[hueco]

Will somebody help me remember the theory behind isolating the neutrals and grounds at a sub-panel? My broken brain can't remember!

 

[Speedy Petey]

ALL sub-panels must have a separate ground and neutral run to them.
Neutrals remain un-bonded (isolated from the panel box).
Grounds terminate on a separate bar screwed right to the panel box.

**This does not apply to some existing installations.

 

[hueco]

Yes, I know that, what I don't recall is the theory behind the rule!

Can you elaborate on it not applying to "some existing installations?"

 

[Arrow3030]

They should not be bonded if there's an equipment grounding conductor to the main. If they were there would be a parallel path as both the neutral and grounding conductor would carry amperage. There are other reasons also.

 

[Speedy Petey]

Yes, I know that, what I don't recall is the theory behind the rule!

Can you elaborate on it not applying to "some existing installations?"

The theory behind it is that if you do not have a dedicated grounding conductor then if there is a compromise on the neutral then all the metal parts, exposed and not, of the system will try to carry the current back to the source and can become live with voltage.
With a dedicated grounding conductor if the neutral is compromised then the system will simply not work properly and will still be safe as long as the ground is intact.

The existing installation comment was that some older installations allowed the use of a 3-wire feeder where the neutral served as both the neutral and the ground. This has not been allowed for a few years now.

 

[Arrow3030]

Another reason is that the grounding conductor has the potential to have multiple paths back to the source. This is because all normally non current carrying conductors that have potential to be energized are to be bonded. If some were to be bonded somewhere other than the main there could be "objectionable current" in the system. For example if a water heater were plugged into a system like this, all the plumbing in the house could be a return path for current. Remember, electricity doesn't just take the path of least resistance, it takes all paths available to it.