Bonding Two Structures Together

[chrisreisinger79]

The factory I work at has recently built a new material silo. It is not attached to the existing factory structure in any manner. The engineer behind the project wants the silo bonded to the factory in case one of the onsite mechanics is welding on the silo but has his ground lead attached to the factory structure. He is asking for the two to be bonded together so that the electrical and electronic equipment in the silo does not get damaged in the above scenario. How would I go about sizing this cable?

 

[rlundsrud]

I beleive you would use table 250.122, the cable is sized based on the OCPD for the feeder that supplies the building. The NEC dosen't AFAIK require any size cable if there isn't a sub-panel in this silo that is supplied by the main building. In that case the engineer can spec whatever size he wants. Perhaps use table 250.122 and base it on the size of the OCPD on the largest welder that might be used.

 

[chrisreisinger79]

There is no sub-panel. Only a control enclosure with a PLC and a PanelView screen.

 

[petersonra]

If you have a control enclosure presumably it is fed by a branch circuit from the main building. Is this in rigid?

 

[chrisreisinger79]

Yes. There are several rigid conduits which stub through the silo and terminate into the enclosure.

 

[winnie]

IMHO in the situation described, the welder should get a gibbs slap for poor 'ground' lead placement.

NEC won't address this issue. The rigid conduit will probably do a fine job of carrying the current.

If as a design issue the engineer wants a separate bonding conductor, I would size it as a welding conductor based on the welding current and the length of the run. The whole point is a conductor that carries welding current, so size it as a welding conductor, not as an NEC mandated EGC.

-Jon