Understanding isolated (unbonded) neutrals and grounds in panels/subpanels

[HMonk09]

I have a question regarding the isolation of grounds and neutrals in a subpanel. First, please bear with me as I describe what I see in the panels associated with my home.

I live in a mobile home. My meter feeds directly into a 200-amp cutoff panel. Within that panel are two, 100-amp breakers. One 100-amp breaker feeds the 100-amp house panel (connects to a 100-amp breaker in the house panel). The other 100-amp breaker feeds another panel (subpanel), also on the pole, to a 100-amp breaker. The subpanel contains two, 20-amp DPDT breakers, one to 240V. greenhouse heaters, the other to a 240-volt circuit in a workshop (two separate buildings).

The main cutoff panel has one neutral bar, no ground bar, a buried ground rod/RMC, and an RMC to the house. The meter neutral wire and the ground wire are connected to the one neutral bar. There is also a 20-amp receptacle breaker in the cutoff panel whose neutral and ground wire also connect to the neutral bar.

At the house panel, (a 1988 ITE panel) the cables from the cutoff panel connect to a 100-amp breaker, the neutral bar, and an isolated (not bonded) ground bar. The neutral and ground wires are wired to their respective bars. It seems to me that the house panel is wired as a subpanel. The first question I have is, can we make any assumptions about the ground wire if everything was done according to code at the time? Would the ground wire attached to the neutral bar in the cutoff panel be attached to a dedicated ground rod (or the RMC), and the ground wire in the house panel be attached to its own dedicated ground rod?

The subpanel to the outbuildings is connected to the cutoff panel by an RMC. From the 100-amp breaker in the cutoff panel, the hot wires connect to the subpanel bus, the neutral wire to the neutral bar, and ground wire (which is attached to the neutral bar in the cutoff panel) to an unbonded ground bar that has a dedicated ground rod. The heater and workshop grounds are connected to the ground bar. Nothing is connected to the neutral bar except the cable from the cutoff panel neutral bar.

First off, does this sound as if it is properly wired? I know that the NEC states that, in a subpanel, neutrals and ground wires should be isolated. But as I described, are they really? Because the neutral wire and ground wire share the same neutral bar in the cutoff panel and, because the ground wire in the subpanel is connected to that same neutral bar in the cutoff panel, the ground wires in the subpanel are indirectly (through the ground cable) connected to the neutral bar in the cutoff panel. In other words, how can the ground wires be considered to be isolated from the neutrals?

Secondly, can we assume there are three grounds: one rod to the subpanel, one rod/RMC to the cutoff panel, and one rod/RMC to the house panel? BUT, if the cutoff panel has a dedicated ground (rod/RMC), why does the sub panel need its own dedicated ground rod if its ground bar is already grounded via the cable from it to the cutoff neutral bar - that has its own ground wire attached?

My web searches have not discussed my scenario and, therefore, I would appreciate it if someone could enlighten me, in general, regarding isolating grounds from neutrals (when they seem to be indirectly bonded) and, specifically, the appropriateness of my panels as wired.

Thanks, Monk

 

[Lucien Nunes]

Neutral and ground are always going to be connected, somewhere, somehow. In fact one of the things that makes a neutral a neutral, is that it is grounded at the source. The answer to your question lies in understanding the significance of where in the system the two should be bonded together and where not.

Power company line plant often uses a combined neutral and ground conductor. At the main panel near the meter, under the NEC it is conventional to bond neutral and ground and they can be then considered to be at the same electrical potential. Your ground rods help maintain that potential equal to that of the surrounding terrain, both towards your wiring system and back towards the power source.

Moving on from the main panel to a sub-panel within an installation, for both safety and functionality reasons it is problematic to combine the functions of neutral and ground in one wire. An obvious example is that if a connection is broken, the current returning along the neutral will make all grounded appliances live downstream of the break. Therefore separate neutral and ground wires are used, respectively serving the neutral and ground bars of the sub-panel which must not be connected together. Both are connected to the grounded supply neutral and a low resistance will exist between all neutral bars and all ground bars. But the normal current flow will be confined to the neutrals, and the safety equipotential will be maintained by the grounds, with no interference between the two functions.