DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Electrical and Wiring > Neutral and Ground on a Sub-Panel




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Old 06-23-2009, 07:47 PM  
wblandry
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Default Neutral and Ground on a Sub-Panel

I've read a couple of threads about neutral and ground in a sub-panel and understand that a sub-panel should have the ground "floating", that is seperate from the neutral. It seems that a main panel, on the other hand, will have both neutral and ground terminating on the same buss bar.

The existing installation I'm looking at has the neutral and ground together on a buss bar even though it is technically a sub-panel. I think I know what to do to seperate them but am not in the same city as the box right now and will not be able to change it right away.

My question is, does this present a real and present danger to people who might use these curcuits? Can the fix wait until I return to the site? (Although I'm far away, the circuits are in use by others.)



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Old 06-24-2009, 12:07 AM  
Nestor_Kelebay
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I'm not an electrician, but my understanding is that the electrical code requires the neutral and ground wires to be separate in all sub-panels. The neutral wire is only connected to the grounding rod in the main panel.

I understand the reason for this is that if there were ever a short circuit and the white wire were to carry full current, then connecting the white neutral wire to the ground wire in any of the sub-panels could cause a shock to anyone working in the main panel that might come into contact with a bare ground wire.

I'm not an electrician, but hopefully an electrician will be able to provide further comment. My understanding is that this isn't an urgent problem, but it's something that needs to be corrected.



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Old 06-24-2009, 05:52 PM  
wblandry
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Nestor,

Thanks for your reply ... I see your point about danger to anyone working in the main panel. Hopefully, as you suggested, a professional electrician will confirm our understanding.

Bill

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Old 06-24-2009, 10:54 PM  
kok328
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what you have read about sub-panel floating grounds is correct but, does not pose an immediate danger. If you think about it, the neutral and ground in the sub-panel connects to the non-floating ground/neutral bar from the main panel.
My only concern is your wording "technically a sub-panel", what are you looking at that leads you to believe this a sub-panel (not that I doubt you, I just want to be sure that you are correct).

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Old 06-25-2009, 06:45 AM  
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Having the neutral bonded to ground in a subpanel allows the current to also pass through the ground conductor onto the main panel. The ground conductor (bare or green wire in some instances) is not intended to carry normal system flow -- but only to shunt off any potential that will pass current to a person (or to another metallic surface or a flammable object) derived from a defective device, appliance, etc. For example, a metal-cased electric drill without the three prong plug presents the obvious shock hazard. Electric drills made solely of plastic usually are touted as "double insulated" and a three-prong plug is not necessary.

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Old 06-26-2009, 10:14 PM  
wblandry
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The "technically a sub-panel" comment was refering to the fact that this is the primary panel on a houseboat (I didn't want to say main so I made up this word "primary").

It is actually fed by a heavy marine cord from a plug on the dock ... which is itself fed by a "main panel" that serves all boats at the dock (to my understanding this is really the "main panel").

Each houseboat, of course, has it's own primary panel which is fed by a hot, a neutral and a ground that originates at that true "main panel".

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Old 06-26-2009, 11:48 PM  
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Your houseboat situation is exactly analagous to the wiring in an apartment block. In my electrical room, I have a main panel that I've never seen the inside of because it's designed so that you can't even open the panel without shutting off all of the power to the entire building. (Opening the panel door requires that the big lever on the side of the panel be pulled down, and pulling it down shuts off all the power to the building.) Cables from that main panel then go through a bank of 24 meters (one for each apartment and three meters for the common areas (hallway lights and plugs, parking lot and laundry rooms) to each of the apartments in the building. Each apartment has it's own electrical panel (technically "sub-panel") where there are a pair of 50 amp fuses for the stove, and four to six 15 amp fuses for the various residential circuits in the apartment.

According to my math, that's gotta be at least a 2500 amp main panel in my electrical room. 21 stoves at 50 amps per stove = 1050 amps. At least four 15 amp circuits per suite = 60 amps per suite X 21 suites = 1260 amps. Seven 15 amp circuits to the 14 car parking lot = 105 amps. Three electric dryers at 30 amps per dryer = 90 amps.
1050 + 1260 + 105 + 90 = 2505 amps

That main panel can give a pretty good shock if you play touchie feelie with the wrong thing inside it.



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