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    jcaron2's Avatar
    jcaron2 Posts: 986, Reputation: 204
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    #1

    Apr 12, 2009, 04:04 PM
    Connecting 100A service to barn
    I'm getting ready to electrify my barn. It's about 500 feet from the existing breaker panel in my house, and I need roughly 80 Amps of capacity. I have a 500 foot spool of 4/0-4/0-2/0 aluminum service entrance cable, which should easily handle my current requirements without an excessive voltage drop. My basic plan is to put an 80 or 100 amp breaker in my existing panel, run a line from there underground to the barn, and tie it into a subpanel.

    If the subpanel were only a few feet away from the main panel, I'd use 3- or 4-gauge copper wire and be done with it. However, since my wire is so vastly oversized, I have serious doubts that it would physically fit into a 100 amp breaker. Plus, I'm unsure whether it's allowable or advisable to run aluminum wire directly into a breaker in my main panel. Should I instead mount some sort of disconnect or stand-alone breaker box on the outside of the house that makes the transition from copper to aluminum? On the other end of the line where it ties into the barn, can I simply sink a new ground rod into the earth and hook it into the sub-panel just as if I was installing a main panel from the meter box?

    Sorry for the multiple questions, but I figured it was better to get them all out at once. Thanks for any advice.
    denman's Avatar
    denman Posts: 48, Reputation: 2
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    #2

    Apr 14, 2009, 01:20 PM
    It would have served your purposes better had you run SER/4 wire to the barn. The aluminum wire should fit into your 100 amp breaker. Make sure you apply an anti oxidant coating in and around the bare wire a each connection. Since you have to separate the neutrals and the ground wires in the sub panel a ground rod (or 2 wired together) should work.
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #3

    Apr 15, 2009, 08:12 AM
    A few points I need to comment on:

    Exactly what type of insulation or cable do you have? If this is true SE cable, then it cannot be buried directly in the ground. It will have XHHW insulation, which can be pulled in conduit in a wet location, such as outdoor or underground, but pulling this cable assembly through conduit will be very difficult.

    If this is located in the USA, and the local code in effect is the 2008 edition, a feeder to a detached building must use a 4 wire system. You only mention a 3 wire SE cable, which is good for the two hots and one neutral. The separate equipment grounding conductor is missing.

    And, even though a 4 wire feeder is required, ground rod(s) is required to ground the equipment grounding bar/conductors in the panel in the separate building.

    The neutral must be kept separate and isolated from any equipment grounding.

    Using the #4/0 to help with voltage drop is fine, but a #4/0 will not fit into a 100 amp breaker or panel lug. You can splice a short piece #2 AL onto the #4/0 at the panel to use into a 100 amp breaker or lug.

    Even if you use a 100 amp disconnect switch, as you mentioned, the lugs will not take the #4/0.

    Also need to be careful the #4/0 does not take up too much room in a panel, esp the existing one in the main building. May need to do your splicing to reduce wire size in a nearby junction box.

    But we still need to confirm the wire you have can be direct buried.

    If it is three wires, all insulated and twisted with not overall covering, then it could be URD cable, which can be buried. And a #2 Al USE conductor can be added for the equipment ground.
    jcaron2's Avatar
    jcaron2 Posts: 986, Reputation: 204
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    #4

    Apr 15, 2009, 10:02 PM
    Quote Originally Posted by tkrussell View Post
    A few points I need to comment on:

    Exactly what type of insulation or cable do you have? If this is true SE cable, then it cannot be buried directly in the ground. It will have XHHW insulation, which can be pulled in conduit in a wet location, such as outdoor or underground, but pulling this cable assembly thru conduit will be very difficult.

    If this is located in the USA, and the local code in effect is the 2008 edition, a feeder to a detached building must use a 4 wire system. You only mention a 3 wire SE cable, which is good for the two hots and one neutral. The separate equipment grounding conductor is missing.

    And, even though a 4 wire feeder is required, ground rod(s) is required to ground the equipment grounding bar/conductors in the panel in the separate building.

    The neutral must be kept separate and isolated from any equipment grounding.

    Using the #4/0 to help with voltage drop is fine, but a #4/0 will not fit into a 100 amp breaker or panel lug. You can splice a short piece #2 AL onto the #4/0 at the panel to use into a 100 amp breaker or lug.

    Even if you use a 100 amp disconnect switch, as you mentioned, the lugs will not take the #4/0.

    Also need to be careful the #4/0 does not take up too much room in a panel, esp the existing one in the main building. May need to do your splicing to reduce wire size in a nearby junction box.

    But we still need to confirm the wire you have can be direct buried.

    If it is three wires, all insulated and twisted with not overall covering, then it could be URD cable, which can be buried. And a #2 Al USE conductor can be added for the equipment ground.

    Thanks!

    Sorry, should have been a little clearer on the wire type. It's triplexed 4/0-4/0-2/0 aluminum with XLPE insulation and says that it's suitable for direct burial, up to 90C in wet or dry locations. Sounds exactly like your description of URD cable.

    Here in North Carolina, I believe our current code is based on NEC 2008, so thanks for clarifying that I need both a ground wire AND ground rods at the barn location. You recommend #2 USE, so does that mean I have to use a ground wire rated at 100A? I was under the impression that the ground wire can usually be undersized (for example, I see 4/0-4/0-2/0-4 as a common size for so-called "mobile home cable", presumably for 200A service. In that case, isn't the ground wire only rated for 75 amps?). Obviously my wallet would be happier if I could get away with #6 or #8 aluminum or even #10 Cu THHN, but that's probably asking a lot.

    Thanks again for the info.
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #5

    Apr 16, 2009, 03:26 AM
    Since the feeder is upsized for voltage drop due to the distance, the equipment ground also needs to be upsized accordingly.
    jcaron2's Avatar
    jcaron2 Posts: 986, Reputation: 204
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    #6

    Apr 16, 2009, 10:38 AM

    Ahh, makes sense... I guess. Just out of curiosity, any idea WHY this rule has been adopted? I can't really imagine any scenario where that additional ground wire would add any advantage in case of a fault, but maybe I'm just unimaginative.

    Reasoning aside, the code is the code, so I'll need to run the ground line whether I want to or not. One other question, if you don't mind: With the ground cable running from the house and tied into the additional ground rod(s) near the subpanel in the outbuilding, does that mean I should AVOID tying the neutral and ground together out there? Before you pointed out the code requirement for the fourth line, my original thinking was that I'd treat this like a brand new service entrance, thereby tying the neutral and the "new" ground together at the panel in the outbuilding.

    I really appreciate the help.
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
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    #7

    Apr 16, 2009, 11:42 AM
    You guess? Don't guess, ask for more info until you get the answers you need. Allow me to explain. That is what we are here for.

    Since the feeders were increased to reduce voltage drop due the impedance of a small conductor, it stands to reason that the equipment ground, if left sized small, would have an impedance greater than effective to allow a overcurrent protection device to operate under a fault condition, so it must be increased also to reduce the impedance to be effective.

    High impedance (AC resistance) is not good to have, esp in a grounding conductor.

    The ground just does not get any respect, just like Rodney Dangerfield.

    As I stated in my previous answer:

    And, even though a 4 wire feeder is required, ground rod(s) is required to ground the equipment grounding bar/conductors in the panel in the separate building.

    The neutral must be kept separate and isolated from any equipment grounding.


    Yes, the neutral must be kept separated in this panel.

    The only location the neutral and equipment ground are connected is at the first means of disconnect for any service. This usually is at the Main Breaker. Can also be in the meter, and at the utility transformer, or at all of these locations, as long as it is not beyond the load side of the main.

    Briefly, the purpose is to insure the impedance is kept low to insure fault current is allowed to rise high quickly to operate any fuse or circuit breaker, and to keep neutral currents from flowing in the equipment grounding system.

    Do a search in this forum for grounding, as I have explained this in some detail previously.

    This is the main reason the fourth wire was added to any feeder to a separate building. Three wire was allowed up to the 2008 edition. This allowed the neutral and equipment ground to share one conductor. This was frowned upon for several reasons, one that I mentioned already.

    Another is that sometimes due to poor installations or connections, the earth is required to act as a conductor, which is not good at all.

    An installation such as this would not be done the same as a new service, as there is the grounding of the neutral done back at the main, and this sub-panel has it's own overcurrent protection device.

    Sizing and arrangement of equipment ground conductors also revolves around AIC (Available Interrupting Current) ratings of systems, which is beyond the scope of this forum to explain here.

    Trying to give the best reasons and answers without just saying "because I said so".

    We all heard enough of that from our parents. But the topic of grounding is complex, and certain things are done for many often mistaken unrelated reasons.


    Hope this helps.
    jcaron2's Avatar
    jcaron2 Posts: 986, Reputation: 204
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    #8

    Apr 16, 2009, 11:54 AM

    VERY helpful. Thanks!
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #9

    Apr 16, 2009, 02:19 PM

    I think a couple of points were missed:

    1. You need to have a disconnect in the barn. It could be the main breaker. The main (in the barn) can be rated higher than the feeder since it's used as a disconnect.

    2. Panels like the above need to have the neutral bonding screw removed and you may have to purchase a "ground bar kit" to give you the extra ground terminals.
    jcaron2's Avatar
    jcaron2 Posts: 986, Reputation: 204
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    #10

    Apr 17, 2009, 08:36 AM

    Okay, so here's my plan. Please let me know if you see problems or have suggestions or improvements.

    1) At the house:

    In my existing main panel in the house, I'll install a 2-pole 100A breaker. From that breaker, I'll run about 30 feet of #2 Aluminum cable from each hot to a separate NEMA 3R pull box on the exterior of the house. There I will splice these to the two 4/0 aluminum wires that will make the run out to the barn. Meanwhile, I'll also run about 30 feet of #4 aluminum from the neutral and ground buses in my main panel (which are tied together with a bonding strap in that box) into the same pull box. There I will splice the neutral connection to the 2/0 wire and the ground connection to a separate #2 aluminum USE cable. All four larger wires will then run from the pull box down into the ground in 2 or 2.5" PVC conduit.

    2) At the barn:

    At the barn site nearly 500 feet from the house, the four large cables will exit the ground through a 2 or 2.5" PVC conduit and enter through the wall of the barn into an insulated split-neutral 150A load center. I think I may have to buy an add-on ground lug to accept the #2 wire. Meanwhile, I'll sink the requisite ground rod(s) nearby and tie those to the ground bus.

    My long-term plan is to have two or three 2-pole circuits (for a hot-water-heater, a clothes dryer, and maybe a plug for a welder), plus a couple of 15- or 20-amp single-pole circuits for lights and outlets. As you can see, I don't plan to run a lot of current through the neutral conductor. Nevertheless, is the 30' of #4 wire at the main panel okay? I can up-size the wire if I buy add-on lugs for my existing panel, or I can reduce the distance to only 5-6feet if I use a lot more care digging the first 20' of trench in order to go under a buried drainage pipe.

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