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RobertS 02-27-2008 09:18 AM

Mobile home electric.
 
Why don't they use a ground rod on a mobile home panel. The disconnect has a grounding electrode with a 4 wire feeder to the mobile home panel with out a ground rod/electrode. The neutral, and ground are isolated, and the ground bar is bonded to the panel box. Isn't it essentialy a sub-panel.

jrclen 02-27-2008 10:20 AM

Quote:

Originally Posted by Robert Shields (Post 102294)
Why don't they use a ground rod on a mobile home panel. The disconnect has a grounding electrode with a 4 wire feeder to the mobile home panel with out a ground rod/electrode. The neutral, and ground are isolated, and the ground bar is bonded to the panel box. Isn't it essentialy a sub-panel.

Another kick at the same cat Robert? You have a 5 page thread going already on this subject. :whistling2:

RobertS 02-27-2008 10:32 AM

Nope, I would like to know the theory behind this, and not just regurgitate some stuff from a book.

InPhase277 02-27-2008 11:30 AM

Quote:

Originally Posted by Robert Shields (Post 102316)
Nope, I would like to know the theory behind this, and not just regurgitate some stuff from a book.

Robert, the code just simply doesn't require another grounding electrode like a 4-wire subfeed would. You are right that the panel inside the mobile home is essentially a subpanel. And you certainly could add another grounding electrode.

Why is it different? I really don't know, but I would venture the guess that the mobile home isn't a detached structure. It is THE structure. If a framed house had it's meter/main located on a pole separate from the building, and a 4-wire feeder to the panel inside, it would be the same situation.

In other words, the code is not considering the pole as one structure and your trailer as a detached structure. The pole and trailer are one unit together.

InPhase277

HouseHelper 02-27-2008 11:47 AM

This is just my opinion:

Article 550, which deals with mobile and manufactured homes, requires a disconnect to be located in sight of but not more than 30ft from any point on the exterior wall of the mobile home. All the rules that apply to a main service apply to this disconnect. Since this disconnect is usually installed within about 6ft of the structure, and the structure is required to be fed by a four wire feeder, a separate grounding electrode is not required at the mobile home.

Stubbie 02-27-2008 12:42 PM

Robert

You can put a ground rod at the mobile home it just isn't required by the NEC as HH has said and Inphase77.. It is generally not allowed by local codes. But it must be bonded to the ground bar of the panelboard in the mobile home not the frame of the mobile home. Most of why and how we run the electrical service to a mobile home is because they are mobile "meant to be moved from location to location" ie they are not on permanent foundations. This is why the service equipment or at least the disconnect is located close to the mobile home ( within 30') every wonder while this close proxomity is required? If we were to move the mobile home we do not want a permanent wiring method to service equipment where we have unfused service entrance conductors entering the dwelling. This of course has nothing to do with a ground rod being allowed at the mobile home but gets you thinking correctly about the intent of the code. The service serves a dwelling/habitable structure that is 'mobile' built on a metal frame, that frame is bonded to the panelboard ground bar inside the mobile home with a minimum #8 copper bonding wire which of course is bonded to the equipment ground of the feeder from the service equipment located outside and within 30 feet of the mobile home. This is considered sufficient shock protection for humans and lightning protection for a mobile home. We treat it pretty much the same as a sub-panel that is in the same structure as the service equipment. Only in this case the service equipment is 'closeby' on a pedestal and a water pipe bond (if present) and ground rod(s) driven or other electrodes and bonded there with grounding conductors completing the required GES. So in a nutshell it is the same thought process for not requiring a grounding electrode for a sub-panel in the same dwelling with the service equipment. Adding ground rods simply isn't needed and would provide very little additional benefit to the electrodes already present at the service disconnect. If your looking for some reason that it would be terribly dangerous to have a ground rod at the mobile home sub-panel I really don't think one exists other than the possiblity of an earth return for neutral current being present between two ground rods that are in close proximity to each other..one at the service equipment and one nearby at the mobile home sub-panel. I would suppose this is the main reason most local codes to not allow electrodes bonded to the mobile homes sub-panel.

RobertS 02-27-2008 01:08 PM

Thank you Stubbie, as usual I think I am making this to complicated. Your explaination is vary clear.

Stubbie 02-27-2008 01:25 PM

Well I'm glad someone thinks I'm clear.....:)

In your other thread I think this got a little 'confuluted' because we were talking detached structure vs manufactured home. The nec really doesn't make a distinction between mobile and manufactured until we get them on permanent foundations. I think if I remember I had a post in your other thread about are we talking a detached building or manufactured/mobile home. You can have a permanent wiring means for the service equipment then because we are then allowed to have it attached or even in the manufactured home. The builders are required to place a metal nameplate on the home (you have probably seen it) it generally will say along with that gob of site installation instructions if means have been provided for attached service equipment and must provide instructions for the intended grounding of electrodes and what kinds of electrodes will be allowed. This usually means that the panelboard inside the manufactured home is to be the service equipment. And grounding is like any other dwelling. One thing they do not want in this case is the ground rod driven underneath the mobile/manufactured home it must be driven outside the perimeter.


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