Wiring subpanel

[Mac25]

I recently had a barn built and I am in the process of wiring the sub panel. I have a 200A service in the house and 100A in the barn. Here is my thought process....run 2/2/2/4 service entrance cable from the house service to barn using a 100A breaker, buried in conduit. It is my understanding that I need to unbond the sub panel, separating the neutral and ground bars? Will I need to add grounding rods for the sub panel after unbonding the neutral and ground? Thanks

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[SPIKER]

I recently had a barn built and I am in the process of wiring the sub panel. I have a 200A service in the house and 100A in the barn. Here is my thought process....run 2/2/2/4 service entrance cable from the house service to barn using a 100A breaker, buried in conduit. It is my understanding that I need to unbond the sub panel, separating the neutral and ground bars? Will I need to add grounding rods for the sub panel after unbonding the neutral and ground? Thanks

Sent from my iPhone using TractorByNet

Sounds OK, so far.

( edit in Yes you must have ground rods for each building that is powered as normal.)

M

 

[Ed of all trades]

My inspector said I had to ground mine.

 

[Inspector507]

Yes grounds rods are required.

 

[s219]

All sounds good to me. Assume the 2-2-2-4 is aluminum, AKA mobile home service wire.

Interestingly, I learned that 2-2-2-4 aluminum is only rated for 90A as a branch circuit, but 100A as a feeder. For a feeder, they assume something like 90% loading (on average) so the amp rating is slightly higher.

 

[BeezFun]

I just had a similar installation done, both the electricians who did the work and the county said that ground rods weren't allowed at the barn. The ground wire from barn had to be carried back to the house and connected to main panel ground bar. The explanation was that the ground rods can be at different potentials because of different soil conditions or corrosion in connections. That can create ground loops that affect electronics equipment, particularly audio and video, and make it possible for the grounded case of electrical equipment to give someone a small shock. I have no idea if this is some local thing, or an NEC thing.

 

[s219]

I just had a similar installation done, both the electricians who did the work and the county said that ground rods weren't allowed at the barn. The ground wire from barn had to be carried back to the house and connected to main panel ground bar. The explanation was that the ground rods can be at different potentials because of different soil conditions or corrosion in connections. That can create ground loops that affect electronics equipment, particularly audio and video, and make it possible for the grounded case of electrical equipment to give someone a small shock. I have no idea if this is some local thing, or an NEC thing.

Bad idea if lighting strikes the barn and gets into the electrical, as it will not have a direct path to ground other than going back to the house. Code as I know it calls for ground wire run from main panel *and* ground rods at outbuildings. That only applies to a feeder, where there is a panel and multiple circuits in the outbuilding. If it's a branch then ground rods are not required.

250.32 Buildings or Structures Supplied by a Feeder(s)
or Branch Circuit(s).
(A) Grounding Electrode. Building(s) or structure(s) sup-
plied by feeder(s) or branch circuit(s) shall have a ground-
ing electrode or grounding electrode system installed in
accordance with Part III of Article 250. The grounding
electrode conductor(s) shall be connected in accordance
with 250.32(B) or (C). Where there is no existing ground-
ing electrode, the grounding electrode(s) required in 250.50
shall be installed.
Exception: A grounding electrode shall not be required
where only a single branch circuit, including a multiwire
branch circuit, supplies the building or structure and the
branch circuit includes an equipment grounding conductor
for grounding the normally non–current-carrying metal
parts of equipment.

 

[BeezFun]

Bad idea if lighting strikes the barn and gets into the electrical, as it will not have a direct path to ground other than going back to the house.

A ground loop might make someone hope to be struck by lightening after spending weeks trying to sort out all the resulting problems with the electronics in homes these days.

A more likely safety problem than lightening would be the possibility of having equipment chassis carrying current because of ground loops, which probably can't kill someone but would sure give them a surprise. It has always seemed incredible to me that ground system resistances are allowed to be as high as 25ohm, which means there is very little chance a circuit breaker would trip even if the ground was properly connected. I guess we should be thankful for gfi.

Concern about lightening damage would seem to argue for one grounding system, not two. If, for example the house is struck by lightening but the service entrance ground system is at a higher potential than the barn ground system, the induced current from the lightening strike now (tries to) flow out to the barn. A strike flowing into one rod would also induce a potential difference of possibly thousands of volts between the two ground systems because of the resistance of the soil, which now shows up on any piece of electronics connected to the other ground. In any case my experience with lightening is that it never does what anyone predicts, never follows any of the intended paths, and seems to always destroy things in a way no one would imagine possible.

Code as I know it calls for ground wire run from main panel *and* ground rods at outbuildings. That only applies to a feeder, where there is a panel and multiple circuits in the outbuilding. If it's a branch then ground rods are not required.

I admit to no knowledge of the code, and the section you quote seems to clearly call for a separate ground system. So I cannot explain why both of the licensed union electricians I hired, and the county's inspection department, told me the opposite.

 

[DarkBlack]

Bad idea if lighting strikes the barn and gets into the electrical, as it will not have a direct path to ground other than going back to the house. Code as I know it calls for ground wire run from main panel *and* ground rods at outbuildings. That only applies to a feeder, where there is a panel and multiple circuits in the outbuilding. If it's a branch then ground rods are not required.

What beezfun said is pretty much OK.

If you think it's about lightning ( is it really? ), Do you really think lightning... that has the voltage potential to force current, the distance through high resistance air, from the ground to the sky... is going to be stopped by a thin layer of plastic around the underground conductors?:shocked:

 

[SPIKER]

BEEZs

The issue is if you run a wire 1000 feet away then connect only the ground wire back to the main panel in the house you have a LOT of potential ground difference in the earth between the two buildings. If you are standing on dirt or concrete (typical floor in an out building) and plug in a drill & stand on the wet floor you have a high chance if a great voltage difference between the GROUND of the CASE of your drill and the DIRT under your feet. That means you CAN get shocked even from the GROUND SYSTEM not just the Conductor or Neutral system! That is why you have to tie the ground rods to the sub panel of the panel but not the neutral of the sub panel. It ties the grounds all together but depends on the insulated/coated neutral wire to carry all of that normal current back to the main panel. The often bare ground wires then are simply keeping all the earth local potential to the main earth.

Mark

 

[MossRoad]

I was required to put a ground rod at the sub-panel in the garage, too.

 

[BeezFun]

BEEZs

The issue is if you run a wire 1000 feet away then connect only the ground wire back to the main panel in the house you have a LOT of potential ground difference in the earth between the two buildings. If you are standing on dirt or concrete (typical floor in an out building) and plug in a drill & stand on the wet floor you have a high chance if a great voltage difference between the GROUND of the CASE of your drill and the DIRT under your feet. That means you CAN get shocked even from the GROUND SYSTEM not just the Conductor or Neutral system! That is why you have to tie the ground rods to the sub panel of the panel but not the neutral of the sub panel. It ties the grounds all together but depends on the insulated/coated neutral wire to carry all of that normal current back to the main panel. The often bare ground wires then are simply keeping all the earth local potential to the main earth.

Mark

What you are describing is how an electric chair is used to electrocute prisoners. Their feet are held at ground and their body is held at some high potential until they die.
I wear rubber soled boots because I want to do everything possible to keep current from running through my body. The last thing I want to do is have my feet connected to anything close to ground potential.
In the scenario you describe, the purpose of the ground wire is to hold the case of the equipment at ground, so that if there is a fault in the equipment that causes the case to come into contact with an energized conductor, the current will be carried back through the ground wire to the ground rods, and not through my body. The last thing I would wish for would be to stand near the ground rod for my electrical system when I happen to pick up a shorted out drill.

 

[buickanddeere]

I just had a similar installation done, both the electricians who did the work and the county said that ground rods weren't allowed at the barn. The ground wire from barn had to be carried back to the house and connected to main panel ground bar. The explanation was that the ground rods can be at different potentials because of different soil conditions or corrosion in connections. That can create ground loops that affect electronics equipment, particularly audio and video, and make it possible for the grounded case of electrical equipment to give someone a small shock. I have no idea if this is some local thing, or an NEC thing.

Somebody is mixed up between ground and neutral.there is a long line of people who can't seem to tell the difference no matter how many times they are told. The neutral floats isolated from ground from the 1st electrical service back to the barn.
The ground is also ran from the 1st electrical service back to the barn. Connected to two ground rods at the barn and to the ground bar, the ground bar only of the barn panel.
No connection between neutral and ground at barn.

 

[scott_vt]

Good Afternoon Beezfun,
" I have no idea if this is some local thing, or an NEC thing."

I am far from an electrician, but when I built my barn a few years back, I ran 150 ft of cable out to the 100 amp sub panel in the barn and used two copper grounding rods outside the corner of the building. So far everything has worked fine, no issues.

My best guess is this is a local code issue in your area !

 

[DarkBlack]

I was required to put a ground rod at the sub-panel in the garage, too.

For 99.99 of people, It won't matter if you do, or don't, it's a code decision.
The reality of small technicalities: Is the building, and home distance and conductivity from the service pole ground, along with your neighbor's impedance on a shared transformer, which many have. Your garage is assumed further, but may be closer. Your neighbor's system may be closer than yours!

 

[MossRoad]

I think its a code decision because it makes sense from a safety standpoint. As others have mentioned, many folks get ground and nuetral confused. They aren't the same thing.

 

[Gunny]

I just finished upgrading my service entrance and installed an additional circuit breaker panel for some future projects. I had to add the second grounding rod and carry unbonded grounds back to the outside box.

Since it had to be inspected, I just laid everything in by NEC code, then called the inspector. Fastest inspection I've ever had, he took a quick look at what I'd done, asked a few questions, slapped the sticker on the box, grabbed the check and hit the road. The first question he asked as if I was going to hammer him on why I had to add the other grounding rod.

Before I started the job, I thought all these new rules and requirements a real PITA, but then I realized that they were in place for a reason and it just wasn't worth taking a chance, because it had to pass inspection and even if I didn't have to have it inspected, on the safety side it just wasn't worth taking a chance. Our house burned down when I was 11 years old because of an electrical problem and when I was thirteen a neighbor died because of a faulty ground when he was milking his cows.

Like several have said, grounding rods and the extra wire to carry the ground back to the entrance panel just aren't that expensive, especially when your home (or garage) or lives are in question.

 

[Al Mac]

As the rules are different between countries. It is all up to the local inspector on what he wants (he has the last say).

Here in my part of Canada a ground rod would not be required for a sub service. In the first post it was said that the cable was a 2/2/2/4 and the 4 is the ground back to the main service. All services only requires one grounding point which at the main service.

If this was a overhead (triplex) line from the house to the barn then a ground rod would be required as the triplex has no ground separate ground wire.




Al

Good Afternoon Beezfun,
" I have no idea if this is some local thing, or an NEC thing."

I am far from an electrician, but when I built my barn a few years back, I ran 150 ft of cable out to the 100 amp sub panel in the barn and used two copper grounding rods outside the corner of the building. So far everything has worked fine, no issues.

My best guess is this is a local code issue in your area !

 

[yelbike]

I thought about it for a second. Glad the inspection is done. Part of the problem is in your question.

A barn or out building is not a sub panel but rather its own service, even if served from your house. So don't remove the neutral to ground strap (jumper). Because the barn is its own service you need another ground source. Here that would mean a ground plate or two driven ground rods, this can be different in different jurisdictions.

For clarity, a sub panel would be in the same building. In a sub panel is where you remove the neutral to ground jumper as you only ground your neutral once per service.

I realize it's a play in n words but it's similar to ground and bond, totally different but still the same.

 

[s219]

I thought about it for a second. Glad the inspection is done. Part of the problem is in your question.

A barn or out building is not a sub panel but rather its own service, even if served from your house. So don't remove the neutral to ground strap (jumper). Because the barn is its own service you need another ground source. Here that would mean a ground plate or two driven ground rods, this can be different in different jurisdictions.

For clarity, a sub panel would be in the same building. In a sub panel is where you remove the neutral to ground jumper as you only ground your neutral once per service.

I realize it's a play in n words but it's similar to ground and bond, totally different but still the same.

That is at odds with NEC code. The neutral-ground bonding should only happen in one place downstream of the main service entrance, and that is at the main breaker panel. All other sub panels fed from the main panel, whether in the same building or in out buildings, should not bond neutral and ground.

I think there is a lot of confusion about ground (and probably neutral). Really, the ground circuit has a couple purposes. One is as a chassis ground, to return any shorted current back to neutral (at that single bonding point). It's like a backup return path to the main service entrance. Another is to serve as an earth ground for stray/excess current that is not part of the normal service (lightning strikes, wiring errors, etc). Rather than think about all the scenarios and second guess things, I just stick to code.