electrical grounding reqd ?

armyho211 · May 24, 2017

added a subpanel to a new metal shed, power coming from house...

the subpanel is grounded to a 5/8 x 8ft ground rod, I intend to wire 4 outlet boxes inside the shed with alum flexsteel conduit, will run 3 # 12 wires, blk,wht, and a bare wire....

QUESTION # 1) since the system is grounded via the ground rod, do I still need to ground the 20 amp receptacle to the metal outlet box with a green pigtail in addition to the ground wire to the ground rod ?

question #2 ) is it okay to mount the outlet boxes directly to the sheet metal siding ? or should I place a piece of plywood between the outlet box and the metal siding ?

APPRECIATE ANY RESPONSES ! THIS WILL BE INSPECTED, SO I NEED TO DO IT RIGHT.....

AntonLargiader · May 24, 2017

Two different grounds here. One is the ground rod system which BTW needs a second rod. But that is totally different from the ground fault path, which is your green or bare wires.

Some receps self-ground to the box, but it depends on the recep and the box system. What boxes/covers/receps are you using?

pattenp · May 24, 2017

#1 - You need to pigtail the equipment ground to any metal box. The grounding electrode (rods) have a totally different purpose then the equipment ground. Rods are for lighting strike protection, equipment ground is to clear fault current (trip breaker).

#2 Can't really say with not seeing the actual way you intend to mount.

armyho211 · May 24, 2017

anton, pattenp - I will be installing 3ea 4" square outlet boxes in series, with 1/2" aluminum flex. the receptacles are 20 amp 120v, 2p 3w. brass, silver, and ground screw. ( LEVITON # CR-20-I )

AS FAR AS MOUNTING THE 4 SQUARE BOX, ANY REASON WHY I CANNOT MOUNT THE BOX DIRECTLY TO THE SHEET METAL SIDING ? OR SHOULD I MOUNT A PIECE A PLYWOOD TO THE SIDING AND THEN MOUNT THE 4 SQUARE BOX TO IT. ?

THANKS !!!!

mm08822 · May 24, 2017

aren't the mounting screws going to stick through the sheet metal siding for either method? Consider small bolts with rubber gaskets under the boltheads. Install bolts sticking into the shed. I would only use the plywood if needed for creating a flat surface to mount box.

If using duplex recepts, you don't need 20a recepts unless you have a load specifically requiring it.

Need 2nd rod.

All ground wires shall be connected to each metal box they enter. Grounding of metal boxes shall be independent of whether any device is installed in the box.

pattenp · May 24, 2017

armyho211 said:
AS FAR AS MOUNTING THE 4 SQUARE BOX, ANY REASON WHY I CANNOT MOUNT THE BOX DIRECTLY TO THE SHEET METAL SIDING ? OR SHOULD I MOUNT A PIECE A PLYWOOD TO THE SIDING AND THEN MOUNT THE 4 SQUARE BOX TO IT. ?

THANKS !!!!

No need to yell. I heard what you said. But just as mm08822 asked, how are you thinking of screwing the boxes straight to the metal siding? You don't want rain water leaking from outside into back of box around screws.

armyho211 · May 24, 2017

pattenp- sorry about the italics, too lazy to re-type... I need to step back and absorb everything you guys are suggesting...

thanks to all, I know I will have other questions to ponder...

wyliesdiesels · May 24, 2017

OP- how many wires did u run between the house and shed?

mm08822 · May 24, 2017

pattenp said:
No need to yell. I heard what you said. But just as mm08822 asked, how are you thinking of screwing the boxes straight to the metal siding? You don't want rain water leaking from outside into back of box around screws.

And those exposed screw tips will make for great meat tenderizing :scared:

matt_i · May 24, 2017

I wouldn't just put a plywood shim between the outer siding and drive the fastener thru. That screams hack to me.

Imo the electrical work should be mounted to the structure of the shed, framing, whatever that may be. There is also the issue of securing the cable...and again framing would be my choice.

slimpickins · May 24, 2017

Armyho,
The question you are asking is more complicated than it seems. You need to check your local code because there *MAY BE* some local variations in the code and how the inspector wants to see it done. Most inspectors will be helpful because they would prefer answering questions and then inspecting a job done right, rather than rejecting a hack job and getting into an argument.... YMMV ... but in my experience inspectors are your friend.

That being said, the *general rule* is that once you are beyond the service entrance to the property you should only have a single grounding point (Ground rods, ground plates, whatever). The code changed in 2008 so again check with your inspector. http://iaeimagazine.org/magazine/2008/09/16/two-buildings-common-service-grounding-requirements/

Installing a second grounding point can result in "ground loops" - the situation where you can experience a current between two ground rods. If you have to install a ground rod for the second building then you probably cannot have (or cannot use) a grounding conductor in the service lines supplying the second building.

In regard to mounting your outlet, it wasn't clear whether you're mounting this inside or outside. It sounds like outside because you wouldn't want the screws poking to the outside of the building. Either way, I don't think your metal box should be electrically grounded to the building. There is a discussion on this over here: http://www.selfhelpforums.com/showthread.php?8555-Wiring-a-Metal-Storage-Building indicating it makes a difference if the building is all metal or metal over wood. There's just so many variables in what you're asking ... and the only person you really need to satisfy is the inspector.

So, again I recommend that you check with your inspector on this situation because you could inadvertently create a ground loop situation. i.e. if your metal building is grounded, and you screw a box to it, then ground that box with the ground wire in your loomex, then you clearly have a potential ground loop using your very small 14 gauge ground wire in your loomex.

Cheers

mm08822 · May 24, 2017

slimpickins said:
Armyho,
The question you are asking is more complicated than it seems. You need to check your local code because there *MAY BE* some local variations in the code and how the inspector wants to see it done. Most inspectors will be helpful because they would prefer answering questions and then inspecting a job done right, rather than rejecting a hack job and getting into an argument.... YMMV ... but in my experience inspectors are your friend.

That being said, the *general rule* is that once you are beyond the service entrance to the property you should only have a single grounding point (Ground rods, ground plates, whatever). The code changed in 2008 so again check with your inspector. http://iaeimagazine.org/magazine/2008/09/16/two-buildings-common-service-grounding-requirements/

Installing a second grounding point can result in "ground loops" - the situation where you can experience a current between two ground rods. If you have to install a ground rod for the second building then you probably cannot have (or cannot use) a grounding conductor in the service lines supplying the second building.

In regard to mounting your outlet, it wasn't clear whether you're mounting this inside or outside. It sounds like outside because you wouldn't want the screws poking to the outside of the building. Either way, I don't think your metal box should be electrically grounded to the building. There is a discussion on this over here: http://www.selfhelpforums.com/showthread.php?8555-Wiring-a-Metal-Storage-Building indicating it makes a difference if the building is all metal or metal over wood. There's just so many variables in what you're asking ... and the only person you really need to satisfy is the inspector.

So, again I recommend that you check with your inspector on this situation because you could inadvertently create a ground loop situation. i.e. if your metal building is grounded, and you screw a box to it, then ground that box with the ground wire in your loomex, then you clearly have a potential ground loop using your very small 14 gauge ground wire in your loomex.

Cheers

OP, hasn't indicated location in profile. In the US, NEC requires the second earth ground point for detached buildings even though one exists at the point of service. I always wondered as well about ground loops as well across long distances, but it apparently it is not a big enough issue and I have yet to hear a plausible explanation. (I still lift gec wires and bonding wires as though they are hot for this very reason.)

One poster on here was thinking he had 3-4 amps flowing on his egc. If he was/is right, lifting that wire would sure give a surprise to someone.

He's mounting inside and running metal flex.

The metal shed frame/sheet metal should have a bonding jumper back to the sub panel grd bar for exactly that reason - to eliminate any chance for it becoming and remaining energized due to some system fault.

Running anything by the AHJ is always prudent, as he gets to pick the color sticker you get at the end of the job.

zmaxmotorsports · May 25, 2017

A buddy sent me this yeaterday.
Supposed to be a house in fremont that took a direct lighting hit last week.
I didn't get the details of what damage was caused by the strike and what happened after.
I guess the picture would help.

wyliesdiesels · May 27, 2017

Oh damn!!!

:shocking:

forAK · May 27, 2017

That looks like a meteor hit it!

sberry · May 27, 2017

Installing a second grounding point can result in "ground loops" - the situation where you can experience a current between two ground rods. If you have to install a ground rod for the second building then you probably cannot have (or cannot use) a grounding conductor in the service lines supplying the second building.

You need both and must have 4 wires.

ard · May 27, 2017

zmaxmotorsports said:
A buddy sent me this yeaterday.
Supposed to be a house in fremont that took a direct lighting hit last week.
I didn't get the details of what damage was caused by the strike and what happened after.
I guess the picture would help.

Another sub-panel ground bar improperly bonded....

slimpickins · May 29, 2017

mm08822 said:
OP, hasn't indicated location in profile. In the US, NEC requires the second earth ground point for detached buildings even though one exists at the point of service. I always wondered as well about ground loops as well across long distances, but it apparently it is not a big enough issue and I have yet to hear a plausible explanation. (I still lift gec wires and bonding wires as though they are hot for this very reason.)

One poster on here was thinking he had 3-4 amps flowing on his egc. If he was/is right, lifting that wire would sure give a surprise to someone.
---I've never seen any current on a ground loop personally, but I've heard anecdotal stories about the reality of it. And exactly on the surprise on lifting a ground wire if you get a few amps of current on a ground loop.

A buddy sent me this yeaterday.
Supposed to be a house in fremont that took a direct lighting hit last week.
I didn't get the details of what damage was caused by the strike and what happened after.
I guess the picture would help.

He's mounting inside and running metal flex.

The metal shed frame/sheet metal should have a bonding jumper back to the sub panel grd bar for exactly that reason - to eliminate any chance for it becoming and remaining energized due to some system fault.

This is what I would always do for exactly this reason. That's the whole reason why grounding conductors became mandatory 50-60 odd years ago. I remember as a kid, my dad retrofitting my mom's washing machine with a grounded plug. Many people got killed before the grounding rules were fixed. My dad also told me the story of a guy that fixed his own hand drill but wired it wrong (before the days of double insulated tools). He went up to work on a tin roof and his drill wouldn't work because he wired it wrong, so he set the drill down (metal body drill in those days) and electrified the whole roof with his improperly wired drill, and another guy died when he had his properly wired and grounded drill and then touched the electrified tin roof.

Running anything by the AHJ is always prudent, as he gets to pick the color sticker you get at the end of the job.

A grounding rod at the second location makes sense to me for buildings that are too far apart. Then you *shouldn't* need the expense of a ground conductor in the long service/sub service run. In the second building, you still have to isolate the ground (remove the bonding screw that ties the neutral to ground), otherwise there is the potential for the neutral wire to carry any ground loop current. This is explained in the first link that I provided in my previous post...

Always check with your inspector.... !!!

ard said:
Another sub-panel ground bar improperly bonded....

Yikes, I've always wondered about installing lighting rods. Could never wrap my head around sticking a grounded conductor in the air above my house -- seems like you're asking for the lightning to strike!!??

Cheers

AntonLargiader · May 29, 2017

slimpickins said:
... Then you *shouldn't* need the expense of a ground conductor in the long service/sub service run.

You need to read some of the threads here on grounding. Wylie talks about it a lot. What you propose is unsafe.

Yikes, I've always wondered about installing lighting rods. Could never wrap my head around sticking a grounded conductor in the air above my house -- seems like you're asking for the lightning to strike!!??

You need to read about that, too. It doesn't attract lighting strikes, it prevents them.

pattenp · May 29, 2017

AntonLargiader said:
You need to read some of the threads here on grounding. Wylie talks about it a lot. What you propose is unsafe.

You need to read about that, too. It doesn't attract lighting strikes, it prevents them.

Nothing prevents lighting strikes. Lighting rods are an effort to redirect a lighting strike safely to ground instead of having it go through the structure.

sberry · May 29, 2017

I agree, slim needs a little help here.

AntonLargiader · May 29, 2017

Right... did a bit more reading myself. A news blurb on how the systems have changed:

http://minnesota.cbslocal.com/2012/05/29/good-question-why-dont-you-see-lightning-rods-on-homes/

mm08822 · May 29, 2017

Slim, the egc is required with the feeder to trip the ocp if the feeder experiences a fault. Eliminating it could let an otherwise high resistance fault sit there brewing away just waiting to seriously shock someone. User protection and system protection is far more important than a possible ground loop. I'm with you on the ground loop concern, but not at the expense of safety.

The egc could be grc/imc/emt/uf and not just an added conductor in pvc conduit. You proposal would eliminate many wiring methods.

As to the metal box being grounded in the shed......you started by saying no it should be isolated from structural steel. I come along and stated it has to be bonded the eliminate any potential differences and risk thereof.
Then you reply to that saying it must be grounded. :headscrat

sberry · May 29, 2017

No reason to insulate a box, bonding it to the steel is a good thing.

mm08822 · May 29, 2017

pattenp said:
Nothing prevents lighting strikes. Lighting rods are an effort to redirect a lighting strike safely to ground instead of having it go through the structure.

Slim, lightning rods atop a structure connected to earth are used to also eliminate any static charge buildup that could otherwise attract lightning to that location. It may seem wrong to create this attraction at point sources (rods) but it can eliminate stray charge present over most of the surface of the building.

If the stray charges remained on the surface, the entire surface becomes a conductor (think of spider web as a lattice of small conductors). Rods and larges conductors to earth create virtually a single path to ground eliminating many combustible paths otherwise forced to carry the strike to earth.

AntonLargiader · May 29, 2017

mm08822 said:
Slim, lightning rods atop a structure connected to earth are used to also eliminate any static charge buildup that could otherwise attract lightning to that location. ..

That's what I was initially getting at, and I'd read that here, but doing a bit of reading this morning suggests that the "dissipation theory" is pretty well discredited now. The dissipation effect is effective for a pretty short distance.

I'm not an expert and don't understand it all myself. Seems that the bajillion amps of a lightning strike would overwhelm any reasonably sized conductor.

slimpickins · May 30, 2017

Anton,

Thanks for the heads up. I see the final edits I made to my last post somehow didn't make it to the board - sorry for the confusion. I was referring back to the first link (http://iaeimagazine.org/magazine/2008/09/16/two-buildings-common-service-grounding-requirements/) in my original post which shows the old and new requirements in the code. The setup I described is indicated as an "exception only" for existing installations (See figure 1 in the link) and has no grounding conductor between the buildings. This uses the neutral for a grounding conductor and is no longer permitted.

The current state is the requirement for a grounding conductor between the buildings, plus ground rods and bonding screws at both buildings.

mm08822 said:
Slim, the egc is required with the feeder to trip the ocp if the feeder experiences a fault. Eliminating it could let an otherwise high resistance fault sit there brewing away just waiting to seriously shock someone. User protection and system protection is far more important than a possible ground loop. I'm with you on the ground loop concern, but not at the expense of safety.

The egc could be grc/imc/emt/uf and not just an added conductor in pvc conduit. You proposal would eliminate many wiring methods.

As to the metal box being grounded in the shed......you started by saying no it should be isolated from structural steel. I come along and stated it has to be bonded the eliminate any potential differences and risk thereof.
Then you reply to that saying it must be grounded.

My comments about grounding the box are in the context of whether the metal exterior is grounded or not - the requirements on this depend on the underlying structure (wood or steel), although I fail to see why you wouldn't ground a metal exterior of a wood structure building. Thus my repeated comment about checking with the inspector.

PS. Re: lighting rods, I'm aware of their purpose and function, dissipation theories, etc. I was just trying to make a bit of humor. Even a tiny thin wire creates a path for lightning to follow:

How big a conductor would you need for it to survive a lightning strike???

Has anyone seen pictures of a house with lightning rods that have been struck by lightning. They often run the conductors inside the attic and attached to the side of the house. These things get hot when they are carrying a bazillion amps. :headscrat

Cheers

AntonLargiader · May 30, 2017

slimpickins said:
The current state is the requirement for a grounding conductor between the buildings, plus ground rods and bonding screws at both buildings.

Not bonding screws; they bridge the neutral to ground which is exactly what all of these new requirements are getting away from. Bonding is only at the first point of disconnect, and not at subpanels.

That article is not the best written thing I have ever read, by a long shot, but it's probably accurate. There's no need to try to reinvent the code on this by suggesting things like not using a ground wire if you have ground rods and so forth. This stuff has been figured out by experts and it is the law of the land.

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