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

[grover]

I have electric service installed on a pedestal next to my cabin.  On this outside pedestal is the meter with a ground rod and and next to that an outside 200 amp breaker panel.  The panel was installed to run tools while building and for power when camping.  The panel has feed through lugs, meaning it has top lugs where the power comes in from the meter, then the main breaker, and at the bottom of the bus bar is a 2nd set of lugs.  This is where I plan to hook up my lines running into the cabin.  There will be 3, 4/0 aluminum wires plus a ground.  2 hot, 1 neutral and the ground.  These will run underground in conduit and attach to my inside 200 amp panel.  The ground bus and the neutral bus in the sub panel will not be connected and the neutral bus is insulated from the box. 

I hope I have described this correctly and I think this plan is all up to code.  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.

[MountainDon]

With a proper size ground wire between the main and the sub you don't need a separate ground rod at the cabin.  But it is a good idea to have two ground rods at the main that are connected from one to the other at least 6 feet apart, but farther is better. 2X the length of the rods is considered better. The ground wire should be continuous from the panel through the first rod clamp and on to the second. There should be less than 25 ohms resistance; more than that makes the second rod mandatory under NEC. Wet ground is better than dry which is why we direct our gutter water to where the rods are. And if you only have one rod there is no effective way to check. Most contractors and inspectors do not test from what I am told.

[flyingvan]

I have your identical setup at the Cuyamaca Cottage.   Don's right, the requirement was two 8' copper rods at least 6'apart.  Two things you didn't mention, but I suspect you have done anyway since you're dead on with what you wrote--
1) The neutral and ground are bridged at the main panel
2) Your ground at the house is bonded to cold water line, gas pipe, and foundation

[flyingvan]

Oh---before you run your wires to the house, consider putting a power transfer switch just below the main.  It's really hard to add later but easy to do now, then if you ever want to install a generator out at the pole it'll be easy.  A 200 amp power transfer switch will cost about $324 new

[Patrick]

This is what my power compant wante done I know your setup is a bit different but this explains the ground rod placement pretty good.http://dahlberglightandpower.com/wp-content/uploads/2013/06/Underground-to-Building.pdf. And dont forget to wad the paste on those aluminum wires.

[rick91351]

Ground rods can be angled if you can not run them straight down by the way.  For example rocks and maybe that stupid piece of concrete no body know why it is there but it is.  You can use a steel post driver - usually a heavy often weighted pipe about three foot long with handles on both sides.  Like a small pile driver you can pound them in until you can get on them good with a double jack.  (Long handled sledge hammer)      Because of rock we had to bed the ground rods for the service for the house and shop in the trenches.  Grounded well because it was where we went through an intermittent creek and it was very wet down there.   One ran toward the Idaho service drop and the other ran toward the shop both at angles.  We got them in per code but every one I tried to drive went down about two feet and that was it so we angled them off into the conduit  trenches.     

[grover]

Thanks for the responses, it sounds like I am on the right track. 
flyingvan, I don't have any metal water lines or gas lines.  Does that create a problem?
I have a cistern with a submersible pump and sched 80 pvc to the house.  That is not plumbed yet and the plan is pex for in the cabin.  There may be some points where copper will need to be used, not sure about that yet.

[flyingvan]

No problem at all----there's nothing for residents to ground themselves to.

[grover]

The 4/0, 4/0, 4/0, 2/0 wire is called "mobile home feeder wire and is rated for direct burial but I would rather put it in conduit just for the extra insurance.  Will this work in 2" pvc conduit or do I need larger?
Link http://www.menards.com/main/electrical/rough-electrical/electrical-wire/4-0-4-0-4-0-2-0-aluminum-mobile-home-feeder-per-foot/p-1469809.htm

[rick91351]

I like your idea of conduit - we pulled similar but not direct burial.  I think it was three inch we used


Re: Rick and Ellen's Homestead
« Reply #133 on: November 26, 2013, 04:43:01 PM »

[MountainDon]

There is a limit to how many wires of what size that can fit into any particular size of conduit. Set by NEC. This also varies with conduit type. It has to do with heat. There are temperature limits for the wires themselves. In conduit they run hotter.

I'm just finishing lunch and am in the middle of a project that keeps taking more time and can not find my bookmark, if I saved one. There is also an old topic here someplace on conduit and the number of wires that can be run. maybe try a search. There are links in it.


Myself I'd run the already approved for direct burial feeder wire in the ground. Buy a roll of "underground cable" marker tape. Partly fill the trench, lay the tape and finish burial.

[rick91351]

If you do buy direct burial it is sort of redundant to buy conduit.  But I like conduit for several reasons.  With a good operator on a backhoe they can usually feel conduit if they are fishing.....   Another reason I like conduit in my mind it will last longer.  I have no bases for that opinion I suppose other than the old funeral thing of ash to ashes and dust to dust.  And what ever you bury will some day return.

That said we have direct burial running all over here.  Near each frost free I ran a direct burial 12 / 2 Romex.  That has been so handy over the years..... Trench was open for the water pipes so we splurged and run the Romex.....  

[grover]

My concern now is mostly the conduit size.  The charts I have found say 4, 4/0 wires max out the 2 inch pvc but my fear is the mobile home feeder being direct burial has more insulation thus greater diameter. 
I also had someone recommend another wire.  It is a 4/0, 4/0, 2/0 and I guess I would have to get another strand of #4 copper or 2/0 aluminum for the ground.  I thought I had read somewhere that in a sub panel the neutral needed to be the same size as the hots.
There is also another 4/0, 4/0, 4/0, 2/0 wire that has the 4 wires wrapped in a grey insulation but it is not rated for direct burial.  Putting the direct burial cable in conduit seems like extra insurance against future problems.

[Adam Roby]

I tend to be the same way, when I did my pool I put underground wire in the PVC piping just as an extra precaution.  If you put a shovel into the ground you will have a hard time getting through the PVC, but the wire will immediately slice through... and then you are digging a new trench.  Another possible positive, if the PVC is big enough, you could technically later run more wire without digging, just leave a string in there for "future use" and if one day you need a second line you do not have to dig, just pull and hope the line doesn't break. 

[rick91351]

How far are you looking at pulling your wire?  It really does not sound all that far or   

When we pulled ours with the larger conduit.  It worked so easy.  We used a lot of lube like a couple bottles -

Some say use dish soap or lard or my thinking why not use the real thing.  Less chance of a reaction with the cable insulation.....  In fact we shoved almost a third of it through before the guy on the excavator had to make his first pull.

Side note - my electrician - 'Electric Ray'  an old guy but keeps on keeping on he does not like direct burial.  But he is old school and kept telling me when we laid this all out out, "Now we can go direct burial but......"  I never heard the reason after the but......

With conduit and that large wire you want the long arching sweeps not the shorter 90's.     

[MountainDon]

I don't understand the ratinal behind putting DB wire inside conduit. If I wanted the insurance of using conduit (more resistant to a shovel poke than DB wire) then why not run THWN inside conduit?  Which is what our 325 foot high voltage DC line at our cabin is.

[rick91351]

------snip----------

That said we have direct burial running all over here.  Near each frost free I ran a direct burial 12 / 2 Romex.  That has been so handy over the years..... Trench was open for the water pipes so we splurged and run the Romex.....

The direct burial 12 /2 was buried like 4 feet so I am not going to stumble on it..... We were using the big excavator of my logging road making neighbor - my ol' buddy - ol' pal.......

[rick91351]

 

I don't understand the ratinal behind putting DB wire inside conduit. If I wanted the insurance of using conduit (more resistant to a shovel poke than DB wire) then why not run THWN inside conduit?  Which is what our 325 foot high voltage DC line at our cabin is.

I see MD's point here and sort of my own as well ties in.  If you are using conduit why use direct burial unless you can get a heck of a deal on it.  With our distance necessitated larger wire.  I had bought the conduit we used several years ago knowing what we were going to have to do.  It sat stock piled here. But then as I say Electric Ray he does not like direct burial and he liked my pile of conduit - 

As far as the fear of a shovel into DB I really don't think that is going to happen.  A non informed backhoe operator will pull everything loose as will track hoes and excavators.  However a good operator on a backhoe that he knows and uses many times can fish and find without to much damage.      One of the guys the works for our telephone company up here is a master of it.  I am first to admit I am not one of those.   

   

[Adam Roby]

The question is, is it legal to pass normal wire in PVC, or does code require it to still be underground rated if buried?  If you are running any amount of length, then the cost factor is major.  In my pool case it was only 50 feet.  The PVC was to prevent a shovel accident, but the underground wire was to protect from water.  Regular wire is not rated for water contact, and who knows how water tight my connections would be with the PVC, especially with frost heave, so why take the chance.  That was my take anyways, kind of a double protection, especially that it was for a pool.  I would not hesitate to do the same for my generator connections... just extra protection.

[MountainDon]

What is normal wire?   Romex?  That can not go in a conduit, underground mainly because conduit is considered to be a wet location and ordinary Romex is only for dry indoor areas. It's the insulation that needs the wet rating.  You can run UF type in conduit underground.

Most THHN sold today is also THWN, where W = wet. Many places don't list the THWN on the tags on the sales shelves. Look at the wire and see if it imprinted THHN/THWN; then it can be placed in conduit.    When I bought wire for the cabin PV to CC lines THHN/THWN was cheaper than any direct burial types even including the conduit. I have not looked recently.


An advantage of conduit over direct burial is depth of trench needed. Most places let conduit be shallower; at least here does.   Individual single conductor wires are also going to be easier to pull, IMO.

[grover]

I'm going to check with some of the electrical supply places today.  The big box stores are limited as to what kind of wire they stock.  The only large service entry wire that HD stocks is 3/0 copper.  That's almost $3 per foot X 3 plus a ground.  That gets expensive fast.

[rick91351]

I'm going to check with some of the electrical supply places today.  The big box stores are limited as to what kind of wire they stock.  The only large service entry wire that HD stocks is 3/0 copper.  That's almost $3 per foot X 3 plus a ground.  That gets expensive fast.

That is why we went with the aluminum.  I never have use it before but it was a lot less.   

[grover]

Electrical supply house recommended 4/0 4/0 2/0 alum and add a #4 or #6 copper for the ground.  Is it ok to downsize the neutral to 2/0?  I'm running this about 70 ft.

[Rob_O]

Around here HD sells 4/0 4/0 2/0 4 USE cable for mobile homes. It is on the website

Putting the underground wire in conduit will keep it from getting destroyed by rocks as the soil settles

[Patrick]

Electrical supply house recommended 4/0 4/0 2/0 alum and add a #4 or #6 copper for the ground.  Is it ok to downsize the neutral to 2/0?  I'm running this about 70 ft.

I asked my master electrician friend about that when I was doing mine and I believe he called it an unbalanced neutral and that I would have to check with the locals if they approved it he said some do and some dont . I ended up using individual strands of 2/0 copper for all conductors and #4 copper for the ground as per his recommendation.