Grounding electrode at sub-panel vs mobile home panel.

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peter52

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A mobile home panel doesn't require earth ground rod/ electrode because of the 4 wire feed 550.33 and exception.. What is the theory to have a earth ground rod/ electrode at a detached sub panel that is also 4 wired.
 
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peter52 said:
A mobile home panel doesn't require earth ground rod/ electrode because of the 4 wire feed 550.33 and exception.. What is the theory to have a earth ground rod/ electrode at a sub panel that is also 4 wired.
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Are you asking about a sub-panel installed in a detached building?

A typical sub-panel installed within the same building as the electrical service is not required to be connect to another grounding electrode at the sub-panel.

Chris

P.S. welcome to the forum.:)
 
The reconnection of an equipment grounding conductor (EGC) to earth through a ground rod happens at detatched buildings where the Main Bonding Jumper is not present.

This ground rod isn't going to do much. At 60 Hertz and around 25 Ohms, the amount of current that flows in it into earth, in a fault involving the EGC, will be very small, as the much lower resistance of the EGC will divide most of the current into the EGC.

But if there is a lightning strike, or some other high voltage surge, impedances shift, depending upon voltage and speed, and the ground can become the easy path for that current.
 
peter52 said:
A mobile home panel doesn't require earth ground rod/ electrode because of the 4 wire feed 550.33 and exception.. What is the theory to have a earth ground rod/ electrode at a sub panel that is also 4 wired.
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Consider 550.32
The service equipment for a manufactured home must be within 30 ft. and grounding electrodes are installed at the service. I can oly deduce that when a service is within this zone it is considerd part of that structure (manufactured home) go figure.

Now read the exception to 550.33 (A).

What all of this means is that when the service falls outside the 30 ft. zone you will be required to install a service disconnect at the manufactured home and you will be required to install grounding electrodes just like you would for a seprate structure.

For the theory part of your question, we install grounding electrodes at seprate structures to maintain the same potential throughout the property that the electric system serves.
 
I was reading the new 2008 code to 4 wire a sub in a detached building. Then I started thinking about mobile homes that have been wired that way for years. It seems the ground rod at the sub-panel is a feeble attempt to control lightening. Not worth the time or money.
 
peter52 said:
I just don't think the ground rod at the detached subpanel is worth the time or money.
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Lighting will strike a building with or without electricity, whether a 60 ampere subpanel or 1200 ampere feeder.
We can help you submit a coded proposal for the 11 cycle to delete a ground rod at a detached building, it would be interesting to see what the CMP replies.
 
This is the only part of the code that doesn't make sense to me. If I put in a 70 foot mobile home and the panel is at the other end of the mobile home the panel could be 100 ft from the disconnect. If I put a detached sub panel in a building 2 feet from my house I need a ground rod. I guess the grounding electrode at the detached sub panel could possibly help cause less(or more) damage to the house. If a mobile home took a hit it probably wouldn't survive. I wonder what Jeff Foxworthy has to say about this. It would be interesting to submit a change, but let me study it some more.
 
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