Sub Panel Ground Question

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  #1  
Old 11-18-05, 07:06 AM
DaGbyte
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Sub Panel Ground Question

Buying a new(er) home. The previous owner obviously had the sub panel professionally done, but the building inspector pointed out a problem. The sub panel is about 25 feet away from the main. The neutrals and ground are not seperate. They are on the same bus. Now, he said that as a rule, they need to be seperate. However, he said there are mixed opinions on this. Can an electrician please shed some light on the issue? Is it more a rule, or code?

Also, in order to get this fixed, how long/much would a good guestimate be? Can't take more than a hour or 2...

-DaG
 
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  #2  
Old 11-18-05, 07:23 AM
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I will let the real experts comment on the code. I do believe that it is a code requirement.

The reasoning is this: There is ONE point in your house which is considered to be the central earth ground, and this is the grounding rod, connected to main panel. All the groundING conductors in the house return current to this point in a fault situation. Now, at the main panel, the NEUTRAL bus, where all the current-carrying neutral conductors from the system are connected, this bus is BONDED or electrically connected to the GROUND. Notice that in this setup, all system current is flowing in the neutral wires, but none is flowing in the grounding wires. SO, every GROUNDING connection throughout the system is at zero volts with respect to any outher ground such as a water pipe.

NOW, if out in a sub-panel you were to bond the neutral and ground busses, then all the current flowing in that subpanel system has to return all the way to the main, and it has TWO parallel paths: the neutral wire and the ground wire. It would divide more or less equally. Due to basic amps X volts, the grounding bus now is no longer at zero volts with respect to true earth ground, creating a potentially unsafe condition.



Sorry, this was a long winded way of saying that bonding the ground and neutral would allow current to flow in the ground wire, and this is bad!
 
  #3  
Old 11-18-05, 07:30 AM
DaGbyte
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Forgot to mention, the sub panel is grounded to a water pipe. Does that matter? I would imagine that the trip back to the main would be an issue tho...
 
  #4  
Old 11-18-05, 08:01 AM
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Is the sub panel in the same building as the main panel, or in a separate building?

If the sub panel is in the same building then the ground and neutral MUST be separated (by code) and the ground is not grounded to anything except the main panel by means of the connecting wire.

The reason for this separation is for the reason 594tough pointed out, to keep the neutral current off the ground wire.

If the sub panel is in a separate building then the need to separate the ground and neutral depends on whether the sub panel is wired with a separate ground or not. If wired with three wires (no ground) then they get connected together. This method is less preferred and has other requirements which must be met. If wired with four wires (which includes a ground) then the ground and neutral get separated at the sub panel. A sub panel in a separate building must always be grounded, usually by means of one or more ground rods.

It does sound like there is a problem with this installation. However, keep in mind that some home inspectors are not up to date on electrical codes and make mistakes in that area.

If I were you I would require that the issues be fixed (when you have properly identified them) and that the panel be inspected by an electrical inspector prior to you closing on the house.
 
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