Code Requirements for Grounding

[Haldor]

Please excuse me if this has been asked before, I searched the forums, but didn't find anything that was directly related to what I am interested in.

I was asked the following questions about the proper way to resolve an issue involving power grounding for technical power to be used with a sound reinforcement system in a school auditorium. The questions asked bring up more questions to my mind about what is the correct way to proceed and I admit I am beyond my depths here. Here is his question to me (please bear with me on the length of the questions, but I didn't want to edit out any possibly pertinent data):

Our local High School has come up with a little cash to make some upgrades in their auditorium. This is an old building with old wiring back before they were installing proper mechanical grounding. So one of the first things I would like to see happen is address that.

If I am understanding this right, we install the ground rod at the service entrance and any sub fed boxes have their ground and neutral separated so there is no cross connection except at the service box. Our problem with this is that the service entrance really isn't a service entrance. This building is fed from the main service entrance/distro gear located in another building. In other words, the two 350 amp primary distribution boxes in this building are already subfeeds. Being such an old building, there is
no mechanical ground pulled from the main to these boxes and certainly no chance of getting one installed. (BTW. Even though we have two 350 amp boxes in this building, they are fed by a single 350 amp single phase feed from
the switchgear).

Would it be worthwhile/beneficial/appropriate for us to install a ground rod at our two boxes is this auditorium? There is an old sub fed box (from one of our two main boxes), located in the lighting booth that I want to replace with a more modern box and have appropriate mechanical grounding available. This box is feeding the sound system, lighting control board, a computer for PPT stuff, an Altinex switching system, and the projector. I want to be able to pull some new circuits from it all the way to the stage for plugging in audio gadgets there. That way I could split the ground and neutral in this box."

This building is located in Illinois.

Please feel free to suggest any pertinent section of the NEC code that addresses this issue. I have already suggested he contact a local licensed electrical contractor to take care of this, but would really like to what regulatory issues are involved. Is it permitted to install a ground rod at the building entrance when the power is being supplied from a second building that has a ground rod installed at it's point of entrance (I assume/hope)? Does it meet NEC and local code to supply power from one building to another in this fashion? If this is legal than what is the proper way to resolve the grounding issues.

Thanks very much for any help you can provide. I am an EE, not an electrician, so I definitely don't want to tread beyond my area of competance. At the same time, I do want to know what the correct solution is, since technical power is such an important issue for sound systems (which I do know a lot about) and problems involving grounding come up fairly regularly.

Phil Ouellette

 

[hbiss]

Re: Code Requirements for Grounding

I don't see any regulatory issues. You can drive as many ground rods as you want but they won't help you. The fact that there are no ground conductors installed with the feeds may also be a non-issue. If they are in a metallic conduit none are required even now. The conduit serves as the grounding conductor.

What will be important is that when you install your circuits and equipment every ground- which means receptacle grounds, the conduit and the racks themselves goes back to one common grounding point on the ground bus in the new panel. When you design your system, if you can see more than one way back to ground you may have problems with ground loops. I think if you are careful about this 99% of any problems will be eliminated.

I'm sure Dereck will have a few more suggestions in the way of isolated power which you may want to consider.

-Hal

 

[ryan_618]

Re: Code Requirements for Grounding

It sounds to me like you might want to read 250.32, seperate structures supplied from a common service. I beleive that is what you are describing.

 

[hbiss]

Re: Code Requirements for Grounding

It sounds to me like you might want to read 250.32

Yes, if this were new construction. Kind of a moot point here. Ground rods are just for lightning protection in that instance and will do nothing for "technical power".

Like I said, it's everything that you do AFTER the new panel that you are installing. If you then have power quality issues they can be addressed with isolated power.

-Hal

 

[ryan_618]

Re: Code Requirements for Grounding

Hal: The original post was: "Please feel free to suggest any pertinent section of the NEC code that addresses this issue." That is what I did. I included this to address the lack of an equipment ground between the structures, which might not be an issue if there are no metallic paths between the structures.

 

[dereckbc]

Re: Code Requirements for Grounding

Although NEC permits supplemental ground rods to be added, I doubt it will give you the desired effect you are looking for. All the noise you are trying to eliminate is called common mode noise, and all the ground rods in the world will not eliminate it.

Not sure how much of a budget you have to work with, but the absolute best solution is a shielded isolation transformer in its many forms. They can offer up to 140 db of common mode rejection ratio. The transformer will establish a new ground point to referrence the sensitve equipment. If your power requirements are 7.2 KW or less, and all the equipment is in the same room, you can install a rack mounted balanced power system. All the electrician needs to do is install a single dedicated 240 volt @ 30-amp circuit to power the transformer, then plug-n-play the equipment in.

Or you could install a conventional dry type transformer, panel, and dedicated 20-amp circuits. Lets us know what your capabilities and budget are, and we may be able to come up with some more ideas.

 

[Haldor]

Re: Code Requirements for Grounding

Thanks for the quick replies. I will look up section 250.32 of the NEC tomorrow.

Ok now I am going to really show my lack of knowledge off, be gentle this isn't what I do for a living.

My understanding is that the only time the neutral wiring is supposed to be connected to ground is at the bonding strap inside the circuit breaker box. This is to ensure that a ground fault will trip the breaker. In addition, the system ground is supposed to be connected to earth ground near the service entry point. The only time you are normally permitted to make an additional ground to neutral bond is at a separately derived power source (isolation transformer). That is about the limit of my knowledge on the subject.

To turn this into a question, does any of this matter with respect to supplying service between two buildings? In short does it make any difference if the service entry at the first building is grounded and the service entry at the second building (powered from the first building) is grounded or not grounded? One additional question is if the feeders are in metallic conduit or not. If the feeders are not in metallic conduit which means there is no ground bond between buildings then is the ground bond at the second building required?

Thanks,

Phil

 

[dereckbc]

Re: Code Requirements for Grounding

Phil, do your homework first, you will get more out of it. Your answers for seperate structures is in 250.32, SDS in 250.30, and N-G bond in 250.24.

You have the basic ideas correctly, so go read and the pieces will fall together, then we can clrear thing up for you. Also read 647, short article, and it should turn on some lights.

 

[Haldor]

Re: Code Requirements for Grounding

Hi Dereck,

You snuck your post in while I was composing my reply.

I will need to get back to my friend who is actually at the site (I'm in another state). I have a pretty good idea about technical power within the building, the part that was beyond my experience was about bringer feeders in from a second building (I don't get involved at that level). I normally spec technical power (as a consumer, not an electrical designer) as follows:

- Technical power wiring shall be done in accordance with NEC and local requirements.
- Technical power wiring shall be installed in dedicated metallic conduit.
- Technical power outlets shall be wired using isolated, dedicated ground, hot and neutral conductors.
- Technical power outlets shall not bond the technical ground to the metallic conduit (use isolated power outlets).
- Technical power shall be sourced from one circuit breaker box (one of the keys to getting a single point ground).
- Technical power circuits shall be powered from a single transformer leg. Lighting control circuits (dimmers) shall be powered from a different transformer leg (ideally from a different circuit breaker box).

None of the systems I've seen installed in this manner have had hum or interference problems due to power noise or grounding. Am I missing anything?

An isolation transformer can be a big help when fixing existing building wiring, but dedicated conduit, dedicated wiring and attention paid to keeping the technical ground isolated during installation works just as well and doesn't cost significantly more, at least in new construction (or during remodels).

Phil