Main service disconnect bonding jumper

[nerspellsner]

I have installed a 400 amp fused disconnect used as the main service disconnect for a single family dwelling. Two ground rods are connected to it at this point. It is feeding a 400 amp automatic transfer switch which then feeds two 200 amp main breaker panels. The grounds and neutrals are seperated in both panels. A seperate ground kit and a seperate isolated neutral kit were included with the 400 amp fused disconnect. The neutral kit included a green bonding screw. Now here comes some bonding questions about this installation. Would this screw be considered the main bonding jumper? Is the screw all that is needed for the connection between the ground and neural? If it is, how can this screw equal the same as a conductor sized for the main bonding jumper? Would installing a conductor between the ground and neutral along with the screw be acceptable or does this create a parallel path? Is a main bonding jumper and a main bonding conductor the exact same thing? Thanks.

 

[Pierre C Belarge]

Yes the green screw provided with the kit is your MBJ...how, because the manufacturer tested it during the design process of the equipment and it passed the minimum standards to become the MBJ. I also agree it would seem skimpy, but it is accepted.

If you wanted to install the screw and a MBJ conductor as well, that would be acceptable. It does create a parallel path, but only within the enclosure.

I would think that the main bonding jumper and a main bonding conductor would be one and the same.

P.S. - I would like to add, that I have seen instances of fault conditions where the green screw (MBJ) has disappeared during a very large fault.

 

[Bea]

Is the GEC connected to the neutral bar if not then there must be a buss or wire of same size connecting the two termianl bars together can not relie on the enclosure to make the connection 250.24(A)(4)

 

[nerspellsner]

Thanks to both of you for your help. Bea, yes the GEC is connected to the neutral lugs. If the GEC was connected to the ground bar instead of the neutral lugs what size would the conductor have to be to connect the two? The GEC is connected to two ground rods. The GEC is a #4 Cu even though it only has to be a #6 Cu. Would the jumper be #4 Cu or would it be 1/0 Cu using Table 250.66? The Disconnect load conductors are 500MCM Al fused at 350 amps. Thanks again.

 

[augie47]

bond

bond

If you did use a wire to tie the grounded conductor to the equipment ground bar I would say it would need to be a minimum of 1/0 per 250.66 & 250.24