Grounding a seperate dwelling


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Old 10-10-15, 11:58 PM
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Grounding a seperate dwelling

Hello fellow tradesmen in cyberspace.

OK, i have a question for a project i'm working on in the next few weekends.

For starters here, my daughter and husband bought a house and it came with an efficiency rental unit on the property.

They were told that the electric needs to be 100 amps and it has to be grounded for the power to be turned on.

The main dwelling has two meters but one service head, and the rental unit meter has a disconnect there at the meter of 60 amps, fed through 1 1/2 metal EMT buried underground. In that conduit is a neutral and two line feeds. About 70 feet of run.

My shortcut plan for a ground back to the house is to avoid trying to pull a ground wire through this conduit. Instead, use a grounded nut on the 1 1/2 connection inside the new breaker panel. Can i get by, by using a wire from the ground buss, fastening it to the ear or "screw cinch down" on the grounded nut? Essentially the EMT back to the house is a ground.

The new breaker panel is not a sub-panel, but a 100 amp service panel.

Also I'm rewiring the entire unit. It was BX'd a long time ago via cloth insulation on the wires.

I'm a drywaller and we removed the ceiling drywall today for a clear shot - no attic crawl space here - if there were one you would need to be 3ft tall.

I have some electrical experience with new service on my home, including a new service head entrance and later added a subpanel. But i'm over my head here and want a home run ( figuratively) the first time around.

Thoughts?

Thanks in advance.
 
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Old 10-11-15, 04:34 AM
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The EMT conduit will serve as the grounding conductor between the two buildings. You will need two ground rods at the remote building.

Depending on their size the feed conductors and perhaps the EMT may need replacing. You need #2 gauge copper wires for 100 amps.
 
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Old 10-11-15, 07:51 AM
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fed through 1 1/2 metal EMT buried underground.
EMT should not be used for direct burial. It will rust out quickly. If it has been buried a while it may be so rusted through it is not providing an adequate EGC. It is very doubtful you can pull new wire through it due to its likely condition. It needs to be replaces with PVC or IMC or RMC. PVC will be easiest and cheapest but you will need four wires. Direct burial of quadplex aluminum mobile home cable without conduit except for protection above ground may be you cheapest/easiest solution other than overhead quadplex.

Note mobile home cable that is not dual rated can not be brought into a building. You must transition to another wiring method where you enter.

Note if you go overhead be sure the quadplex is listed as UV (sunlight) resistant.
 
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Old 10-11-15, 08:41 AM
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I will second Ray's comments. Since you are not setting another service I don't see why the feed needs to be 100 amps.
 
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Old 10-11-15, 12:24 PM
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Just to clarify that tubing is rigid., not light duty.
 
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Old 10-11-15, 12:48 PM
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If it isn't threaded it is tubing, EMT. If it uses set screws it is EMT. Compression fittings it might be EMT.
 

Last edited by ray2047; 10-12-15 at 06:03 AM. Reason: Clarification.
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Old 10-11-15, 05:42 PM
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It is threaded at 1.5 inches off memory.
 
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Old 10-12-15, 06:07 AM
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It is threaded at 1.5 inches off memory
Excellent. Your original post said
the EMT back to the house
so that was what I was going on. Your probably good to go if threaded.
 
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Old 10-12-15, 08:12 AM
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Best practice in this case would be to install a bonding bushing on both sides of the threaded rigid conduit to ensure a good ground connection at the panel, but it is not strictly required.

The threaded rigid is usually good for at least several decades underground, beyond that it will show some decay but depends entirely on your local soil conditions. It's feasible to last much longer in the right area. Once you get the subpanel hooked up, you can measure the resistance (ohms) from the subpanel neutral bus to the subpanel metal ground bar to roughly gauge the quality of the conduit ground. You should get a reading no higher than the low single digits. Anything above that is the result of a significantly corroded pipe.
 
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Old 10-12-15, 06:19 PM
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Thanks for the input everyone.

I'm starting to get worried here, and i emailed them last night that they should go to the planning commission and get exactly what they need in terms of what is specified and required. I just remembered too, that i didn't see a building permit anywhere, but when they went to this county office, a desk lady told them what was generally required - no inspector on hand ATM. Probably under the assumption that they'd hire it out via a contractor. I tell you this house has been a headache for them so far. They have to put in about 20k in foundation repair and that's the tip of the iceberg. Their house electric is screwed up too. I saw a few breakers with doubled up wires and there isnt a ground rod outside either. They are grounded inside here via 3 wire "rope" or romex etc and have 100 amp. That was required for the bank loan.

They have a 220V dryer with no spaces left for central air upgrade next year. They have a vintage 1957 furnace in there - saw the nameplate. Ouch! For the heating this winter.
 
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Old 10-12-15, 06:59 PM
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Some wires allow two conductors, depends on the brand and wire sizes.

The ground rod has nothing to do with grounding a receptacle.
 
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Old 10-12-15, 07:14 PM
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Adding ground rods can be thought of as a separate project. It is the same regardless of the age of the house or the age of the other components in the electrical system.

Before installing something new and big like central air conditioning or something new and complex like lights and receptacles throughout the basement, you need to do a load analysis, which is paperwork. Going about that is the same regardless of the age of the house or the age of the other components in the electrical system. Now, the results of the load analysis (which results include a required number of amperes or watts) might or might not require replacement of some existing components in the electrical system.

There are many situations where, if you run out of slots in the panel for new circuits, you can install a subpanel next to that with only two or three reroutings of existing wires to the subpanel. Many times, installing a subpanel is a separate project that is the same regardless of the age of the house or the age of the other components in the electrical system.
 

Last edited by AllanJ; 10-12-15 at 07:30 PM.
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Old 10-12-15, 07:33 PM
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Ok let's talk science-y stuff.

From i what understand, the ground on the outlet ( or anywhere ) is to provide a path for the current back to neutral so the appliance or equipment is not energized. - it is tied back to the neutral effectively blowing the beaker - instead of remaining energized with a potential.

I suppose the earth rod then is there in case there is a smaller potential on an appliance chassis, meaning not enough to trip the circuit, but enough there for a shock, and the 8ft rod in the ground would induce the current to follow the lower resistance here - and bypassing you?
 
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Old 10-12-15, 08:23 PM
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Earth rod is to drain atmospheric charges.
 
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Old 10-12-15, 08:26 PM
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The resistance of the earth is too high to trip,a breaker. Also the current is trying to get back to the source.
 
 

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