What’s the story on when bonding bushings are needed? Are they needed whenever metallic conduit is being used as the EGC from a sub panel to the main panel and you have concentric or eccentric knockouts?
At less than 250 volts to ground they’re basically never needed except for bonding a metal service raceway that has concentric or eccentric knock outs.
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Thanks Robert. So the feeders for the sub would not be considered a service correct? Only the service coming from the meter?
That’s correct. For feeders a bonding bushing would never be required when the voltage is less than 250 volts to ground. A service raceway is one that has service conductors in it which would be the conductors on the line side or upstream from the service disconnect.
So for example if you had an overhead service down to a meter pan and then a metal nipple into a main breaker panel (that main would be the service disconnect) that metal nipple would require to be bonded by a means other than standard locknuts. If there are no concentric or eccentric KO’s then a bonding locknut can be used on either end, only one is required.
With concentric or eccentric KO’s a bonding bushing would be required to bond one end of that service raceway.
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Yes it does, and your previous explanation helps as well, thank you. I do have a few questions though.
- What is the idea behind requiring the bonding bushing or bonding locknut on only one end under certain circumstances?
- In reference to the picture below (red arrow), what is a “female entry?”
- In reference to the picture below (blue arrows), I assume these questions would be in reference to the sub-panel and not the main in my situation?
Also for anyone wondering the difference between a bonding locknut and a bonding bushing…
Bonding Locknut
Bonding Bushing
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When you have a metal raceway that contains service conductors between the meter enclosure and the panel those conductors have no overcurrent protection (the OCPD is on the primary of the transformer) so the theory is that if there is a fault to that metal raceway the bonding connection has to be better than what a standard locknut can provide.
The reason that they’re only required on one end is because the neutral is already bonded to the metal meter enclosure, the neutral is also bonded in the panel because that is the service disconnect. (All metal parts upstream of the service disconnect are bonded to the neutral, there are no equipment grounding conductors in this part of the system). Since both the meter pan and the panel are bonded directly to the neutral a fault to the metal panel enclosure is not relying on the metal nipple to carry the fault current back to the transformer, the neutral is doing that. That is one reason why the nipple between the meter pan and the panel can be a non-conductive raceway like PVC. A fault to the panel enclosure still has a path back to the transformer through the neutral.
Just for the record this is not my flow chart, it was made by one of the members at Mike Holt’s Forum. I’m guessing that this is a term used to describe a threaded hub in a box or a conduit body. Think of an aluminum outdoor metal box with threaded entries called an FS box.
The GEC reference is for when a grounding electrode conductors is run inside of a ferrous raceway like EMT. In that installation the GEC is required to be bonded directly to the raceway on both ends to mitigate what is called the choke effect.
The second blue arrow in your question is the metal raceway that we discussed prior. It’s called a service raceway because it contains service conductors. Service conductors only exist upstream of the service disconnect so if you had conductors from the service panel feeding a sub-panel those conductors would be feeder conductors because they have an OCPD ahead of them. Being that they are not service conductors no bonding bushings or other bonding means besides a standard locknut would be required to bond a metal raceway.
If you follow the flow chart you can also see what happens when your system voltage is over 250 volts to ground (one example is a 480Y/277 volt system). In that case a bonding bushing may be required for a feeder.
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Perfect, Thank you @rmeier2 !
You’re welcome! 