can of worns - old panel now subpanel

[rob-1]

Hi all,
not an electrician but long history of electronics and trouble shooting with major "telephone co" b4 retiring.
Bought an old house to fix up and sell. the former main service entrance panel is now a sub-panel which is being fed by a new main about 50 ft away on the same structure.
The former main panel's (here to forth sub-panel) neutral bus bar is bonded directly to ground by a ground wire connecting the neutral buss bar to a cold water pipe coming out of the ground. In addition, the sub-panel itself is grounded by a ground wire coming from the main. This ground is connected directly to the panel.
FYI: H-H-N + ground feeding the sub-panel. Also, the sub-panel's feeder breakers are 100amp and so are the feeder breakers on the main panel.
Question:
From what I've read there is a problem here because i understand that the neutral and ground are NOT to be bonded together in a sub-panel. Lastly, there is a 30 amp double pole breaker in this sup-pan. Two hot wires on the breakers and a green wire connected to the neutral buss bar. I'm wondering if who ever ran this circuit was trying to get a ground off the neutral bar. If it had been a white wire to the neutral bar for this circuit I wouldn't have gave it a 2nd thought.
Well, I know this is a handful so my gratitude in advance to all "pros" who can chime in.
Oh, I know someone may say "get an electrician out there". That's fine, but I still want to understand this.
Thanks,
Rob-1

 

[russ]

Re: can of worns - old panel now subpanel

Your grounds have to terminate in your new panel with the new service main.
You also have to bond the panel to the neutral there.
You will have to remove the old grounds from the old neutral bar, ane remove the bond between the old neutral bar and the old panel.

As for the 30amp circuit is it a neutral or ground, you have to find out the circuits purpose.
If it's aground you have to remove it from the neutral bar, and install it directly to the panel to ground it.

Russ

[ March 06, 2003, 09:37 PM: Message edited by: russ ]

 

[bphgravity]

Re: can of worns - old panel now subpanel

You are exactly right. You have a whole bunch of parallel fault paths according to your explanation of the installations. What can happen is a ground fault in a conduit or equipment will flow back to the sub panel and then split between the grounded and grounding conductor, thus spliting the fault current to each. In some cases, this could restrict the OCD from tripping and opening the circuit. Not good. Since you have the four wires comming in from the main, all you need to do is separate all the whites from the greens, and insulate the neutral bus.

 

[don_resqcapt19]

Re: can of worns - old panel now subpanel

bp,

In some cases, this could restrict the OCD from tripping and opening the circuit.

How?
Don

 

[bphgravity]

Re: can of worns - old panel now subpanel

I guess the more important matter is that only fault current should flow on metallic parts and when grounded and grounding conductors are in parallel, normal operational current will flow on metallic parts like the conduit and the panelboard. As far as restricting breaker tripping, from Rob's explanation of how the grounding system is wired, some of the current will go to the water pipe and never make back to the source from high impedance leaving only half of the current to flow back to the source completing the circuit. There is a possibilty that the current being split reduces the flow back on the grounded conductor enough to either extend the length of time of trip a few cycles or not at all.

 

[rob-1]

Re: can of worns - old panel now subpanel

OK guys, thanks but got a follow up.
What about the grounded water pipe at the sub-pan. There is a grd wire from the water pipe to the neutral bar at this sub-pan. So when I install a ground buss bar & mount it to the panel (which gets its ground from the main feeder cables & which I will not bound to the neutral buss bar at the sub-pan) do I also remv the ground wire going from the water pipe to the neutral bar at this sub-pan? If i do, then I'll no longer have a bond btwn water pipes and ground. Remember, this sub-pan use to be the main serv. panel. Pretty sure that why the neut was going from neut buss bar to water pipe ground. Once I remv the water pipe to ground bond I don't think there be such a bond any longer.

 

[don_resqcapt19]

Re: can of worns - old panel now subpanel

bp,

... some of the current will go to the water pipe and never make back to the source from high impedance leaving only half of the current to flow back to the source completing the circuit. There is a possibilty that the current being split reduces the flow back on the grounded conductor enough to either extend the length of time of trip a few cycles or not at all.

Again, I ask, how?

I agree that thare are issues with parallel paths and normal circuit operation. You are correct that current will be flowing on paths where it shouldn'd be under normal operating condition. However, this will in no way impair the fault clearing in the event of a ground fault. If the current is flowing on any path, then that path must lead back to the source. If it doesn't lead back to the source, then current won't be flowing on it. These are parallel paths back to the source and will in no way reduce the flow of current back to the source. No matter how high the impedance of a path is, if it is placed in parallel with another path, the total impedance of the two paths will always be less than the lowest impedance of any single path.
Don

 

[bphgravity]

Re: can of worns - old panel now subpanel

Thanks Don, I think I am confusing the problem with high-impedance earth path when no neutral is connected. I see your point and appreciate your explanation.

 

[charlie b]

Re: can of worns - old panel now subpanel

Originally posted by rob-1:Do I also remove the ground wire going from the water pipe to the neutral bar at this sub-panel? … If I do, then I'll no longer have a bond between water pipes and ground.

That connection belongs at the main service. So does the connection between the neutral bus and ground bus. This panel used to be the service panel, and that is why these two connections were made there. But since it is now a sub-panel, you must remove both connections. While you are at it, you should verify that whoever installed the new main panel made these types of connections (i.e., first from neutral bus to ground bus, and secondly from this point to planet Earth) at the new main panel.

 

[hornetd]

Re: can of worns - old panel now subpanel

Originally posted by rob-1:
OK guys, thanks but got a follow up.
What about the grounded water pipe at the sub-pan. There is a grd wire from the water pipe to the neutral bar at this sub-pan. So when I install a ground buss bar & mount it to the panel (which gets its ground from the main feeder cables & which I will not bound to the neutral buss bar at the sub-pan) do I also remv the ground wire going from the water pipe to the neutral bar at this sub-pan? If i do, then I'll no longer have a bond between water pipes and ground. Remember, this sub-pan use to be the main serv. panel. Pretty sure that why the neut. was going from neut. buss bar to water pipe ground. Once I remove the water pipe to ground bond I don't think there be such a bond any longer.

You do have to remove the old Grounding Electrode Conductor (GEC) from the sub panel neutral buss bar. But there is nothing that prevents you from then connecting it to the sub panel Equipment Grounding Conductor (EGC) buss bar. That having been said you should check to make sure that the other end of that old EGC is connected to a metal underground water pipe within five feet of were that pipe enters the building. I say that because you are using it as a supplementary grounding electrode. If it is instead a bonding conductor to ground the interior metallic plumbing and the underground supply is non conductive or not electrically continuous to the point of attachment to the bonding conductor you can not terminate the bonding conductor in a sub panel in a house.
--
Tom

 

[rob-1]

Re: can of worns - old panel now subpanel

Thanks all. Guess what? I went over to the new main panelborad and saw that the neutral bus bar IS NOT bonded to the ground buss bar.
Question.
Since here is no bonding bar (btwn neutral bar and ground bus bar) existing in this panelboard, can I just run a bare ground wire btwn the neutral buss and ground bus bar. If so what size wire? I'm thinking a #10. Lastly, is it OK that the ground bus bar is mounted directly onto the panelboard (that is it is in direct metal 2 metal contact - not "floating" like the neutral bar). I know that this setup (grnd bus attached in direct contact to panelboard) is probably OK because now I also bonding to the panelboard, just that probably is not good enough. Thanks again all for all of your input and time.
Sempre Fi, God & country, apple pie.

 

[russ]

Re: can of worns - old panel now subpanel

250.102(C) same as the ground conductor run to the water pipe.
Which is based on the size of the conductors feeding your main breaker, Table 250.66.
If this is a 100 amp service, your service wire is probably #2 copper or smaller, = # 8 copper ground

Russ