Grounding a detached garage

[wuntunearlybko]

i will preface this thread by saying sorry for beating a dead horse. i have searched this forum and spent seemingly hours reading and still cant draw a conclusion.

I just had a completely detached garage built. i have installed a 100A subpanel that will run off of my 200A main in the house. now from what i read i will need to keep the ground and neutral buss bars separated/un-bonded in the sub-panel since the main panel is already bonded (am i correct??).

1) will i need to install ground rods at the garage?
2) the concrete crew left a piece of rebar sticking out of the top of the retaining wall that i know extends to below the bottom of the slab (over 9'), totaling about 15'. is this for grounding? can it be used for grounding? here is a pic... you can barely see it between the trees

-cutts-

 

[Norcal]

Re: Grounding at detached garage

If it is only 15' it is too short, you need 20' in the footing. See 250.52(A)(2).

 

[wuntunearlybko]

Re: Grounding at detached garage

If it is only 15' it is too short, you need 20' in the footing. See 250.52(A)(2).

i will have to measure and get exact measurements... i am just guessing it is 15'. it very well could be 20'

if that is the case, i am assuming it can be used as a ground rod?

-cutts-

 

[Stuart in MN]

Re: Grounding at detached garage

Yes, the neutral and ground are kept separate at the panel in the garage. Contractors these days will commonly leave a rebar sticking up through the slab for use as a ground connection (it's called an Ufer ground) but you should check with them to make sure that is what it is.

 

[wuntunearlybko]

Re: Grounding at detached garage

Yes, the neutral and ground are kept separate at the panel in the garage. Contractors these days will commonly leave a rebar sticking up through the slab for use as a ground connection (it's called an Ufer ground) but you should check with them to make sure that is what it is.

thank you for the clarification! always trying to learn new things and want to do this correctly.

-cutts-

 

[VHF]

If you aren't 100% certain whether you have a proper Ufer ground you could connect the rebar to your ground bus and drive a couple ground rods.

Either a Ufer ground or a pair of ground rounds 6' apart will meet NEC minimum requirements for a detached garage, but the Ufer ground must be used if available.

As already discussed, netural and ground need to be kept separate at the garage sub-pannel, and 4 wires need to be run from the house.

 

[Falcon67]

If you aren't 100% certain whether you have a proper Ufer ground you could connect the rebar to your ground bus and drive a couple ground rods.

Either a Ufer ground or a pair of ground rounds 6' apart will meet NEC minimum requirements for a detached garage, but the Ufer ground must be used if available.

This is what the larger city around here requires - UFER plus two rods if the UFER is in place. On new construction, no UFER means the inspector makes you punch a hole in the slab and tie into the rebar.

 

[Coolerman]

I've never quite understood this method of grounding to the re-bar inside a concrete slab. Concrete doesn't conduct current very well and unless they welded all the re-bar together there is no solid connection between sections of re-bar in a slab. The re-bar does not touch the ground directly, so how is a good ground obtained this way?

My contractor did not bring out a piece of re-bar to connect to so I guess I'll be driving rods...

 

[malibu101]

I've never quite understood this method of grounding to the re-bar inside a concrete slab. Concrete doesn't conduct current very well and unless they welded all the re-bar together there is no solid connection between sections of re-bar in a slab. The re-bar does not touch the ground directly, so how is a good ground obtained this way?

My contractor did not bring out a piece of re-bar to connect to so I guess I'll be driving rods...

Not to be a smartbutt.
But here is some basic info- http://en.wikipedia.org/wiki/Ufer_ground
The link says that concrete has better conductivity than most types of soil.

 

[Charles (in GA)]

Concrete is a very good conductor.

Charles

 

[Gary S]

Concrete is a very good conductor.

Charles

Yes, I discovered that after I built my garage and was in the process of installing the phone line. I couldn't kneel on the concrete and touch the phone wire without getting an electrical shock in my knee where it touched the concrete. I had to put down a rubber mat to kneel on to finish the wiring because it wasn't pleasant.

 

[Aceman]

A ufer ground comes from the footing or foundation. Simply putting bar in concrete doesn't make it automatically qualify as a ufer.

Second, if you don't have proof that piece of bar coming out of the top of the wall goes INTO the footing and is over 20' of length, it's worthless. Did anyone take pictures before pouring?

 

[wuntunearlybko]

A ufer ground comes from the footing or foundation. Simply putting bar in concrete doesn't make it automatically qualify as a ufer.

Second, if you don't have proof that piece of bar coming out of the top of the wall goes INTO the footing and is over 20' of length, it's worthless. Did anyone take pictures before pouring?

it def goes into the footing. i dont have visual proof but i went home for lunch that day and they had at least 2' dug for the footing. that rebar goes to the bottom of the footing.

-cutts-

 

[Norcal]

See 250.52(A)(3), This is a copy & paste from the 2008 NEC.

(3) Concrete-Encased Electrode. An electrode encased
by at least 50 mm (2 in.) of concrete, located horizontally
near the bottom or vertically, and within that portion of a
concrete foundation or footing that is in direct contact with
the earth, consisting of at least 6.0 m (20 ft) of one or more
bare or zinc galvanized or other electrically conductive
coated steel reinforcing bars or rods of not less than 13 mm
(1⁄2 in.) in diameter, or consisting of at least 6.0 m (20 ft) of
bare copper conductor not smaller than 4 AWG. Reinforcing
bars shall be permitted to be bonded together by the
usual steel tie wires or other effective means. Where multiple
concrete-encased electrodes are present at a building
or structure, it shall be permissible to bond only one into
the grounding electrode system.

 

[matt151617]

Can you call the contractor and verify with them? Driving ground rods isn't hard (depending on your soil type/composition), but save yourself the time and money and ground to the slab if you can.

 

[Coolerman]

Not to be a smartbutt.
But here is some basic info- http://en.wikipedia.org/wiki/Ufer_ground
The link says that concrete has better conductivity than most types of soil.

No offence taken, I didn't know how it worked, that is why I asked!

 

[Charles (in GA)]

Yes, I discovered that after I built my garage and was in the process of installing the phone line. I couldn't kneel on the concrete and touch the phone wire without getting an electrical shock in my knee where it touched the concrete. I had to put down a rubber mat to kneel on to finish the wiring because it wasn't pleasant.

You just think you were getting a shock from the current in the phone line, wait till you are touching it when someone calls and a ring current is sent on the wire.

Charles

 

[coolreed]

You should test the conductivity of your soil. Depending on how that comes out will determine how many copper ground rods you should install in the soil near your panel. Run a 1/0 copper grounding cable form your grounding rod(s) to your panel. I would secure the grounding cable to the grounding rod(s) using Caldwelding. Also, if you have to run more than one grounding rod connect them together using 1/0 cable caldwelded to each rod.

You are building a nice garage so don't skimp on the electrical hardware.

 

[Morrisman]

Our whole house is grounded through an 8' long copper rod driven into the ground vertically. Nope, we're not in the USA, but it seems to work, and it seems to be the way most all houses are done round here.

There is also a ground wire that comes in with the main power lines, but that is not actually connected to our house, just wrapped around the insulation block where the line comes in from the overhead poles.

We have 220v mains to the house, through two live wires, so I derive my 110v by connecting between one of the two power lines and the ground system.

Most of the power outlets in the house are just dual pole, no ground connection. I insisted on adding it to the kitchen and laundry room though, where the white appliances are.

 

[pattenp]

What's the point of running such a large size wire? NEC only calls for #6 copper for the grounding electrode conductor..

You should test the conductivity of your soil. Depending on how that comes out will determine how many copper ground rods you should install in the soil near your panel. Run a 1/0 copper grounding cable form your grounding rod(s) to your panel. I would secure the grounding cable to the grounding rod(s) using Caldwelding. Also, if you have to run more than one grounding rod connect them together using 1/0 cable caldwelded to each rod.

You are building a nice garage so don't skimp on the electrical hardware.

 

[Speedy Petey]

You should test the conductivity of your soil. Depending on how that comes out will determine how many copper ground rods you should install in the soil near your panel. Run a 1/0 copper grounding cable form your grounding rod(s) to your panel. I would secure the grounding cable to the grounding rod(s) using Caldwelding. Also, if you have to run more than one grounding rod connect them together using 1/0 cable caldwelded to each rod.

WHAT?????? You CANNOT be serious.
This is a little detached garage, not a commercial radio tower.

 

[Speedy Petey]

Either a Ufer ground or a pair of ground rounds 6' apart will meet NEC minimum requirements for a detached garage, but the Ufer ground must be used if available.

The Ufer must be used "if present", not if available. Like Falcon said, many a contractor has had to chip away to get at the rebar because no one left it accessible. This has been code for a while new.

And if a Ufer is used NO other electrodes are needed.

 

[wuntunearlybko]

thank you all for the advice and discussion. i really love this site!

i confirmed with my contractor this morning that it is to be used for grounding. it is 25' and extends down into the ground a couple of feet below the bottom of the footing.

-cutts-

 

[Kevin C]

thank you all for the advice and discussion. i really love this site!

i confirmed with my contractor this morning that it is to be used for grounding. it is 25' and extends down into the ground a couple of feet below the bottom of the footing.

-cutts-

The rebar for the ufer that is used to connect to the earth should be totally encased in concrete. Only the connection end should be exposed.