I'm installing a new 200 amp electrical service to my shop and have a question about the equipment ground. The meter and main breaker are about 125' away from the branch circuit panel on another building, and will be connected by 3-3/0 cu current conductors in a 2" underground PVC conduit. The neutral will be bonded to the ground rod at the meter can (only) as per local requirements.

What size grounding conductor should I use between the meter/main and the branch circuit panel?
Should, or could I put another ground rod at the branch circuit panel and bond all the equipment grounds together? (I know, not the neutral) Pros/cons?
Should I bond the BC panel to the metal skin of the building? The building is wood frame, and the panel is mounted to wood. The skin will be inadvertantly connected to the ground, but only thru a few 12ga branch circuit grounding conductors at locations where the metal outlet boxes are mounted to the skin.
I read a post a while back about suplemental grounding electrodes, and I'm still not sure what's best.
I'm out in the county, so there's no inspector I can call on cept the REA guys that only care what you do up to the meter/main.
Thanks, Rob

 

You don't want to groud the steel in this application, You could be well served to use #2, I don't have my code book handy.
A ground rod at the branch circuit panel is not a good idea either although some electricians do this.

 

Why is a ground rod at the circuit panel a bad idea?

 

Since you have a evidently have a breaker at the meter end, in series with the connection to the shop, the shop is connected to a "feeder" circuit.

In this situation the equipment grounding conductor is sized according to article 250.122

Assuming you have a 200 amp breaker supplying the feeder itself, article 250.122 calls for a minimum of 6 AWG copper or 4 AWG aluminum equipment grounding conductor. The intent of this code section is to provide a sufficiently hefty conductor to carry the available fault current long enough to trip the breaker protecting the branch circuit.

You can go a little larger if you would like; maybe 4 AWG copper. But any conductor 125 feet long is pretty worthless for lightning or transient protection. It does not matter how big you make it, the dynamic impedance is too high.

A ground rod is not required in this circumstance for electrical safety. You are allowed to add one, but in a dry place like West/Central Texas, a ground rod is an iffy proposition -- typical resistances would be well in excess of 30 ohms. Too little contact area, too high inherent resistance in the dry ground.

The best solution for lightning safety would be a Ufer ground (connect to rebar in the footer).

If you have an electrical ground rod, and you also have a lightning protection system, then they are required to be bonded together. The way I read the code, you need not bond the lightning protection system to the equipment grounding conductor if you do not have a specific electrical ground rod. But I find this section (250.106) a little confusing.

 

Just a clarification. The additional ground rod at the branch circuit panel that i'm asking about, would be in addition to the grounding conductor between the main and the BC panel.

 

Thanks fefarms, I was hoping a #6 would be adequate since I have a big roll just laying around. (wish I had 400' of 2/0 or 3/0 just laying around!)

There is no lightning protection system for the shop building, I'm just a big fan of grounding. Electrolisis, ground fault current path, static charge, ect.

 

I mis-remembered article 250.32. A grounding electrode IS required if you have a panelboard in a remote building, whether or not a separate equipment grounding conductor is supplied. (It is not required only in the case that you have just a single branch circuit to the remote building and supply an equipment grounding conductor as part of the branch circuit.). So a chicken house with a couple of lights and one recptacle might not need a ground electrode, but a full-fledged shop will.

The grounding electrode should be bonded to the equipment grounding conductor and to the panelboard enclosure, and to such things as metal water pipes and structural metal framing. The grounding electrode can be a driven rod, underground metal piping, a Ufer ground, a ground ring, etc.

If you use 6 AWG wire for an equipment grounding conductor, it must be bare or have green insulation. The re-identification exception of 250.119 doesn't kick in until 4 AWG.

 

Disconnect on the pole, This would not qualify as a seperately derived system, since it has its own meter and Main. And if the he is running the #6 ground, back to Main, he has ground rod installed there, He may put in another ground rod, as long as the 2 are bonded together, but is it necessary for a second ground rod on this system? why?

 

The new meter and 200A main circuit breaker are only for the shop, and no other loads. They are located on the far end of a pole barn that is next to my shop. Not on a pole only. So which is it, to rod-or Not to rod?

Also, are you saying that green tape is not OK on #6?

 

New codes require colored ground and Neutral, for #6 conductors and smaller

 

If I'm not mistaken, your meter and service qualify as an AC system and need to be grounded at the service. Since you have 3/0 wire coming into the service entrance, you should have, at least, #4 wire for ground. I'd use #2 for overkill, just like I usually see #6 ground wire for 2 or smaller, when I think 8 is code. If you want to use beefy power wire, better use a beefy ground.

The ground wire can be coated or bare. If coated, it must be green or you it can be marked with green tape at the exposed insulation.

You're spending a bit of cash on that 3/0 wire, if you sink a ground rod directly below the service entrance, you'll only need to buy a few feet of #2 bare to connect up the ground.

Doesn't really have to make sense, just has to follow code and be something the inspector is used to seeing. That ground wire is usually the first thing the inspector see's when he walks up to the box. The trick is not to suprise him. You want him to drive up, spend 10 minutes looking everything over, and drive away. If he has to question anything and pull out the NEC, just means more possible work for you. Take a look at some of the newer installs in your area. Try to make yours look similar.

 

Keep'em comming guys, I'm still a little confused on the grounding issues. (Not to mention current code/NEC) I do so little electrical work anymore, that I'm a little rusty. (IBEW/NECA JATC '74)

As fefarms pointed out, The grounding conductor should be sized to handle the max ground fault to trip the over current protection device. And as I have stated, I like grounds, grounds are good. I'v seen way more weird events with "grounds" than I've ever seen on the current side of a circuit.

But, as I understand electrical theory, the potential to ground from a 120/240 1PH system is only there because the neutral of the secondary of the distrabution transformer is connected to earth (ground). Without this neutral bonding (or any other point of the transformers secondary windings) there would be no potential to ground. With out any secondary connection to earth, you have an isolation transformer. No potential to ground.

So I guess what I'm trying to think out here is that the potential is really just trying to get to the neutral, and will use the earth if it's condusive.
Under current practices of bonding the neutral to earth, it seems to me that any/all the additional earth bonds would be a plus, in the event that the grounding conductor should loose continuity.

Somehow I think there's more to it than this, otherwise more grounding and more than one point of ground bonding the neutral should be good.
Anyone care to enlighten me/us here?
Thanks, Rob

 

You've almost got it...

You understand how the two "hots" come in from the transformer with "neutral" to complete the circuit. There only needs to be one neutral wire, because of the phase difference between the two hots. The neutral is shared between the two hots as they flip flop between the two phases and is only used by one hot at any one time.

That's why the neutral can be sized the same as the hot, (not always, but for the sake of discussion). This can work fine and it's easy just to refer to neutral as ground - but you should never forget it's a reference and just used to complete an ideal circuit.

The code is there for safety on those, "what if", situations. What if there was a power surge or lightning strike which made both your "hots" in phase? Now your neutral wire is only half as big as it needs to be and, like in the case of a lightning strike, the current will be looking for a parallel path to ground. What if the neutral got energized or somehow became open? Same thing, the ground becomes a backup.

They size these ground wires partly on what they can take in a worse case. If something strange happened, what size wire would it take to make the return path seem normal. Because if it's not normal, like if the wire "feels" too small, it's a resistance. You might be touching something on the grounded side of a piece of equipment and turned a series circuit into a parallel circuit - even if it is a shorted ground, it can be a resistance.

Took me a while to understand this. Like, why the heck do I need to ground the metal coverplate on a recept. with 12 when I'm doing EMT? Why can't I use 14? Because the NEC says so - it rarely says why, but there's always a "what if" somewhere, if you think hard enough.

 

Maybe it's too late in the day, I'm a morning kind of guy, but....SAY WHAT?

My utility electrical service is single phase. I bet your's is too.
The primary side of the transformer of your typical residential system is also single phase (even if 3 phsae is around). Another good bet is that one leg of that primary phase is grounded. But the primary is totaly isolated from the secondary in an AC transformer, except in what is called an autotransformer.
The secendary side of the transformer (the side we use, 120/240VAC) is single phase 240 volt with a center tap on the winding for 120V. It's this center tap that we call the neutral. "Neutral", only because it's connected to earth (bonded,grounded). Common leg is also another term that is probably more accurate.
Anyways, the potential from any side of the secendary only seeks ground because the center tap is bonded to ground.
It' too late, I need my sleep. Will check in the Am to see if this makes any sence.

 

What fefarms said is true, BUT there is a formula based on the length of conductor,reistance per foot to figure this, so taking your linemans and cutting into a 12/2 romex conductor, that is say 125' long, and you cut it live at 100'(example) at that instant the amps would be like 7000, when the breaker trips. I won't go into th whole formula, because it varies from the size of breaker to size of conductors.

This is pretty irrelevant to your situation but I find it interesting.

Also, you'll have half the current on the primary that you have on the secondary.

anyway back to the original question, If funds are not an issue it wouldn't cost that much to go above and beyond the code, Put in the ground rod at the shop, bond the panel with the little green screw, and pull in a #2 copper, you won't regret it.

If you look at a 4 conductor direct bury alum cable, the ground will be #2