3 wire feeds to sub panels

[Robert Sole]

Does anyone know when 4 wire conductors were required for the feed between the service panel and the sub panel. I inspected a home today that was built in 1974. It has one service panel with a 225 AMP breaker and 3 sub panels fed from the main with a 100 AMP, a 125 AMP and a 150 AMP breaker. All three panels are fed with only 3 conductors so the neutrals and grounds are connected to the same buss in the panels.

I know this is not permitted now but was it then?

Appreciate any help.

[James E. Braun, CMI]

I have never seen a 4 wire single phase panel feed. When did they come out with that?

[Robert Sole]

That is what I am not sure about. Now the neutrals and grounds are only permitted to be connected together at the service panel. Otherwise you run the risk of the panel being energized. That is why you should not see a bonding screw installed in a distribution panel.

[Jeffrey R. Pope]

I believe it was '96 when a four-wire feed became the norm, but don't quote me on that. However, the requirement for isolating the neutral has always been there. You cannot ground the neutral past the service equipment - period (some exceptions do apply ).

[Robert Sole]

Thanks Jeffrey but how do you isolate the neutrals and grounds without the 4th conductor?

[Jeffrey R. Pope]

The three conductors should be H+H+N, with the conduit used as the EGC or ground. If the panel is wired with two hots (120V ea) and a ground, it's wrong. The third conductor should be insulated (not bare) and land on an isolated bus in the downstream panels (i.e sub panels).

All egc's should simply terminate in the downstream panels. If the sub panels are not bonded to the service equipment by metallic conduit (essentially the fourth conductor), then the three-wire feed is wrong.

[Frank P. Newman]

Questions that would be related to the original post:

1. If the subpanel is non-metallic (a few are) does that change anything?

2. If an older subpanel is in a separate building, should a separate ground be driven for that panel?

[Jeffrey R. Pope]

1) The only change is in regards to the equipment-grounds from circuits supplied by the sub panel. There needs to be a means to ground them at the sub panel without using the neutral. This can be done by adding a bus bar that is bonded to the metallic (assuming) conduit supplying the sub panel.

2) Older or not, separate buildings require their own GES.

[Frank P. Newman]

1) understood, but in this part of the country if there is any conduit at all it will rarely be metallic.

2) If the sub panel in a separate building is fed by a 4 wire circuit, no additional GES would be required (other than via the ground conductor feeding the panel)?

[Jeffrey R. Pope]

If the conduit does not serve as a means to bond the equipment, a four-wire feed would be necessary.

Regardless of the feed (3-wire or 4-wire), a separate building requires its own grounding electrode system (GES) via ground-rod, water pipe, ufer, etc.

[Robert Sole]

Thanks for the feed back. In most cases around here, the distribution panel is quite a distance from the service panel and is fed only with the wire (no conduit). You conformed what I was thinking, that it needed the 4th wire to provide a return for the ground to the service panel.