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New 100 amp underground service to outbuilding; 3 or 4 wires?

New 100 amp underground service to outbuilding; 3 or 4 wires?

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  #1  
Old 11-28-10, 04:15 PM
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New 100 amp underground service to outbuilding; 3 or 4 wires?

Hello folks,

I'm guessing this might be a local code question, but before I go to the county electrical inspector, i want to know what others think...

I'm going to run 100amp service to an outbuilding.
What i'm wondering is if I need to run 3 or 4 wires to the new outbuilding panel. In other words, 2 hots and the neutral for a 3 wire run, or do i need to run 2 hots, the neutral AND a 4th lead for gound wire?

For the 3 wire run i plan on setting the gound rod outside right near the service panel.

Do I need to run the ground all the way from the main panel, hence, the 4rth wire?
Thanks!
 
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  #2  
Old 11-28-10, 04:51 PM
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Not necessarily local bur national requirement. You must use 4 wires and have at least one ground wire at the detached building. A separate ground and neutral bar with the ground bar bonded and the neutral bar isolated. If you have more then six breakers in the sub you must have a disconnect at the sub. That can be a main breaker in the subpanel.

Note above is based on 2008 NEC. Local code may be based on older versions of the NEC and may permit 3-wire if there are no other metallic pathway. But why not go by the latest safest rules.
 

Last edited by ray2047; 11-28-10 at 06:39 PM.
  #3  
Old 11-28-10, 05:02 PM
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thanks ray2047,
just the info i needed.
i thought i heard somewhere that the new code states a 4 wire system. The local home depot has 4 wire aluminum...they called it mobile home wire, so i wasn't sure if that was the same wire for outbuildings.
Also thanks for the grounding clarification.
 
  #4  
Old 11-28-10, 06:48 PM
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they called it mobile home wire, so i wasn't sure if that was the same wire for outbuildings.
It is correct. URD Underground Residential Distribution cable also known as mobile home case because of one of its common use. Usually it is rated for direct burial or use in conduit.
 
  #5  
Old 11-29-10, 09:01 AM
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Originally Posted by Solar1 View Post
i thought i heard somewhere that the new code states a 4 wire system.
As of 2008 code, all outbuildings must be a 4 wire feeder. Previous versions allowed 3 wire in some circumstances.

The local home depot has 4 wire aluminum...they called it mobile home wire, so i wasn't sure if that was the same wire for outbuildings.
Also thanks for the grounding clarification.
If it is URD, it can be used for outbuildings, but that cable can only be used outside or underground due to its fire rating. It cannot be used inside a building, even in conduit. It also probably cannot be used at its advertised amperage due to temperature rating. They are probably selling #2 aluminum as 100A, when in fact it is only rated for 90A for this application. We'll have to know the fine print on the wire or cable jacket to tell you the exact usage of it.
 
  #6  
Old 11-29-10, 02:15 PM
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hi, if it is urd, how do I change the wire to interior wire?, outside?
Does the junction box need to be a switchbox, or can it be a rainproof box and use split bolts to join them?
is there a different kind of wire to order so i don't need to change wire at the interior?
thanks
 
  #7  
Old 11-29-10, 02:43 PM
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The underground cable can be used indoors only if it carries the XHHW or THWN mark which means it has passed the flame test for premises wiring. Many cables carry multiple ratings so you'll have to read the actual cable or the mfr spec sheet. You will probably find URD, USE, or RHW instead meaning outdoor use only.

Some jurisdictions allow up to 6' of outdoor cables inside (such as to connect to a panel mounted on an exterior wall, however many do not allow it at all so a call to the inspector might help if the panel is close.

That means you would need to have a junction box on the outside of the building where you can splice to/from the URD. Type SER cable can be used for the interior portion or you can run conduit and pull individual THHN or XHHW wires. If you want to stick with the same conductors for the whole run (no splicing), use a fully assembled conduit system and install XHHW or THWN which are allowed both in- and outdoors.
 
  #8  
Old 11-29-10, 03:09 PM
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thanks much Ben.
My friend got anxious and already picked up the wire! He just called me. He told me the home depot guy told him they don't stock the mobile home stuff anymore because of the nec code changes. He did tell me the guy said it as direct burial. he is going to check the cable (or I will tomorrow) to check the rating markings. Thanks for that additional info. If is is not rated as you mentioned for interior use, How should I change to the interior wire...in other words, what kind of box or switch do i need to use?
thanks again, jim
 
  #9  
Old 11-29-10, 05:03 PM
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Use an exterior rated junction box like a PVC 6x6". The wires can be spliced with split bolts and heavy tape or with the more expensive insulated set-screw connectors like Polaris. I assume this is aluminum wire, so be sure to get connectors rated for aluminum and use non-oxidation paste on the wires.
 
  #10  
Old 11-29-10, 07:12 PM
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THANKS BEN!!!
you know your stuff!
i'll find out more about the wire tomorrow.
you gave me some great info to work with
j
 
  #11  
Old 11-30-10, 06:29 AM
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Hello Ben,
I'm heading out to my freinds house to look at the wire.
One thing I don't think i asked was with the 4 wire service, do I still need a ground rod at the outbuilding?
thanks, jim
 
  #12  
Old 11-30-10, 06:57 AM
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A ground rod will still be needed at the outbuilding, even with the 4 wire feed.
 
  #13  
Old 11-30-10, 02:35 PM
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thanks for the ground rod answer...

i looked at the aluminum cable today it does show USE-2 SUN RES, but it also shows XLPE...I'm hoping the XLPE means it might be rated for indoors (in conduit) ???
maybe one more question...does the new 100 breaker panel need to be main breaker type, or can a main lug breaker panel be ok per nat code?
 
  #14  
Old 11-30-10, 03:23 PM
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I hope i'm not putting too many topics in one thread, but the seem to relate to each other directly. I read in another forum thread that the neutral and ground must NOT be bonded in the subpanel. Is this ture, and if so, how does one "un-bond" in the sub panel? Isn't there a bonding wire across the two neutral buss bars in the breaker panel "bonding" the two buss bars together? On one buss bar is usually the ground lug and the other has the neutral lug?
Now I'm confused.
 
  #15  
Old 11-30-10, 03:25 PM
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Six breakers or less you don't need a disconnect. However you will probably find a 100a main breaker panel with a breaker package included is cheaper then a main lug and buying individual breakers and of course you won't be limited to six breakers because the main breaker serves as a disconnect. You will probably need to buy a ground bar also but they are cheap.
 
  #16  
Old 11-30-10, 05:51 PM
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how does one "un-bond" in the sub panel?
Typically, the neutral bus isn't bonded to the panel can in a new panel, but there will be a strap or bonding screw left unattached that can be used to bond the neutral bus to the panel can if the panel is used as a main service panel and first overcurrent protection of the main. There should be some basic instructions in the new panel that describe how to bond the neutral bus to the can, just don't do it and be sure it wasn't done at the factory. This used to be something that always had to be done in the field, but that may have changed. Read the instructions!
 
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