Electrical Sub-Panel question

   / Electrical Sub-Panel question #1  

Shimon

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I'm installing a 100A main lug sub panel in a shop and I was planning on running three 2 awg wires (2 hots, 1 neutral) through 1.5" conduit about 35' from a 100A breaker on the main panel.

I just want to make sure I'm grounding this correctly. I'm going to run 4 awg copper from the sub panel to a separate grounding rod. And the ground and neutral buses in the sub panel will not be connected together. So basically, there is no ground wire connection between the main and sub panels since the sub panel will have it's own grounding rod.

Does this sound right?
 
   / Electrical Sub-Panel question #2  
You are only allowed to run a separate ground if it is a separate structure that you are wiring.

A structure can only have a single ground point. It is done this way to avoid having a difference in potential (i.e. a voltage) between the two ground lines. Because of diferences in soil types - no two rods bond to the earth in the same way. Permitting multiple grounds could cause everything from small electric shocks to ground loops (which can really screw up electronic gear).

Joe
 
   / Electrical Sub-Panel question
  • Thread Starter
#3  
Thanks.

The sub-panel is going to be in a shop which is a separate structure from the house (where the main panel is). The buildings are fairly close (about 15' apart) but I figured a separate rod would be better than running a ground wire through the conduit.

I just want to be sure that the ground and neutral are not supposed to be connected in the sub-panel (I know they are in the main panel...but I understand they shouldn't be in the sub-panel).
 
   / Electrical Sub-Panel question #5  
Unless you want to dig the service back up if you ever sell, you might want to read this...

WAC 296-46B 250

There is lots of information if you look for the WAC's on the web. Your 1½" conduit will handle the number of conductors you listed w/ a ground.
 
   / Electrical Sub-Panel question #6  
I had the sub panel put in my shop which is 75' from the house. The main house panel is grounded, sub panel is not. The sub panel is bonded to the main panel.
 
   / Electrical Sub-Panel question #7  
Ditto for my 40' run from main in house to sub in detached garage. Building to building distance less an issue than length of run.
 
   / Electrical Sub-Panel question #8  
Shimon,
I have refrained from replying to electrical posts recently unless I see something that is dangerous.
Let me see if I have this correct. You are going to run 3 wires to the separate building, no ground. Then you are going to run a #4 wire to a ground rod and connect it to a ground bar/buss. And the neutral is not connected in any way to this ground? Is this what you are going to do?

If so, you are using that ground rod and the earth as your sole means of ground. Sure glad I'm not the one using any electric in that building.

If you only run 3 wires to a separate building, the neural really needs to be connected to that ground rod. If you run 4 wires a ground rod should be installed and connected to the 4th wire and then no connection to the neutral.
 
   / Electrical Sub-Panel question
  • Thread Starter
#9  
Thanks for the replies.

I guess I misunderstood the grounding issue. I thought I read that if you have a separate structure it needs its own ground rod and I also read that a sub-panel should not have a neutral to ground connection but I think I'm confusing two issues.

I would rather just run a ground wire in the conduit and not have to worry about driving a grounding rod and leave the ground and neutral bus bars separate in the sub panel.

Thanks for assistance.
 
   / Electrical Sub-Panel question #10  
Shimon,
You still need to run a ground rod. It is tied only to the ground wire coming from the other structure and the gound lug on the breaker box (i.e. it is not bonded to neutral).

Driving a ground rod is easy. All it takes is a 10 lb mallet. Make sure that you put the wire clamp on the rod before you start driving. The reason is that the mallet will flatten the top of the copper rod - making the installation of the clamp difficult to do "after the fact" (you don't need to connect the wire until you are done driving the rod home)

Joe
 
 
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