Detached building: Subpanel grounding and feeding


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Old 09-30-08, 01:16 PM
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Detached building: Subpanel grounding and feeding

Here is my situation: I built a detached garage and I want to add power to it. I have 200A underground service from the power company coming into my meter main attached to the outside of my house. There is a 100A 2-pole breaker in the meter main that feeds my house panel. There is space to add another 100A 2-pole breaker in the meter main.

My plan is to put a panel in the garage and feed it directly from the meter main. I have already bought the 100A 2-pole breaker (not yet installed) and buried the cable from the meter main to the garage. When I was figuring out what I needed for cable, I asked the guy at home depot and they sold me #2/#2/#2 AL SEU cable. The cable is buried, but not connected at either end yet.

After reading more about subpanels on the forums, it seems I should have buried a 4-conductor cable rather than a 3 conductor.

I plan on connecting the 2 hots of the 2/2/2 to the 2 poles on the new 100A breaker and the other conductor to the neutral bar in the meter main. On the other end, I plan on connecting the 2 hots of the 2/2/2 to the main breaker in the subpanel (which I haven’t bought yet) and the other conductor to the neutral bar in the subpanel. I plan on making sure the neutral and ground are NOT bonded in the subpanel. I plan on grounding the subpanel by connecting the ground bar to two 8’ ground rods (already installed 8’ apart) and back to the ground bar in the subpanel in a loop. I have no metal piping in the garage to ground to.

So, my questions are:
1. Will my plan work?
2. Is it SAFE?
3. Is it up to code?
4. Is a 100A or 125A panel with a main breaker OK for my use?
5. What size of copper wire do I need to ground my subpanel as described above?


I live in a rural area of Alaska. I assume that the borough government (similar to a county) has incorporated the NEC by reference, but there are no local codes, no inspections, no inspectors, no real requirements, etc. Therefore, I am not too concerned about my installation meeting code, as long as it is safe.

Thoughts and suggestions are welcome!
 
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Old 09-30-08, 01:54 PM
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Based on some quick searching, I believe that Alaska adopted the NEC 2008 edition as of June this year; which is bad news for your installation in terms of code-compliance. However I should note that your installation would have been allowed under the NEC 2005. So the question about safety is one of tiny percentages (allowed in May, not in June). Compliance with '08 code does provide a somewhat safer installation, but as-is yours certainly is not unsafe. Since there seems to be no code enforcement in your locality it's up to you.

I'll touch on the important points between '05 code and '08 code:

#2 aluminum used to be allowed for 100A subpanels under an interpreted wording of a code article. They have now explicitly changed the wording so that #2 aluminum is good for 90A to a subpanel.

Three-wire feeds (no ground) used to be allowed to an outbuilding provided that no other metal pathways like a water line or phone line were present. Now, four-wire feeds are mandatory, no exceptions.

The ground rods are required in either case, and they should be connected to the subpanel ground bar with #6 copper wire. When a three-wire feed is used, the neutral and ground bar must be bonded; when a four-wire feed is used, the neutral and ground bars must not be bonded.

The rating of the main breaker in the subpanel doesn't matter, because it is only acting as a building disconnect. The overcurrent protection is provided by the breaker in the meter/main panel which feeds this outbuilding.

All of the 120V receptacles in the garage should be GFCI protected, either by GFCI breaker or by GFCI receptacle with downstream "LOAD" protection wired. This also includes special-purpose outlets like garage door opener and lighting.

I believe your installation is safe as there are millions just like it throughout the USA, and given the lack of code enforcement and inspection, not worth the cost to bring up to '08 code for very marginal improvements in safety. Something you may wish to consider as an easy thing to fix is returning the 100A breaker for a 90A breaker if it's available for your meter/main panel.
 

Last edited by ibpooks; 09-30-08 at 02:05 PM. Reason: added info
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Old 09-30-08, 02:17 PM
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Thanks for the quick reply, Ben. A good idea about the 90A breaker vs the 100A. I'll see if I can return the 100A for a 90A. And, I will leave the neutral and ground bonded together since it is a 3-wire feed.

Regarding the 3-wire vs 4-wire...I don't have a metal waterline connecting the house and garage, but I will have a phone line and coax TV cable. Will that change the safety of my proposed grounding system? Or will it still be 'essentially safe', just not to code? Do I need to do the loop as I proposed, or can I just go from the ground bar to the two ground rods without coming back to the panel?

I suppose I can buy a bunch of copper wire and dig another trench so I can use a 4-wire feed, but I've got about 100 feet of trench, and I'd rather not do that if it isn't necessary.

Thanks again!
 
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Old 09-30-08, 02:56 PM
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Originally Posted by jrypka
Regarding the 3-wire vs 4-wire...I don't have a metal waterline connecting the house and garage, but I will have a phone line and coax TV cable. Will that change the safety of my proposed grounding system?
That does complicate it, because that hasn't been allowed for quite a few code revisions back. It can present a safety issue where objectional current could travel on the phone or cable wire which may damage equipment or cause a shock when you unscrew the coax connector from the TV. The more likely problem is poor signal quality (crackly phone, fuzzy TV) or damage to the equipment from electrical storms. I can't say one way or the other what the chances of trouble are, just that there could be problems.

Do I need to do the loop as I proposed, or can I just go from the ground bar to the two ground rods without coming back to the panel?
Loop is not required.

I suppose I can buy a bunch of copper wire and dig another trench so I can use a 4-wire feed, but I've got about 100 feet of trench, and I'd rather not do that if it isn't necessary.
It's usually customary for all 4 wires to be in the same trench, but I can't really think of a specific problem with the ground being in a neighboring trench. The ground should be #8 (or larger) copper or #6 (or larger) aluminum with a direct-burial rated insulation (UF, USE) if you choose to install it.

Perhaps explore some wireless options for the phone and TV? There's pretty good cordless phone technology out there that would probably work just fine, but I'm not certain about the CATV. Companies like SmartHome have a lot of that sort of stuff which might help you out.
 
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Old 09-30-08, 03:36 PM
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[QUOTE=ibpooks;1438606]All of the 120V receptacles in the garage should be GFCI protected, either by GFCI breaker or by GFCI receptacle with downstream "LOAD" protection wired. This also includes special-purpose outlets like garage door opener and lighting./QUOTE]


I wouldn't use receptacles with the lights. Especially if you are going to install fluorescent fixtures. JMO.
 
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Old 09-30-08, 05:04 PM
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I asked the guy at home depot and they sold me #2/#2/#2 AL SEU cable. The cable is buried, but not connected at either end yet.
You direct buried aluminum SEU? If so you have a problem.. can you clarify?

This would be SEU.....

 
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Old 09-30-08, 05:43 PM
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I was wondering if they meant USE and not SEU too.

If you have SEU it is not meant nor allowed to be buried.
 
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Old 09-30-08, 06:10 PM
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OK, I guess I didn't get the nomenclature right, but at least I think I buried the right stuff. It does NOT look like the picture posted by Bruto. I'm struggling posting a picture, but it looks like the 4/0 4/0 2/0 x 500' triplexed wire shown here:

http://electrical.hardwarestore.com/14-48-entrance-cable/4-0-4-0-2-0-x-500''-aluminum-triplexed-wire-658724.aspx

I think the only differences between that link and what I buried was (1) the size and (2) mine had one conductor with a green trace rather than yellow. The other two wires just had black insulation. Also, I don't remember if it was #2 or 2/0, but they said it was rated at 100A.

Any other opinions on my grounding situation? I can get by without the phone line, and I buried the coax that has the ground wire with it. Will that get me around the issues that ibpooks noted?

Thanks again! -Jeff
 
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Old 10-01-08, 11:07 AM
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If the cable has USE, USE-2 or some reference to direct burial its fine. Though USE without the (-2) is not allowed to enter a building and is usually terminated to 3R enclosures that are on the outside of a building. Only way to know for sure is to post the specifications written on the outer insulation so we can tell if you have direct burial wire.
 
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Old 10-05-08, 11:10 PM
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Sorry it took me a while to repost...its been a bit crazy around here.

Anyway, I checked the wire, and it is Type USE-2, so I should be OK for burial and entering a building.

So, if I forget about the phone line, do you think I'll be safe to go with the grounding system I proposed, or will I have a big fire or shock risk?
 
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Old 10-06-08, 08:15 AM
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Originally Posted by jrypka
Any other opinions on my grounding situation? I can get by without the phone line, and I buried the coax that has the ground wire with it. Will that get me around the issues that ibpooks noted?

Thanks again! -Jeff

Talk to the electrical inspector. The coax cable presents a problem with grounding. Normally, the grounded conductor (neutral) is floated (isolated) at the subpanel when there are other conductive paths between two buildings.
 
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Old 10-06-08, 10:46 AM
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Well, thats the problem. We have no electrical inspector. I guess I can call an electrician, but I figured I'd post here instead because I trust the experience of the electricians on this board more than Joe-Blow electrician in BFE Alaska.

So, from what I understand from you guys, I either need to dig another trench and run a fourth conductor for a ground, or I need to forego the phone and TV cable and ground with the ground rods like I was planning? Any other ideas that don't result in me digging another trench and buying more wire?

I thought I had this all figured out before. I guess I shouldn't have asked so many questions and just proceeded as planned. Ignorance is bliss!
 
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Old 10-06-08, 11:37 AM
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Given that your trench is already filled in, I would probably leave it as-is and proceed with the phone and cable. While the ground wire does add some margin of safety, there are millions of installations without it that are just fine. I certainly wouldn't lose any sleep over the safety of this install, but if you do have problems with the phone or cable after it's hooked up, you've at least got a good starting point for the diagnosis. You should also be more aware of symptoms of a broken neutral in the outbuilding like brightening of lights when heavy load is switched on (instead of the usual dimming), bizarre behavior of 240V appliances, power outage in part of the building without tripped breaker. As these (somewhat rare) problems become more dangerous without a ground wire present.

With the coax, I would recommend that you install a grounding block and bond it to the outbuilding ground rods where the coax enters the building.

 
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Old 10-06-08, 12:06 PM
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Thanks for the tip on the coax ground block. Do I just attach the ground wire from my coax to this, and then run copper from there to the ground rod?
 
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Old 10-06-08, 01:35 PM
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Yes, the coax ground wire and outer shield conductor by way of the F-type screw terminal are grounded. Something like a #12 copper runs from the ground block to a clamp on the ground rod. It doesn't have to attach to the rod itself; you can clamp it to the bare GEC which goes to the ground rod using a brass/bronze copper split bolt.
 
 

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