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