Detached Garage Sub-panel

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wa5yom

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I know this has been covered before, but I need clairification.

I have an older home (built in 1940), but it has a new 200 AMP service installed and inspected 2 years ago.
I just had a new detached garage built with GES (rebar conected to rod).
I intend to install a 100AMP subpanel (fed from the main panel) to a 60 amp breaker.

I will use underground direct burial cable from the main to the subpanel, about 50ft.

The subpanel ground bus will be grounded to the GES and the neutral will not be connected to the ground bus in the subpanel. The ground bus will connect to the grounn bus at the main panel.

What is the correct wire to run from the main to the sub and what size ground is required from the sub panel to to the GES?

And, have I configured this correctly?

Thanks,
Tim
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
agree.. your equipment ground should be based on 250.122 corresponding to your breaker supplying the garage/subpanel. You grounding electrode conductor should be sized per 250.66.
 

wa5yom

Member
Well, it is a 200 AMP service with a 60 amp breaker feeding the 100 amp frame sub panel in the garage.

The direct burial cable is about 75 ft.

I expect that #6 wire for the feeder and #8 for the subpanel ground to the ground rod would be OK...

Correct?

What other information would you need?

One concern I have, the trench is 24 in deep.......the water table is high...the last 10 feet has a couple of inches of standing water....

Even with direct burial cable.....the water concerns me......
 

ActionDave

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Durango, CO, 10 h 20 min from the winged horses.
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Licensed Electrician
Well, it is a 200 AMP service with a 60 amp breaker feeding the 100 amp frame sub panel in the garage.

The direct burial cable is about 75 ft.

I expect that #6 wire for the feeder and #8 for the subpanel ground to the ground rod would be OK...

Correct?

What other information would you need?
None.

One concern I have, the trench is 24 in deep.......the water table is high...the last 10 feet has a couple of inches of standing water....

Even with direct burial cable.....the water concerns me......
I don't know what can be done about it. Feed the garage overhead I guess.

Think about this, how much wire is underground in your county.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
I would install larger than necessary PVC conduit with individual conductors.
 

wa5yom

Member
Well, I am in Oklahoma, high content of clay soil. Most homes since the 60's have underground service feeds.

So, UF cable will not work in this environment?

I considered PVC, but the stub installed through the footing is hardly accessible. I am not sure I can attach to it.

The first 15 foot or so of the trench is relatively dry, some moisture but not standing water,

I could route that portion of the trench in a different direction which would allow me to enter the house through a wall with a L and run into the attic which would eventually get to the service, which is mounted on an outside wall, like many homes from the 40's.
 

John120/240

Senior Member
Location
Olathe, Kansas
Wet location rated conductors in PVC would be acceptable. As for moisture in the conduit I would drill a weep hole in the elbow before it enters your garage. You would need to dig around this elbow & install some loose gravel to aid drainage. Then duct seal could be used to seal your conduit where it enters your panel.
 

wa5yom

Member
Wet location rated conductors in PVC would be acceptable. As for moisture in the conduit I would drill a weep hole in the elbow before it enters your garage. You would need to dig around this elbow & install some loose gravel to aid drainage. Then duct seal could be used to seal your conduit where it enters your panel.

Hum...that is interesting...

If I connect to the conduit entering the garage.....it is below the footing and would be a straight connection...not an elbow....

But that leads me to an idea......since I can be in pvc at 18"....that does not mean it has to be sealed conduit....right?

Most of my trench is at 24"..the last 10 ft or so is 18".....what if I just lay come PVC in the trench from the connection at the footing (but not connect it, maybe leave about a foot of cable exposed), to the service, when I turn the pvc up 90 degrees, drill the drainage hole there and proceed into the main 200 amp service and seal it as you suggest...

And I could add a little gravel at each end....

With this option I would not have to dig anymore.....but I could probably finish the trench today...in a few hours....

Even though the cable withstands the moisture.......there is just something about dropping it in a trench with standing water.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
The conductors THWN are rated for wet location. Finish the conduit all the way then install your conductors. Regardless of water table, when underground, you really can not prevent water from getting in there. Drill drip holes in the 90 and you will be OK.
 

iwire

Moderator
Staff member
Location
Massachusetts
Well, I am in Oklahoma, high content of clay soil. Most homes since the 60's have underground service feeds.

So, UF cable will not work in this environment?

Sure it will but most of us in the trade doing work for ourselves would likely choose to use a raceway so you will only have to dig once if the conductors fail.


I considered PVC, but the stub installed through the footing is hardly accessible. I am not sure I can attach to it.

Perhaps abandon that and make a new way in through the sill. I don't like making holes in foundation due to possible water issues with it.





I could route that portion of the trench in a different direction which would allow me to enter the house through a wall with a L and run into the attic which would eventually get to the service, which is mounted on an outside wall, like many homes from the 40's.

Sounds like a good idea.

If you do run a raceway THWN would meet the code but a more durable selection would be USE/RHW. http://www.southwire.com/products/renew-rhh-rhw-use-copper-600v.htm

It is listed for direct burial but can be installed in a raceway and run inside the buildings on each end.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Well, it is a 200 AMP service with a 60 amp breaker feeding the 100 amp frame sub panel in the garage.

The direct burial cable is about 75 ft.

I expect that #6 wire for the feeder and #8 for the subpanel ground to the ground rod would be OK.

If you use a #8 GEC it would require protection from physical damage so #6 might be a better choice.
 
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