New sub panel grounding to original panel


Old 06-18-07, 10:46 PM
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Join Date: Jun 2007
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New sub panel grounding to original panel

To accomodate three new 15 and two new 20 amp circuits from a kitchen remodel, a SD Homeline 6 breaker lug box was added next to the main Wadsworth 100 amp panel. The sub is powered by a 2 pole 40 amp breaker through 8ga stranded. The sub panel has a grounding bar to accomodate the green ground wires from a couple of 12ga aluminum armor cable (aka BX) circuits coming in. The netural buss is not bonded to the box in the sub panel.

The main Wadsworth does not have a grounding buss bar per se in it. If it did I would connect the 8ga from the sub panel grounding buss bar to it and be done. (Right?) The Wadsworth's netural buss does have what looks like a bare 10ga solid copper wire connected to it which goes out a conduit and connects to a water pipe via a copper strap and warning tag.

My question is where should I attach the green 8ga wire coming from the grounding buss bar in the sub panel? The enclosure of the main box? To a known ground like a water pipe?
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Old 06-18-07, 11:39 PM
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you connect it to the neutral/ground buss in the main panel
Old 06-19-07, 05:27 AM
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At the main panel the neutral and ground buss are one in the same. Connect the neutral and the ground for the sub panel to this common buss.

At the sub panel keep the neutral and ground separate, using two different buss bars. The ground buss bar needs to be bonded to the metal sub panel enclosure and the neutral must not be.
Old 06-19-07, 01:02 PM
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Thanks for your replies but I can get a little more help?

Can you help me understand the use of grounding terminals on receptacles and switches in light of this grounding and netural buss being the same in the main panel? I mean, why have a separate green ground wire going from a receptalce to the metal outlet box (in a system of conduit and metal boxes) or to a green wire in the run if it all joins to the netural at the main anyway? Is the grounding terminal's use then designed for use with PVC pipe, non-metallic cables and plastic boxes, where you don't have connectivity through what is housing the wire back to the main?

In my case the bare grounding wire from the water source contacts both the main enclosure through the conduit connection and then its attachment to the netural buss bar. The sub panel is mechanically and metallically attached to the main so the sub's enclosure is part of the main's ground and netural. As soon as the sub panel's netural 8ga is attached to the main's netural buss bar, doesn't that ground the sub panel's netural as well? I'll freely admit that I am not an electrician so can someone help me get my head around this?
Many thanks.
Old 06-19-07, 01:42 PM
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Grounding is a very complicated topic. In fact, there are entire books written on the subject. In residential wiring, it boils down to two distinct types of grounding.

The first is the grounding electrode system. This includes the wire from the water service to the main panel and any copper rods driven outdoors. This system provides some lightning protection and defines a reference voltage of zero volts relative to the charge of the earth. It also provides a bond to the plumbing so that if a hot wire touches a pipe somewhere in the house, the power can flow back to the main panel and trip the breaker instead of electrocuting someone in the shower.

The second grounding system is the equipment grounding system which consists of the green ground wires to receptacles and switches, metal conduits and boxes, etc. The purpose of the equipment grounding system is to provide a redundant backup path for the electricity to take back to the main panel if the neutral or hot conductors in the circuit become broken. For example, if a stove has a malfunction which causes the metal frame to become energized; a properly grounded circuit will allow the power to flow back to the main panel via the green wire or metal conduit and trip the breaker rather than leave the stove frame energized to shock the next person who touches it.

It is important that the ground and neutral be bonded only in one location (the main panel), so that the redundant return paths are maintained throughout the entire building's electrical system. You're correct in observing that this is a bit of a technicality when the main panel and subpanel are directly next to each other and fastened together with steel conduit; however the code defines the rules for subpanels the same whether they are 6 inches or 600 feet apart.

Even though the neutral and ground are electrically connected in the panel, their functions are different. The neutral wire is supposed to have current flowing during normal operations. The ground conductor is only supposed to carry current during fault conditions (neutral or hot break). To ensure that no current flows in the ground conductor during normal operation, the neutral and ground can be connected in only one place. If they were connected in a subpanel, half of the current between the main panel and sub would flow on the ground and half would flow on the neutral.
Old 06-19-07, 03:32 PM
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Join Date: Jun 2007
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Thank you very much ibpooks for taking the time to provide me with that detailed explaination. I now have a much better understanding of this issue.

Thanks to all for their help.

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