Long underground cable run.

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Old 11-03-09, 01:09 PM
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Long underground cable run.

Hi all, first time poster so be gentle. Here are some facts about the run.

Desert property. Edison just upgraded us to 200 amp service. New 200 amp service panel and sub panel installed and permits signed off. There is a manufactured home and a garage on the property.

We want to run electrical service to three RVs on the property. The run will be 175 feet to to first trailer. The other two are within 40 feet of the first. The RV's are all set up with 110v 30 amp plugs.

I have found some surplus cable and was wondering if it will work. It is 270' of 4/0 triplex direct burial (sweetbriar) service cable. They want $400 for it. I was told it was a good deal. my plan is to do all the labor and have an electrician do the final hook up.

Will this cable work and is the price decent?

Brian (sorry for the long post)
 
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Old 11-03-09, 01:35 PM
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That cable will be more than adequate for 120V circuits at that distance -- the price is very good also. Your trenches will need to be 24" deep, and you will need to use very large splice boxes to accommodate the #4/0 as it is a very large conductor. The cable cannot be used indoors, so if you need to go inside to get to the service panel a transition to a different wiring method will be required. Conduit sleeves are required down to the bottom of the trench.

I would run this cable to a 100A subpanel located centrally between the RVs installed on a post. Then run 30A circuits to each RV using #10/2 UF-B cable. Transition to the #10 is required as the #4/0 is simply too large to make the final connections to the RVs as is will not fit in the panel box, on the 30A breaker lugs, or at the receptacle. This should require about 200' of your #4/0 cable leaving 70' left you can sell for scrap to buy the #10 copper UF-B.

Note in this configuration you will have one hot, one neutral (tape the conductor white), and one ground (tape the conductor green). Only half of the slots of the subpanel are powered as it is a 120V only panel.
 
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Old 11-03-09, 02:52 PM
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Thanks for the quick response. The service panel is located outdoors on a pole. When you say splice box is that an in-ground plastic box with a external cover or is it just a large junction box mounted above ground on a pole?

Also, how would the electrician make the connection to the main panel. Would it also use a splice box and run smaller wires up to the panel or does it need to the heavy cable all the way up to the panel?

Brian


Originally Posted by ibpooks View Post
That cable will be more than adequate for 120V circuits at that distance -- the price is very good also. Your trenches will need to be 24" deep, and you will need to use very large splice boxes to accommodate the #4/0 as it is a very large conductor. The cable cannot be used indoors, so if you need to go inside to get to the service panel a transition to a different wiring method will be required. Conduit sleeves are required down to the bottom of the trench.

I would run this cable to a 100A subpanel located centrally between the RVs installed on a post. Then run 30A circuits to each RV using #10/2 UF-B cable. Transition to the #10 is required as the #4/0 is simply too large to make the final connections to the RVs as is will not fit in the panel box, on the 30A breaker lugs, or at the receptacle. This should require about 200' of your #4/0 cable leaving 70' left you can sell for scrap to buy the #10 copper UF-B.

Note in this configuration you will have one hot, one neutral (tape the conductor white), and one ground (tape the conductor green). Only half of the slots of the subpanel are powered as it is a 120V only panel.
 
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Old 11-03-09, 03:38 PM
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Originally Posted by brian2071 View Post
When you say splice box is that an in-ground plastic box with a external cover or is it just a large junction box mounted above ground on a pole?
It could be either.

Also, how would the electrician make the connection to the main panel. Would it also use a splice box and run smaller wires up to the panel or does it need to the heavy cable all the way up to the panel?
If the panel is good sized and not too full, he might be able to run the #4/0 all the way up in to the panel. Trust me though it's a bear to work with so he may prefer to run a smaller cable like #1 into the panel then splice to the #4/0 in a separate, less crowded box. Another issue is that the #4/0 may not fit into the breaker lugs requiring a smaller wire anyway.

The goal overall is to use as few splices as possible as it really racks up the material cost and labor to make the connections, so he'll probably go right to the breaker if possible but a junction box may be needed.

Also I forgot to note in my previous post that the conduit from the panel down to the bottom of the trench must be 2".
 
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Old 11-04-09, 12:32 PM
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Well it doesn't look like the deal I found on the 4/0 triplex direct burial cable is going to go through. The guy doesn't want to ship it.

So, if I need to go buy the stuff from a supply house what size wire should I get. Do I need to use 4/0 or would something smaller be adequate?

Also, since I will be purchasing it new, would direct burial cable or individual wire running through conduit be more economical?

Brian
 
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Old 11-04-09, 02:26 PM
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Originally Posted by brian2071 View Post
Well it doesn't look like the deal I found on the 4/0 triplex direct burial cable is going to go through. The guy doesn't want to ship it.
That's too bad.

So, if I need to go buy the stuff from a supply house what size wire should I get. Do I need to use 4/0 or would something smaller be adequate?
Much smaller wire will be adequate. See below.

Also, since I will be purchasing it new, would direct burial cable or individual wire running through conduit be more economical?
Direct burial cable is cheaper; conductors in conduit are a little better protected.

Based on the new information, I would recommend 2-2-2-4 direct burial aluminum (URD) -- usually sold as 100A mobile home feeder (MHF) cable. This size is also available in a USE-2 type sometimes dual rated as one of the above.

Feed this from a 60A double-pole breaker in your main panel. Set up a subpanel on a pole near the RVs and from that install 30A single pole breakers and run #10-2g UF-B underground to each RV receptacle near the parking spots. Also see the sticky subpanel thread at the top of the forum for some good drawings and information about the grounding rods and other details required for the subpanel.
 
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Old 11-04-09, 03:55 PM
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Since there will be 3 trailers all with 30 amp receptacles would the 60 amp DP breaker be enough? I'm probably wrong in my thinking but I figured we would have to use at least a 90 amp breaker, (3 trailers x 30 amps).

Also when checking on the wire you suggested the counter person said that we should be using heavier wire for a 175 foot run due to voltage drop. I'm not taking his word for it but just making sure. I would love to use the smaller wire.

Brian
 
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Old 11-04-09, 04:55 PM
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As you stated, the RVs are set up with 120v 30A connections. The subpanel that Ben recommended would be 240v @ 60A (14,400w), so really this is equivalent to FOUR 120v 30A feeds (14,400w) since the subpanel has two legs (similar to your main panel).

This is assuming that each RV is actually pulling the full 30A. Code-wise, you could connect more RVs anyway since it would be rare for each to pull the full load... but long story short, you're good with a 60A subpanel.

Same answer for the voltage drop. Voltage drop is calculated based on the actual load. Since your load is only 10,800w (110v * 30A * 3 RVs), it's well within the load based on the wire size and length.
 
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Old 11-05-09, 08:53 AM
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The original circuit was 120V with all of the RVs on the same leg (3 * 30A = 90A). However now that you're paying retail for wire it's more economical to run a 240V circuit with 30A load on one leg and 60A on the other. The voltage drop on the 60A leg will be pushing the upper border of what code recommends (5%), but it would be rare for two RVs to be drawing maximum load at the same time. In the typical case the load will be quite a bit less than 60A with an acceptable 2-3% voltage drop using the #2 aluminum cable.

As Zorfdt pointed out, this 240V circuit will also leave some spare capacity for perhaps a fourth RV, lighting or general-purpose receptacles that you might need in the future.
 
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Old 11-05-09, 09:39 AM
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Now I understand what you're saying. Makes perfect sense. Actually we have two sheds by the trailers that we would like to put some lighting and receptacles in, so that could be the fourth leg. Or we may put in a small window A/C in one of the trailers and could run a separate circuit for that.

So if we use the 2-2-2-4 wire, will we be able to go directly to the lugs on the breaker without splicing? I thought I saw a breaker that had a max wire size of #2.

Is the #10-2g UF-B cable just a flat cable with 2 conductor wires and a ground? If this is correct Lowes had some in stock.

Finally, If all is correct this would be the procedure:

1 only 240v-60amp DP breaker in the service panel. Come off it with 2-2-2-4 MHF wire in a conduit that leads 24" underground. Run the line to a sub panel with 4 only 120v-30 amp breakers. From that we will use the #10-2g UF-B down conduit 24" below grade to each trailer and come up through conduit to 30amp RV receptacles.

Please correct me if I'm wrong.

Brian
 
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Old 11-05-09, 12:05 PM
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The shed would be on a 20a breaker supplied by 12-2 UF. Conduit is not necessary but provides more protection. Usually 18" is deep enough for conduit. Would suggest you also run a " conduit separated about a foot from the main conduit for phone and data wiring. For the smaller conduit you can just fill in about a foot then run it and finish filling the trench.
 
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Old 11-05-09, 01:19 PM
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Originally Posted by brian2071 View Post
So if we use the 2-2-2-4 wire, will we be able to go directly to the lugs on the breaker without splicing? I thought I saw a breaker that had a max wire size of #2.
That's correct; many modern brands can take a #2 directly to the 60A breaker lugs. If you know the make of the main panel I'm sure one of us can check for you. You might need to get a lug kit to land the #2 on the neutral bar as that usually maxes out at #4, but it is an inexpensive item.

Is the #10-2g UF-B cable just a flat cable with 2 conductor wires and a ground? If this is correct Lowes had some in stock.
Yes, it is commonly stocked at home centers. It is important though to get UF-B with black, white and bare wires -- sometimes it's black, red, bare which is not what you want. Note that UF-B and NM-B are different cables even though they look very similar on the shelf.

1 only 240v-60amp DP breaker in the service panel. Come off it with 2-2-2-4 MHF wire in a conduit that leads 24" underground. Run the line to a sub panel
Correct so far.

with 4 only 120v-30 amp breakers.
Here you only want 30A breakers for the three RVs. Your shed or other general-purpose circuits will need 20A breaker(s). For those ones you could use the slightly cheaper #12-2g UF-B, but it is also okay to use the #10-2g if you have some left over from the RVs.

From that we will use the #10-2g UF-B down conduit 24" below grade to each trailer and come up through conduit to 30amp RV receptacles.
Sounds about right.

Your subpanel will also need ground rods and a bare #6 copper wire to connect them. Please see Bruto's very helpful drawings here: http://forum.doityourself.com/electr...-drawings.html

Your installation most closely matches the bottom drawing "Four Wire Feeder, Subpanel in a different building than the service equipment". Note that your subpanel needs to have isolated ground and neutral which requires removing the bonding screw/strap and purchasing a ground bar kit for the panel. A minor difference between this drawing and your installation is that a main breaker is optional (not required) in your subpanel because you have fewer than six circuits in the panel; you can use a MLO (main lugs only) panel instead. I would probably use a simple 4 space/8 circuit or 6 space/12 circuit outdoor panel like this one: Square D Company at Lowe's: 100 Amp, 6/12 Main Lug Panel
 
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Old 11-05-09, 02:42 PM
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Originally Posted by ibpooks View Post
Your subpanel will also need ground rods and a bare #6 copper wire to connect them.
You say rods. Will the subpanel need more than one?

If I have researched correctly, the rod needs to be 5/8" diameter x 8' long driven below grade with a clamp designated for direct burial. Is that correct?
 
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Old 11-05-09, 03:13 PM
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There are situations where one rod is acceptable, but the problem is that it requires a rather expensive tester to prove that one rod is enough given the soil conditions, dampness, etc. It's much easier to just install two rods six feet apart which means the test is not required. Your specs on the rods are correct.
 
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Old 11-05-09, 03:35 PM
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Ok...two rods it is.

When coming out of the main panel with the conduit does the end (24" below grade) where the wire will come out need to be sealed somehow or is it permissible to be left open and should it have a 90 at the bottom? Would 1-1/4 Dia. conduit be large enough?

Brian
 
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Old 11-05-09, 07:00 PM
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1-1/4 would be the smallest size to use, 1-1/2 would make the pulling easier and the increased cost is minimal. Use plenty of cable lubricant when pulling.

You should use a factory-made 90 degree bend on the buried conduit. It is not necessary to seal the lower end although I would recommend using a duct seal product on the exposed ends in the panelboards.
 
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Old 11-06-09, 11:47 AM
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I'll be having a 1-1/2" conduit coming into the sub-panel bringing in the feed from the main panel. Can 2 of the branch circuits run down that same conduit (if I use a larger diameter conduit) into the trench or should it be in a separate conduit?

Also, the plan was to mount the sub-panel on one side of a 4X4 post near the first trailer and put that trailers RV receptacle on the other side of the post. Can I use waterproof flex conduit to run from the sub-panel around the post to the RV receptacle?

Brian
 
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Old 11-06-09, 12:36 PM
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Originally Posted by brian2071 View Post
Can 2 of the branch circuits run down that same conduit (if I use a larger diameter conduit) into the trench or should it be in a separate conduit?
Separate would be ideal, but if you needed to you could double them up. Because this conduit is just a short piece for physical protection the usual conduit fill rules do not apply. You can fill it up so long as the cables can be installed without damage. Don't underestimate the stiffness of the #2 though -- I think it would be difficult to get #10/2 in there with it even in a large pipe.

Can I use waterproof flex conduit to run from the sub-panel around the post to the RV receptacle?
Yes.
 
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Old 11-06-09, 01:06 PM
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The only question I think I have left is: Would you feel safe using the direct burial cable for your own property or would you lay conduit and pull individual wires?

Brian
 
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Old 11-11-09, 10:50 AM
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Originally Posted by ibpooks View Post
I would probably use a simple 4 space/8 circuit or 6 space/12 circuit outdoor panel like this one: Square D Company at Lowe's: 100 Amp, 6/12 Main Lug Panel
I went to Lowes today to check out this panel and it looks like the bottom of the panel has a knock out that will only accept a 1-1/2" fitting. Will 1-1/2 conduit be large enough to house the 2-2-2-4 MHF wire? Or would I use 2" conduit and use a 2" to 1-1/2 reducer bushing where it enters the bottom of the panel?

Brian
 
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Old 11-11-09, 11:10 AM
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Originally Posted by brian2071 View Post
The only question I think I have left is: Would you feel safe using the direct burial cable for your own property or would you lay conduit and pull individual wires?
In my opinion, either way would be fine. If you go at either with a backhoe, you're going to be replacing the cable.

Take pictures while you are trenching and laying the cable so you can refer back to exactly where it is. Some people like to lay a strip of yellow caution tape 6" below ground as a warning flag if you start digging there again.

Also, 24" down is pretty deep. Deeper than planting flowers, grass, or most small bushes.
 
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Old 11-11-09, 12:21 PM
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Originally Posted by brian2071 View Post
Will 1-1/2 conduit be large enough to house the 2-2-2-4 MHF wire?
Yes it should be.

Would you feel safe using the direct burial cable for your own property or would you lay conduit and pull individual wires?
I feel safe with direct burial as long as it is at the correct 24" depth -- that is much deeper than you will ever go casually digging for landscape. It would really only be a problem for a fence post or tree spade, and in those situations you need to mark the utilities first anyway.
 
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Old 11-11-09, 12:38 PM
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Sounds good, 1-1/2" conduit is what I'll use. From what I could figure out at Lowes is to connect the conduit to the panel you put a PVC Junction adapter through the knockout hole from the inside of the panel and glue a coupling on from the outside sandwiching the bottom of the panel in between the two. Is this correct? I saw some couplings with male threads but didn't see any type of nut to use with it. So thought the adapter was the way to go.

Also, when the cable comes into the panel does it need to be clamped? Using the adapter/coupling method I see no way to clamp it.
 
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Old 11-12-09, 02:38 PM
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Sounds good, 1-1/2" conduit is what I'll use. From what I could figure out at Lowes is to connect the conduit to the panel you put a PVC Junction adapter through the knockout hole from the inside of the panel and glue a coupling on from the outside sandwiching the bottom of the panel in between the two. Is this correct? I saw some couplings with male threads but didn't see any type of nut to use with it. So thought the adapter was the way to go.

Also, when the cable comes into the panel does it need to be clamped? Using the adapter/coupling method I see no way to clamp it.

Now my problem is finding the 2-2-2-4 MHF. It looks like my local electric supply houses don't carry it.
 
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