted 70A sub panel. Is there not supposed to be a individual grounding electrode for each sub panel?
I would direct you to 10-208 for 2 or more buildings fed from a single service, and the rule talks specifically about buildings housing livestock. Essentially separate grounding electrode at each building, each panel neutral bonded to this electrode.
And you need the bond wire for the receptacles otherwise the 6/32 screws used to mount the recetacle become the bonding method which is not approved as per 10-804. Maybe refer also to 26-714 if the outdoor receptacles are part of a single family dwelling on the farm, ie GFCI protection.
Building housing livestock must be grounded and the neutrals must be bonded. Outlets require a bonding jumper from the box to the outlet as Jay said Unless there is a pole mounted transformer there is no requirment to Ground any of the overhead wiring except in each building that houses livestock and at the main service all this is fed from. The bare neutral is grounded through the service when distributed overhead with Triplex.
Re: Farm Sub Panel Grounding
#202642 08/16/1110:19 AM08/16/1110:19 AM
Just to clarify the above post. Buildings housing livestock must have a ground electrode intalled. If the building does not house livestock the extra electrode is not required. All buildings must be grounded.