Jump to content

Sub Panel / Grounding Electrode Question


Brian G
 Share

Recommended Posts

Question:

If you have a main and sub panel in the same building, are you allowed to have a grounding electrode at each one? They would both be bonded to the same grounding system of course, but would a second electrode in a different location create any problems (differences in "potential", ground fault paths, etc.)?

Brian G.

Always Gotten By Fine With One Rod Myself [^] [;)]

Link to comment
Share on other sites

There's nothing I know of that says you can't. (but why would you?) I think it would be no different than 2 grounding electrodes for the service equipment.

http://uploads/inspecthistoric/200638185551_outbldgsub.gif

I've been having problems with sub panels in outbuildings. I'm doing inspections on farms for new buyers that I've inspected before (prior to the '99 NEC when sub feeds were treated as SECs and the neutral conductor was required to be bonded to the outbuilding's enclosure and connected to a grounding electrode). The sellers (my clients before '99) are pissed that I'm saying it's wrong now.

Link to comment
Share on other sites

Originally posted by Brian G.

Question:

If you have a main and sub panel in the same building, are you allowed to have a grounding electrode at each one? They would both be bonded to the same grounding system of course, but would a second electrode in a different location create any problems (differences in "potential", ground fault paths, etc.)?

Brian G.

Always Gotten By Fine With One Rod Myself [^] [;)]

Well, I can't find any prohibition against it. The grounding connection to the earth really has nothing to do with equipment grounding, so, if there's going to be a problem, it'll have to do with outside influences on the system such as lightning strikes, power surges, stray currents in the earth, etc.

You might want to post this on the ASHI forum. Douglas might have more insight. He really knows article 250. I fall asleep every time I wander into it.

- Jim Katen, Oregon

Link to comment
Share on other sites

Originally posted by Brian G.

Question:

If you have a main and sub panel in the same building, are you allowed to have a grounding electrode at each one? They would both be bonded to the same grounding system of course, but would a second electrode in a different location create any problems (differences in "potential", ground fault paths, etc.)?

Lets take the extreme case where you have one set of ground rods and the service equipment at one side of the house and the sub panel and the other ground rod(s) on the other side. The assumption is that you have a four wire feed to the sub panel running through the middle of the house. Lightning strikes the ground on one side and, because of differing potentials, grabs the easy path through the house on the grounding conductor between the two panels, frying and/or energizing the hot and neutral feeders as it goes.

I can't quote code for this, but it just seems wrong to have a supplemental grounding electrode connected to the main grounding electrode by means of what is essentially an equipment grounding conductor run with feeders through a home. I realize a similar situation can exist with a separate building, but those feeders are normally mostly buried (or dangling) outside.

I think I'd remove the sub-panel GEC as it's not normal, required, or needed.

Just my opinion of course. If I was actually capable of having a total understanding of article 250, I'd probably be teaching quantum mechanics and string theory rather than crawling under houses. [:-boggled

Link to comment
Share on other sites

  • 2 weeks later...

I just got in my new, spiral-bound copy of the 2005 NEC Handbook, so I went digging. I don’t know that this is exactly dead on point (specifically about sub panels), but I do believe it addresses the issues we’re looking at in this situation.

Article 250 – Grounding and Bonding

Section 250.54 Supplementary Grounding Electrodes

Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53 C or the resistance requirements of 250.56, but the earth shall not be used as an effective ground-fault current path as specified in 250.4(A)(5) and 250.4(B)(4).id="maroon">

Crystal clear, right? [:-boggled But then comes the commentary, and I think the first sentence covers it (underlining is mine).

Grounding electrodes, such as ground rods, that are connected to equipment are not permitted to be used in lieu of the equipment grounding conductor, but they may be used for supplementary protection at electrical equipment locations. For example, grounding electrodes may be used for lightning protection or to establish a reference to ground in the area of electrically operated equipment. Sections 250.4(A)(5) and 250.4(B)(4) also specify that the earth may not be used as the sole equipment grounding conductor or effective (ground) fault current path. Supplementary grounding electrodes are not required to be incorporated into the grounding electrode system for the service or other source of electrical supply.

So as I read that, there seems to be no concern for any differences in “potentialâ€

Link to comment
Share on other sites

Hi,

Don't know if you're interested, but one of the downloads that we have in the downloads library here on TIJ is an Army Correspondence Course on grounding - Install System, Equipment and Component Grounds. Here's the shortcut.

Once you get done with that one, you might try some of the others. Just go to the blue menu bar above where it says 'resources' and choose downloads and check out TIJ's library.

ONE TEAM - ONE FIGHT!!!

Mike

Link to comment
Share on other sites

Brian,

Your a godsend. I definitley need a NEC spiral and was shy about purcahsing the 05 at prices I was seeing advertised with ICC literature. Speaking of ICC, I have to test for the IRC electrical soon and that NEC 05 may come in handy since I'm a little rusty on my load calculations.

Thanks for the tip !!!!

Brad

Link to comment
Share on other sites

Glad to be of help there Brad. This is the biggest spiral bound book I've ever seen. The spiral is about 2" in diameter with over 1300 pages. But hey, it lays flat and you can fold it around to take up only a large-ish page-sized space.

This is my first handbook. I love it already. True, the commentary isn't code itself, but boy does it help clear things up (see the above). Charts, illustrations, photos, diagrams...yowsa!

Brian G.

Breaking the Code [:-magnify

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...