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Grounding an outbuilding

  1. Jan 21, 2005 #1
    I've just added a log cabin on my property. It came prewired and all I have to do is add the lights, outlets switches and run a service from my house. The building has 3 circuits as follows:

    Ckt #1 = (4) 15A outlets #12/2 wire with a ground
    Ckt #2 = (4) 15A outlets #12/2 wire with a ground
    Ckt #3 = (2) ceiling fans with lights (1) 100watt bulb each and (1) outside light 100watt max.

    The circuits terminate out the back of the building as 3 sets of wires.

    My plan is to mount a 4x4 exterior grade JB on the back wall drop 3 sets of direct burial wires down in a 2" piece of conduit to 10" below ground, then run in a 24" deep trench back to my house approximately 60 ft away. Once in the house through a hole in the foundation, I will connect the wires to a JB inside that has a single outlet on it.

    At this point I only plan to connect the 3 sets of #12 wires and grounds together in the outlet JB as I don't plan to put too much on the circuit. I may seperate them in the future.

    My question is, do I need to drop a ground at the building, or is grounding in the house enough? Also, does my plan sound okay at this point?
  2. jcsd
  3. Jan 21, 2005 #2


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    I’m NOT an electrician; so take my free advice for what’s worth.

    Install a 50 amp, dual (240volt), breaker in the main breaker panel of your residence. This breaker will have the two handles physically connected together and is designed to connect to both busses at once in the panel, ergo 240 volts.

    Use 75degree C cable with either of two gauges, I would use the #6 cable since you have a 60foot run, providing a little less voltage drop, but #8 will be just fine. The cable must be the 4-conductor type, 2 hot legs (black and red or blue, neutral (white), and grounding conductor (green or no insulation). I would use direct burial cable (make sure its rated for burial) personally, but you can use the conduit. You can’t (by code) run cable in a conduit so you’ll need to use individual wires of the stranded type to make the pull reasonable easy. I believe, not sure, you can run cabled wires in conduit for a short distance from a little below ground to the sub-panel.

    You MUST install a sub-panel in the cabin when more than one circuit is desired. The sub-panel may come with the neutral terminal bar connected to the grounding bar. That connection MUST be undone.

    You MUST drive in a grounding rod and connect it to the grounding bar in the sub-panel.

    The grounding wire from your residence MUST be connected to the grounding bar in the sub-panel.

    The neutral wire from your residence MUST be connected to the neutral bar in the sub-panel.

    The two hot legs are connected to the power busses in the sub-panel.

    Install circuit breakers in the sub-panel for each of the circuits you plan to use. Usually lighting (fan) is on a separate circuit.

    If you use 12-gauge wire, get overly deep receptacle boxes if you need to connect more than two cables (three conductors per cable) in the same box. 12-gauge wire is tough to stuff in a normal box behind the duplex outlet, inspectors look for 6inch exposed length for the individual wires extending from the box. I used 12-gauge wire in my kitchen addition on the 15 amp circuits but 14-gauge is the usual.

  4. Jan 21, 2005 #3
    I'm pretty sure you are required to have a disconnect AT the building so you will have to have some type of breaker panel on the back of your cabin. You can't just run 3 UF (underground feeder) cables to your house.

    Is this considered living area? If so the size of the service entrance will need to be sized based on the number of square feet as well as a number of other things.

    I will elaborate:

    Figure 3 watts per square foot for basic lighting and convenience outlets.

    Add 1500 watts for each kitchen small appliance ciruit (microwave, toaster oven), and 1500 watts for the laundry circuit.

    Add ratings for permanent items such as water heaters and clothes driers.

    Use the larger of 2 wattages for heating and cooling.

    Multiply the number of outside receptacles by 180 watts.

    Add all together for the gross load.

    Figure the first 10,000 watts of the gross load at 100%.

    Subtract 10,000 watts from the gross load and figure the remaining load at 40%.

    Add the 2 above results to estimate the true electrical load.

    Now I know all of this sounds crazy for what is considered a small cabin I am sure. But if it is on a foundation and is considered a living area you need to adhere to it.

    Of course you can cheat and call it a lounge or whatever and use it for a guest house or whatever later on.

    The code requires any outbuilding with more than two circuits to have a service of 60 amps or more. Two circuits or less only needs a 30 amp service.

    I have an old reference book and it lists the maximum one way distance for a 60 amp load of 60 feet for number 8 wire. BUT this is only in free air. You will be burying wire or have it in conduit and the code at that time did not rate number 8 wire for more than 45 amps when in conduit, cable, or direct burial. So for 60 amp buried you would have to go to number 6 feeding the building.

    You can either run plastic conduit with 4 conductors inside from your house to the cabin or run USE (Underground Service Entrance). You are required 4 wires because you are no longer able to share a neutral and ground in this situation. The ground wire can be a size smaller than the other 3 conductors. In any case the ground will be covered with an insulation and will not be bare. You MUST keep the ground and neutral separate at the service on the back of you cabin. There will be a neutral bus bar and a ground bus bar. They CANNOT have continuity between them before starting the job. The conduit will need to be one inch or larger for 4 number six wires.

    Just run the 4 wires back to the main breaker panel in your house and purchase a 60 amp 2 pole breaker and hook it to the two hot wires feeding your cabin. Hook the neutral and ground coming from your cabin to the neutral/ground bus bar in the panel in your house.

    You will be required to install a ground rod at the cabin and also ground hot AND cold water pipes to the ground bus bar.

    I may have left something out and I am sorry if I have. Just ask, glad to help.

    BTW, the cabin is considered pre-wired and you have to install alot of it anyway? So basically they ran wires for you and that is it?

    Oh yeah, how big is it?
    Last edited: Jan 21, 2005
  5. Jan 22, 2005 #4
    Thanks guys. I was afraid that was what I would have to do.

    The cabin is considered by the local zoning officials as a home office. There is no water service in the cabin at this time. It is 30' x 12' (about 24' of usable length with a 6' porch with a loft over it) and has a propane fireplace for heat. The three seperate circuits were just pigtailed out the back of the building.

    So my revised plan is:

    To put a 60 amp load center at the cabin, connect it to the house using 6/3 UF with a ground wire and direct bury it at 24" below ground.

    At the building, I want to rise the 6/3 UF in a 2" PVC conduit (to protect it above ground) with (2) 90 deg elbows forming an inverted U shaped bend at the top, and the length extending down about 10" underground.

    At the house I will feed it from a 60 amp 2 pole breaker in my home panel (I have a 200 amp service with 2 pole room available because I have electric heat that we don't use. I am going to take out one of the 20A 2 pole breakers and replace it with the 60A 2p.

    Then I need to sink an 8' long grounding pole and connect the ground wire to that out by the cabin and also connect the ground wire (that runs underground with the (2) hot legs and the neutral) back at the house panel.

    Does this sound okay? Does the load center need a main disconnect or is the one in the house all it should have? Can I get an outdoor approved 60 amp panel board? Or will this need to be in the cabin?

    Thanks again.
  6. Jan 22, 2005 #5
    I believe your load center will require a main disconnect. I don't believe you can get UF in number six. It will be USE and is not in cable form. They are individual conductors. USE is suitable to run underground with no fusing or protection at the source. You are not required to run USE if it is fused or protected with a breaker at the source which yours will be. But I don't know of any wire you can by that is #6 and suitable for direct burial that is not USE. You can run THHN or the equivalent for a number 6 size in a conduit unfused at the source. However, it is always wise to fuse it. Do some price checking as I am sure USE is expensive and you may choose to go the conduit route.

    Have you gotten a permit or anything because if the local authorities already know you are adding a building then you will have to get a permit to wire it. You really should regardless. Talk to your inspector. Have a good solid plan when you call him. Kiss his butt a little, some of those guys like to have their ego massaged and they really are there to help you. As far as having the service inside or outside I am not sure if there is a national requirement. Again, check with the inspector. There is NOTHING to stop you from having an outside disconnect, but you may be required to have something inside also.

    One last thing. Find a good book at Home Depot or something. Some are better than others. Some are really pathetic. I have several that are several ages. The older they are the more informing they are I have found. Unfortunately things change and new requirements are made every year so while the old books are accurate in what they do have, they are missing things that were added after they were printed.
  7. Jan 22, 2005 #6
    Thanks again. Yes I have gotten permits for this work. So far I haven't spoken with the electrical inspector. I've been trying to get a professional to do this work, and I can't get one out to look at it.
  8. Jan 22, 2005 #7


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