Electrical question-panels, sub-panels, and ground rods

[grover]

I've decided to go with the 4/0 4/0 4/0 2/0 mobile home feeder wire and put it in conduit at least 18 inches below grade.  I have not decided yet whether to go with 2 1/2 or 3 inch conduit. 
This is my question now...I am thinking about going through the concrete block down below grade and come into my crawl space into an LB then go up and along the bottom of my floor joists and up into my panel.  The reason for this is for the future possibility of putting a deck on that portion of the cabin and an LB on the outside would be in the way.  I haven't found anything that says I cannot do that.  The only thing I have seen is I need to put the conduit inside another piece of conduit where it goes through the block.

Any problems with this?

[Patrick]

Thats similar to how i powered my shop ran the conduit up from the house panel through 15' of floor joists outside and buried then back up and in to the shop inspectors seemed mostly concerned how deep the pipe was in the ground. it is common to uses pipe within pipe going through masonry.Use lots of that electrical lube when your pullin the wire helps alot best to use 2 or 3 people 1 to pull 1 to push and 1 to lube the wire Ive seen people pull it through using a pick up truck also.

[_JT]

My question is do I need to put in another ground rod at the cabin and connect it to the ground bus in the sub panel?  The main panel and meter is about 30 feet from the cabin.

As has been said, the answer here is no, you don't need to. More importantly (the reason why I'm posting), doing so would be not only a code violation but potentially a major safety hazard. You want one AND ONLY one ground connection on each power system. If you drive a separate ground rod at a sub panel, you've created parallel ground paths, meaning it's possible for current to use the ground as a return, meaning anything grounded could become a current-carrying conductor (such as grounded metal equipment housings, like your microwave, toaster, washer, etc). Bad news!

Code specifies (in a residential application) one neutral to ground connection: at the main service entrance (which you have). Do not make a second one anywhere on the same system.

[rick91351]

As has been said, the answer here is no, you don't need to. More importantly (the reason why I'm posting), doing so would be not only a code violation but potentially a major safety hazard. You want one AND ONLY one ground connection on each power system. If you drive a separate ground rod at a sub panel, you've created parallel ground paths, meaning it's possible for current to use the ground as a return, meaning anything grounded could become a current-carrying conductor (such as grounded metal equipment housings, like your microwave, toaster, washer, etc). Bad news!

Code specifies (in a residential application) one neutral to ground connection: at the main service entrance (which you have). Do not make a second one anywhere on the same system.

  Thanks well explained..........

[MountainDon]

As has been said, the answer here is no, you don't need to. ....................
................Code specifies (in a residential application) one neutral to ground connection: at the main service entrance (which you have). Do not make a second one anywhere on the same system.

Just to be clear on this......  The presence of a ground rod does not necessarily mean there is also a bond between the neutral and the ground. It is absolutely true true that there should only be ONE BOND in a system, but I'm not certain if the presence of a ground rod (W/O a Bond connection).      If the subpanel is detached from the main panel is it not OK to ground the sub panel to Earth as long as there is No Bond at the subpanel?

I'm asking because the OP was questioning the sub ground rod and Did state that the sub panel would Not have any Bonding. 

[rick91351]

MD I did not catch that about the non bonding issue.....      Not following you on this one...... 

[flyingvan]

  Keeping up with current events---

I was required to bond my subpanel at the house to the foundation, gas piping (which is buried in the yard) and cold water lines, so I suppose grounding each panel is a good thing.  I'll echo what was said, too---the ground and neutral are only bonded at the main panel, nowhere else.

   Objectionable Current--
http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/ObjectionableCurrent~20020610.htm

[MountainDon]

If I understand Grover's situation there is a pole with meter and a main panel in the ground outside. This was the construction meter/panel system.   Now the cabin is built and the cabin will have a panel inside, connected to the outside panel. Outside = Main; inside = sub panel. Or am I misinterpreting something? 


The Bond is the connection between the neutral bus in the main panel and the ground bus in the same main panel. There must be only one Bond in a system. That should be at the main. If the main was removed and there was only a panel inside then the Bond would be there.    Anybody with information that indicates that is wrong can, should, please point out the error in my thinking so I can correct myself, my info. Thanks. References or explanation of where to look would be appreciated.

[_JT]

Just to be clear on this......  The presence of a ground rod does not necessarily mean there is also a bond between the neutral and the ground. It is absolutely true true that there should only be ONE BOND in a system, but I'm not certain if the presence of a ground rod (W/O a Bond connection).      If the subpanel is detached from the main panel is it not OK to ground the sub panel to Earth as long as there is No Bond at the subpanel?

I'm asking because the OP was questioning the sub ground rod and Did state that the sub panel would Not have any Bonding.

Excellent question, Don. I wasn't addressing the OP because I wanted to interject and emphasize the neutral/ground bonds. The answer to your (and his) question is yes, it's fine to install a ground rod at a subpanel so long as you make sure you remove the bonding strap between neutral and ground bus. In fact, in some localities I believe the ground rod is required (for a subpanel in a detached structure).

ETA:

Code Select Expand


250.32 Buildings or Structures Supplied by a Feeder(s)
or Branch Circuit(s).
(A) Grounding Electrode. Building(s) or structure(s) supplied
by feeder(s) or branch circuit(s) shall have a grounding
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 grounding
electrode, the grounding electrode(s) required in 250.50
shall be installed.

ETA2: Apparently, it gets even more interesting than I'd previously thought. In some cases (depending on what version of the code you go by and how old your structure is) it's ok to bond neutral and ground again in a subpanel for a detached structure, provided it's a 3 wire feeder (that is, 2 hots and a neutral; no ground connection to the main panel). I was not aware of this wrinkle, although I maintain that best practice is to isolate ground and neutral at the subpanel and drive a new ground rod (possibly 2), connected to the ground bus of your panel.

[_JT]

The Bond is the connection between the neutral bus in the main panel and the ground bus in the same main panel. There must be only one Bond in a system. That should be at the main. If the main was removed and there was only a panel inside then the Bond would be there.    Anybody with information that indicates that is wrong can, should, please point out the error in my thinking so I can correct myself, my info. Thanks. References or explanation of where to look would be appreciated.

Correct. With a caveat. The 'main panel' is the panel that has your main breaker for your entire supply (commonly 200A 2P for residential supplies). Whether that panel is located inside the house or outside the house doesn't change that, and you'll only have one main breaker, regardless of how many subpanels there are or where they're located.

Section 250 of the NEC has all the info you could ever want on grounding, although you must supply your own headache medicine.

[rick91351]

I'm OK    I remember my buddy the electrician - (now owns a coffee shop and no electrical licence.)  (( Says less headaches and more $$))   When I ran my sub panel to the RV pads he had me do exactly what you posted.  I removed the bonding strap and drove in a ground rod.  At that time I think you could get by with just the one.     

[MountainDon]

Thanks for the clarifications _JT.  Much appreciated. Electrical can be confusing.

[_JT]

Indeed! I do it for a living and still get twisted up around the code requirements fairly regularly.

[flyingvan]

A marginally related question I've had (since Rob_O brought it up in the portable generator thread)--- My backup generator is isolated by a power transfer switch, but only the hots---not the neutral.  I took a close look at the wiring schematic for the generator and I don't see anywhere where the neutral and ground are bonded, so it's still only at the main panel. 
Without the neutral being switched, is there any situation where I'd be energizing the neutral beyond the cottage?  It doesn't seem like it---I'd think the sum of the two legs would always equal the neutral at the generator, and NEC doesn't require the neutral to be isolated, but I want to be sure---don't want to harm any repairmen, and our power gets knocked out often

[_JT]

Just so I'm clear, the question is: is it possible for your generator, while running, to energize the neutral at your house, possibly backfeeding back through your transformer and energizing the overhead powerlines?

If your neutral and ground are properly bonded at the main service entrance as required by code, the answer under all circumstances I can think of is no. The reason that ground connection is required in residential applications is so that, if there is a ground fault (hot wire shorts directly to ground), the fault current is high enough (because the impedance between neutral and ground is very low [direct bond]) that a circuit breaker trips. The breaker protecting whichever hot contacted ground. So energized neutrals are not something we should ever see on a properly connected residential system.

If things have gotten horribly miswired, I suppose it would have to consider that a possibility. Though still an unlikely one.

[flyingvan]

Yes, you answered my question, thanks! 

  I'll throw myself under the bus here with the only wiring mistake I made (that I'm aware of----it's been completed for well over a year and all is well so far)  and I corrected it before I actually ran the generator----


 

[dakramer]

A panel in a detached structure is required to have a made electrode (rod, pipe, plate ufer etc) NEC 250.52. If the ground is not brought over do not separate the ground and neutral from each other since you are not allowed to use the earth as a ground conductor since it will not reliably trip a circuit breaker. If the ground is brought over use a separate ground bar for the ground and isolate the neutral.

[dakramer]

NEC 250.52 requires another made electrode at the separate building. a ufer is the preferred method.

[dakramer]

ground rods are not to be driven at an angle to exceed 45 degrees. If a rock bottom is encountered they can be laid in a trench at least 30 inches deep. 

The downsized neutrals are permissible if the load on the neutral does not exceed the capacity of the wire, do a load calculation to determine the loads on the various conductors.  typically a few  of the loads will be fairly large 220v loads that do not use the neutral, also the neutral only carries the unbalanced load of the other two conductors. harmonic loads such as computers and fluorescent lighting can cause a large neutral load but these are generally only seen in commercial installations some can require oversize neutrals.

[dakramer]

On the question of how many ground rods are required, the NEC requires a ground of 25 ohms or less, that could be one electrode. How ever The IRC may require two, this is a requirement put there in order to be different than the NEC it protects them from copy right infringement. IRC requirements will be more stringent than NEC or uniform building codes because they have to make changes to protect themselves from copying the other published codes. Writing code books is a multibillion dollar industry. hundreds of thousands of jurisdictions buying books at more than 100.00 each and thousands of contractors buying them as well. If you use the IRC your structure will cost thousands more than if you follow the NEC and the rest of the uniform building codes. the authority having jurisdiction chooses what codes to use it's usually a Hodge podge of codes based on the preferences of the head inspector.

[Don_P]

Thanks for resurrecting this, my barn is wrong 
I ran 4 wires; 2 hots, a neutral, and a ground from a breaker in the main panel. There is a bonding jumper in the panel in the barn from neutral to ground. So as I'm understanding this, the current would be just as happy to take either path back to the main panel. Or, if I grab Dad's old metal bodied non double insulated saw and walk outside barefoot, the return path is just as happy through me. I'm understanding that I need to disconnect the bond and isolate the neutral from the ground. Then, it is a good idea to drive another ground rod at the barn hooked only to the ground.

At last check I think 48 states were under the IRC, code adoption is at the state level rather than the jurisdiction level. An AHJ can try anything, you have to know when he is piling it on and call it sometimes. I can't speak to the NEC but I'll bet it is incorporated into code by direct reference as are other documents like the NDS, copyright isn't an issue... Somewhere in the depths of one of the later chapters of the IRC. But no doubt about it, the ICC is the international cash cow. Although, I haven't had to buy a codebook in well over a decade, it has been freely available online. IMO all laws should be chiseled in a stone set in the village square for all to see.