Can I use Main Lug Load Center box as a subpanel box?

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Old 11-21-05, 04:23 AM
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Can I use Main Lug Load Center box as a subpanel box?

I bought a Murray "Main Lug Load Center" panel box to replace my old main panel box with, but I've decided to try to use it as a subpanel box instead. I'm adding some circuits and it'll be a lot easier right now to put a 2 pole 30amp breaker in the main panel box and run it to the subpanel. I see one problem with doing this using this Murray load center box, there is no separate grounding bus bar in it.

One question I have is can I just buy a grounding bus bar and fasten it inside the Murray box? Or is this the wrong way to do this?

Another question has to do with the grounding itself: my system has a disconnect box below the meter, and disconnect is connected to the grounding rods(2 of them, 6 feet apart) by a pipe and by a wire that comes out of this pipe and goes to a terminal in the disconnect box. Where this pipe enters the disconnect box there is also a screw terminal that connects a thick wire to the other wall of the disconnect box, right next to the conduit leading to the old main panel box. The old main panel box is grounded to the disconnect box only via this offset conduit connection, there is no ground wire going from the disconnect box to the old main panel box. I'd like to ask for some advice on this...what's the best way to provide another ground to the old main panel box so I'll have a ground source to connect to the new subpanel box. The old main panel box doesn't have a ground lug anywhere that I can see, and it doesn't have a grounding bus bar either. I'm wondering if I can add a ground terminal or bar to the old main panel box and
run it to a ground clamp on the outside of the pipe leading to the 2 grounding rods?

Sorry for all the words, it's hard to describe this situation. I hope I made sense, and I'd welcome any advice about this.

BTW, you can probably guess that this is an old house(a small one) and the wiring is 2 wire without grounds.

John
 
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Old 11-21-05, 08:03 AM
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One question I have is can I just buy a grounding bus bar and fasten it inside the Murray box? Or is this the wrong way to do this?
Yes, you will need to purchase a ground bar for the murray panel. Make sure it is a bar for that panel which will align with the pre-drilled swaged holes you will see in the back of the can, usually there are a couple of paired hole locations. You will attach it to the panel via the factory screws that come with the ground bar. Do not buy any ole ground bar and drill your own holes.

Another question has to do with the grounding itself: my system has a disconnect box below the meter, and disconnect is connected to the grounding rods(2 of them, 6 feet apart) by a pipe and by a wire that comes out of this pipe and goes to a terminal in the disconnect box. Where this pipe enters the disconnect box there is also a screw terminal that connects a thick wire to the other wall of the disconnect box, right next to the conduit leading to the old main panel box. The old main panel box is grounded to the disconnect box only via this offset conduit connection, there is no ground wire going from the disconnect box to the old main panel box.
This appears fine the conduit (if metal) is your ground path between the panel and service disconnect. If you have metal water pipe... ie copper ... and it is in contact with the ground for at least 10 ft. after leaving the house you should also have a grounding electrode conductor running to this water pipe. This would be your main grounding electrode and the rods are supplemental. Just depends on your situation.


I'd like to ask for some advice on this...what's the best way to provide another ground to the old main panel box so I'll have a ground source to connect to the new subpanel box. The old main panel box doesn't have a ground lug anywhere that I can see, and it doesn't have a grounding bus bar either. I'm wondering if I can add a ground terminal or bar to the old main panel box and
run it to a ground clamp on the outside of the pipe leading to the 2 grounding rods?
You dont need a ground lug in the main panel to ground the sub. You only have the ground and neutral bonded at one place and that is the main panel. Since you dont have wiring with ground and you only have the one bar in the main panel this is also your ground bar since both the grounding electode system and the neutral (grounded conductor) are connected to it.
You will simply run a #10 awg copper four wire feeder to the sub from your 30 amp breaker in the main. Connect the ground wire to the added ground bar you installed in the sub, at the main panel just connect the ground wire to the neutral/ground bar that already exists. Do not place the ground wire under a screw with a neutral wire. This ground wire between the main panel and sub is actually your equipment ground and not a grounding electrode conductor. Fault currents are intended to flow on this equipment ground wire back to the main panel where it is bonded to the grounded conductor.The fault current then returns via the neutral to the serving transformer. It does not go to ground via the grounding electrode system as many people think. If it flowed to ground it may not allow enough current flow in a fault condition to trip the breaker.

In the sub the neutral and ground are seperated. The ground bar is in solid contact with the can and the neutral is on insulated standoffs. You do not want the two bonded together in the sub. If you have a green bonding screw it will not be used... throw it away.
 

Last edited by Roger; 11-21-05 at 08:19 AM.
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