Out-Building Electrical Grounding

[james_98188]

I've built a 10X12 storage shed in my backyard on a concrete slab, about five feet behind my garage with the idea of putting an air compressor in there such as to pipe shop air into the garage without having to deal with the noise.

I've got the compressor in there now, an upright 5 HP, 60 gallon, 2 stage Ingersoll-Rand. And now, I've got to sift through all the electrical codes associated with running power to an out-building. I'm figuring:

1) A dedicated 220 volt circuit for the compressor,
2) A dedicated 220 volt circuit for a convection heater,
3) A 110 volt circuit for overhead lighting and a workbench power strip.

As best as I can figure out, the electrical codes for an out-building require two 8 foot metal grounding rods, at least six feet apart, pounded into the ground.

My first thought was this:

"****, EIGHT FEET long, pounded into rocky soil??? The contractor who built my retaining wall in my backyard to level and stabilize the yard due to the erosion that was happening because of the adjoining ravine had a devil of a time pounding in the 45 ten foot long 2 1/2 inch diameter metal pipe pilings down into bedrock." (The pilings have brackets welded to them to support the first course of pressure-treated lumber.)

Then my SECOND thought was this:

"****, there's ALREADY 45 ten foot long 2 1/2 inch diameter metal pipe pilings pounded down into the bedrock to support the pressure-treated retaining wall, why can't I just use two of those as ground rods?"

Any reason why not? My shed's just about four feet away from the wall.

 

[theoldwizard1]

Talk to the person who has to approve it, your local building/electrical inspector.

I know there are "alternatives" to the 2 copper grounding rods.

 

[Aceman]

The code says you can use those pipes if they're "galvanized or otherwise metal-coated for corrosion protection."

 

[james_98188]

Okay, that answers the question Aceman, thanks; they're not coated with anything.

 

[Speedy Petey]

The best alternative would have been to use the rebar in the slab (if it exists) as a grounding electrode. In fact, if it exists you MUST use it, and no other electrodes are required at all.

Also, I know many inspectors ignore this, but you can only run ONE circuit or feeder to a detached structure. In you case you would need a small panel in the shed with branch breakers for the separate circuits.

 

[Spudland_Dave]

How would you ground a "temporary" garden shed? I have a 12x16 garden shed that I built 4 years ago, always wanted to run power to it, but never have (wire is sitting in a coil in shed rafters..LOL). One of these days I will.. This shed sits on concrete blocks...Pressure Treated framing/flooring.

I guess I wouldnt really need a "Panel" in there.. 15A Circuit for lighting and 20A circuit for 2-3 receptacles would be more then enough.

 

[pattenp]

How would you ground a "temporary" garden shed? I have a 12x16 garden shed that I built 4 years ago, always wanted to run power to it, but never have (wire is sitting in a coil in shed rafters..LOL). One of these days I will.. This shed sits on concrete blocks...Pressure Treated framing/flooring.

I guess I wouldnt really need a "Panel" in there.. 15A Circuit for lighting and 20A circuit for 2-3 receptacles would be more then enough.

Just run one 20A circuit for outlets and have a couple switched outlets with plug-in shop lights. Don't mess with a grounding system.

 

[Charles (in GA)]

Just make sure there is a disconnect. On a low amp circuit, it can be a double pole snap switch rated for the 20amp and that is all that is needed.

Charles

 

[Journaler]

How would you ground a "temporary" garden shed? I have a 12x16 garden shed that I built 4 years ago, always wanted to run power to it, but never have (wire is sitting in a coil in shed rafters..LOL). One of these days I will.. This shed sits on concrete blocks...Pressure Treated framing/flooring.

I guess I wouldnt really need a "Panel" in there.. 15A Circuit for lighting and 20A circuit for 2-3 receptacles would be more then enough.

My 8x16 shed (on concrete blocks also) is grounded via 6' or 8' (don't remember) copper-ish rod in the ground.
It's got a 50A, 220V circuit running to a panel within the shed. It costs relatively little to put a small subpanel in there. It was a slight pain at first, but having that subpanel made adding external shed lights/outlets a breeze.

 

[Falcon67]

>"****, EIGHT FEET long, pounded into rocky soil???
This is where you get a reason to buy a new/replacement hammer drill

 

[james_98188]

The contractor installed a rebar mesh in the concrete slab, but didn't provide a "protrusion" to connect to it. (Wish I was more familiar with the electrical codes when I had them do the work.)

And yes, I'm planning on installing a "subpanel" in the shed. I'm not yet sure though, how power gets run to the subpanel. Does it get a separate meter? Branch off the main panel? Branch off the meter?

The best alternative would have been to use the rebar in the slab (if it exists) as a grounding electrode. In fact, if it exists you MUST use it, and no other electrodes are required at all.

Also, I know many inspectors ignore this, but you can only run ONE circuit or feeder to a detached structure. In you case you would need a small panel in the shed with branch breakers for the separate circuits.

 

[james_98188]

Excellent, that's exactly what I'm doing. The NEC specifies an eight foot rod IF one proves it provides adequate grounding, otherwise, TWO eight foot rods located at least six feet apart.

How does the power get to your subpanel? From the main panel? From the meter? A separate meter? How much of the work did you do, vs. how much of it did you have to have the power company do?

My 8x16 shed (on concrete blocks also) is grounded via 6' or 8' (don't remember) copper-ish rod in the ground.
It's got a 50A, 220V circuit running to a panel within the shed. It costs relatively little to put a small subpanel in there. It was a slight pain at first, but having that subpanel made adding external shed lights/outlets a breeze.

 

[pattenp]

It's most common to feed the sub-panel off of the main service panel via a 2 pole breaker in the main panel.

 

[james_98188]

Thanks pattenp, that seems easy enough. However, it appears that I should be using a 40 amp 2 pole breaker for my 2 stage 230 volt compressor. I also want to put a 4000 watt 230 volt convection heater in there, which equates to 18.2 amps. Add in maybe 3 100 watt light bulbs and a power strip; maybe an additional 7.3 amps, for a total of 25.5 amps. I see Home Depot has a 50 amp breaker, but even that doesn't seem large enough to support the shed. Would TWO 2 pole breakers be allowed?

It's most common to feed the sub-panel off of the main service panel via a 2 pole breaker in the main panel.

 

[VHF]

No, normally only a single feeder is allowed to supply an outbuilding. (There are exceptions for a separate circuit for lights that are switched remotely.)

Just go up to a 60A breaker if needed support all the loads in the shed. Or 70A or 90A or full 100A if needed!

Moving up to a larger breaker and running heavier wire will reduce the voltage drop your compressor sees when it kicks on.

 

[pattenp]

Thanks pattenp, that seems easy enough. However, it appears that I should be using a 40 amp 2 pole breaker for my 2 stage 230 volt compressor. I also want to put a 4000 watt 230 volt convection heater in there, which equates to 18.2 amps. Add in maybe 3 100 watt light bulbs and a power strip; maybe an additional 7.3 amps, for a total of 25.5 amps. I see Home Depot has a 50 amp breaker, but even that doesn't seem large enough to support the shed. Would TWO 2 pole breakers be allowed?

I assume you're in the US so no to the two 2 pole breakers. Only one power feed can be run to an outbuilding. A 60A feed will most likely do.