Replies

1. 
   CPopejoy | Apr 21, 2002 10:27pm | #1

   Dave,

   It depends.  If the subpanel is in a separate building from the main panel, and there are no metallic paths between the two buildings (metal water pipe, telephone or any other wire), then you can treat the subpanel like a service in terms of grounding (install ground rods for subpanel, tie neutral and ground bars together at subpanel, etc).  In this case, you don't run an equipment grounding conductor (ground wire) from the main to subpanel; the earth (zero) reference potential at the subpanel is created by the grounding electrodes (rods or other) at the subpanel location.

   However, I strongly recommend an alternative approach--running a ground wire be run from the main panel (service) to the subpanel along with the hots and the neutral.   This ground is connected to grounding electrodes at the subpanel.  The neutral bar at the subpanel is not tied to the ground bar (this is called a "floating" neutral).  This approach is safer, because any ground fault current will return to the service (and to the utility neutral) via the grounding conductor.  Without a grounding conductor, there's the possibility that leakage current will flow through the ground.  This is not uncommon.  Also, if any low-voltage wire is run between the buildings in the future, it will not become the return path for ground fault current.

   BTW, you need the grounding electrodes at the subpanel building because if ther's a lightning strike (or high-voltage powerline cross), you want that energy to flow to ground through the shortest path possible--at the subpanel.

   IF the subpanel is in the same building, you do not need a ground rod, and you must run a 4-wire feeder (2 hots, neutral, and ground) from main to sub.

   Cliff 

1. 
   davebogaty | Apr 22, 2002 01:38am | #2

   Cliff, I appreciate the info. The subpanel that I an planning on installing is adjacent to the main panel. This may be over kill, but is it within code to install the four connectors to the main panel (two power, neutral and ground) and also install a copper ground at the subpanel. This would provide a ground from the main panel (present condition) and an additional ground from the subpanel.

1. 
   CPopejoy | Apr 22, 2002 03:27am | #3

   Dave,

   If you want to install another ground rod, terminate it at the ground bar on the main panel.  Then run your 4 conductors from main to subpanel as you intend, and split the ground and neutral buses at the subpanel.

   Of course, your proposal is functionally equivalent (tie the new ground rod to the subpanel ground bar and run a grounding conductor back to the ground/neutral bar in the main).  The difference between the two approaches?  In your approach, if there's ever any increase in impedance (AC resistance) in the path between the main panel grounding electrode and the subpanel ground rod, well then, you have a problem.  Any current leakage to ground (ground fault) in the subpanel will try to get back to the main panel via the earth between the two grounding electrodes.  This may impose a voltage on drainpipes, water pipes, and other metallic things in the ground.  You get a chance of shock/electrocution.

   There are a lot of causes for increase in resistance in that path.  Corroded terminations, broken or damaged conductor, work in the panel by someone who doesn't know that you did something that's against Code...

   So I suggest you play it straight and do it right.  Bring all the groundng electrode conductors (old and new ground rod wires) to the main panel ground bar.

   Cliff

1. 
   davebogaty | Apr 22, 2002 05:14am | #4

   Thanks again, what I was trying to say (which doesn't always come out in the typing) is to do what you said - that is take the ground back to the main panel, but also have an additional ground terminating outside on at a grounding rod. That is having a ground from the main panel and one from the sub terminating outside on grounding rods. That is a redundant grounding system.

   When I built the house 4 wire 220v was not a requirement, but I installed them due to the added safety.

   I know that it is excessive but when I look at the typical attachment of a ground to copper piping I wonder how adequate an approach this is. My water line extends probably 12" outside the house before it transition to pvc.

   Cliff I really appreciate your insight.  

1. 
   davebogaty | Apr 22, 2002 05:19am | #5

   Cliff, I re-read your comment and saw the error of my way. But going back to my comment on the connection to the water line there is no problem with providing two grounds off the main panel is there? Other that foolishness.

2. 
   CPopejoy | Apr 22, 2002 07:21am | #6

   Dave,

   It's a good idea to provide two grounds.  In fact, the Code says that you have to do so, unless you can show that the resistance of a single ground rod is less than 25 ohms.  Because testing to demonstrate this is a PITA, it's normal to drive two rods and run the grounding electrode conductor in series from rod 1 to rod 2 to the main panel ground/neutral bar. 

   The rods have to be at least 6 feet apart (per Code) and most building depts like to have them at least 2 feet away from the footing/foundation.

   So your gut feeling that two is a good idea is indeed well-founded!

   Cliff