I come across quite a few subpanels fed via SEU 3 wire here. I had assumed (key word?) that many older installations that included 2-wire nm followed suit with such a sub-panel feeder. (in my defense,i was not around...)
The issue comes up from time to time, as to these older subpanels being kosher, my generic answer so far is that if it met the codes at the time , yes.
A little digging here has me wondering about this.....
art 250 , "Location of Grounding Connections"
Secondary alternating curent circuits which are grounded shall have a connection to a groundingelectrode at each individual service, except as provided in section 2521.
The connection shall be made on the supply sideof the service disconnecting means.
Each secondary distribution system which is grounded shall have at least one additional connection to a grounding electrode at the transformer or elsewhere. No connection to a grounding electrode shall be made to a grounded circuit conductor on the load side of the service disconnecting means, except as provided for in section 2524.
Sparky: Looks like no one wants to touch this topic. There are "quite a few" 3 wire sub-panels floating around here in NJ, and we are a stste that has had inspections for a long time.
I can't find any documentation to either support or refute the situation.
In 21 years I know that any sub-panel that we installed was 4 wire for single phase, and five wire for 3 phase/4 wire. It was always "hot/hot/neutral/ground" or"h/h/h/n/g"
Another "old-timer" favorite used to be the "bare" neutral for the service. Never was fond of that either. Kind of goes with the "undersized neutral" school of thought. I have a tough time trying to figure the economics of having 3-500 MCM feeders, and a 250 MCM neutral. Yes, it's great if the majority of the connected load is 3 phase stuff, but why in an office bldg., or a resi complex?? Seems to me that it's more of a pain in the axx to have two sizes of wire. But, that's my personal humble opinion.
Hey, if you read this soon...Happy New Year, and be safe. John
Re: Were 3 wire subpanels ever acceptable??#82948 12/28/0209:26 AM12/28/0209:26 AM
I've seen a bunch of these installations. Most of the time, there aren't any branch circuits with an equipment ground involved being fed from thee panels. These installations are usually 40 or 50 years old & involve branch circuits that are wired in NM cable with no equipment ground.
There is a potential hazard if someone installs a branch circuit that does use an equipment ground, but only if the neutral that feeds this panel gets loose or opens up completely.
I usually recommend that the feeder and/or panel be brought into compliance.
Few things are harder to put up with than the annoyance of a good example.
Re: Were 3 wire subpanels ever acceptable??#82949 12/28/0210:07 AM12/28/0210:07 AM
Three wiring a subpanel to a separate building is code compliant. The separate building can not have any metalic connection to the first building (phone, TV, water, etc.). Also, this second building panel needs its own set of ground rods, along with all equipment grounds, bonded to the neutal. There would be no parallel path since the fourth wire wasn't run. A recommended method of wiring this subpanel is to use a fourth wire, add ground rods, and leave the neutral-to-ground bond unattached. Here again, there would be no parallel path and future changes (that would add metalic paths) are not going to make the electrical installation illegal.
If the subpanel is in the same building then there is no choice. You must use a four wire system and leave out the bond. Additional ground rods are not needed at a subpanel if it is in the same building as the main.
Allot of electricians seem to have a hard time grasping the concept of creating a parallel path. Its actually extremely simple. Think of it more as creating a loop than separate paths. This loop is made up of neutrals (grounded), grounds (grounding), and bonds (grounded to grounding connection). If you can find a complete loop between two points (the main and subpanel in this case) then you have a "parallel path".
Re: Were 3 wire subpanels ever acceptable??#82950 12/28/0212:44 PM12/28/0212:44 PM