3 Wire Subpanel Feed

[CNewhouse]

The subpanel at today's inspection was in the same building as the service panel. The subpanel had a 3 wire feed and bonded neutrals. This has never been allowed, correct? If I'm understanding this correctly, this could only be done pre-2008 when the subpanel was in a detached building and had its own grounding electrode? What is the concern, functionally? Would there be an issue clearing faults? I understand the implications of bonding neutrals downstream of the service equipment in general, but get fuzzy as to why it was accepted in detached structures under certain circumstances (but not in the same structure). 

Edited February 26, 2023 by CNewhouse

[Marc]

9 hours ago, CNewhouse said:

The subpanel at today's inspection was in the same building as the service panel. The subpanel had a 3 wire feed and bonded neutrals. This has never been allowed, correct? If I'm understanding this correctly, this could only be done pre-2008 when the subpanel was in a detached building and had its own grounding electrode? What is the concern, functionally? Would there be an issue clearing faults? I understand the implications of bonding neutrals downstream of the service equipment in general, but get fuzzy as to why it was accepted in detached structures under certain circumstances (but not in the same structure). 

One of the concerns in this panel is that neutrals and ground wires are joined together. Bonding the EGC system at more than one point places some ground wires in a parallel circuit with a neutral, resulting in neutral circuits in ground wires. Ground wires are smaller. They weren't intended to carry neutral circuits.

[CNewhouse]

Thank Marc. And yes, that is the portion that I do understand. There is also the concern of bonded systems/materials becoming energized, if I understand correctly. I am confused as to why it was once allowed in detached buildings though. I assume having a dedicated grounding electrode is the difference? And that a large gauge GEC will clear faults via that electrode and prevent issues?

[Jim Katen]

I suspect the reason is that bonded sub-panels in the same building are more likely to cause problems with parallel paths. Most buildings - and particularly dwellings - are full of conductive paths. No so much with separate buildings. 

That might be why the rule morphed from "allowed in separate buildings" to "allowed in separate buildings only when there are no continuous metallic pathways between the two buildings," to "not allowed in separate buildings." 

[CNewhouse]

That makes sense. Thanks, Jim.

[Marc]

6 hours ago, CNewhouse said:

Thank Marc. And yes, that is the portion that I do understand. There is also the concern of bonded systems/materials becoming energized, if I understand correctly. I am confused as to why it was once allowed in detached buildings though. I assume having a dedicated grounding electrode is the difference? And that a large gauge GEC will clear faults via that electrode and prevent issues?

Electric currents flow in the earth, creating gradients in electrical potentials as they go.  The scenario you described is based on allowing separate equa-potential regions to exists in close proximity. Later code cycles improved on that by creating a single, larger equa-potential region.

JMHO