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Thread: Wiring 100Amp Panel for Garage Workshop

  1. #1
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    Wiring 100Amp Panel for Garage Workshop

    I would like to get some advice on wiring a 100 Amp subpanel for my Garage/Workshop. I recently completed my garage, after about 2 years of planning, negotiating contracts, contractors, inspectors and the like. Now the next step is wiring the Garage. After Calling about 6 different electrical contractors and actually getting one somewhat reasonable quote $2750.00 and one outrageous quote $10432.00. I have decided to try and do this myself. I have several friends who are electricians or have done a substantial amount of wiring and have done quite a bit of research myself. I am reasonably confident I can do this as I have done basic wiring for outlets and lights and met code and passed inspection, but never have installed a Main Panel or Subpanel. My problem is I am getting somewhat conflicting information on what type of wire to use. I am somewhat meticulous about how things are done and I also want this done to code and have as little problem as possible for inspection.

    I need to run a feeder from my 200 Amp Main Entrance Panel to my 100 Amp Subpanel to be located in my garage/workshop on opposite ends of the house. The feeder must run 42ft across the crawlspace and then thru approximately 37ft of 1-1/2" schedule 40 conduit under the garage floor with about 5 additional feet on both ends to get to the panels. This gives me a total of about 90ft we might as well just say 100ft. Going thru the crawlspace is not as bad as it sounds since it is paved and well lighted. I have decided to use Copper even though it is much more expensive because the diameter is smaller for copper and I am bit concerned about the pull thru the conduit as it has two 45's and three 90's. Also, Aluminum has some drawbacks in regard to installation when anti oxidizer is improperly applied creating problems down the road. I just would like to avoid that and just spend the extra money for copper. With the amount I am saving doing this myself, I will still be way ahead. I am using a 100 Amp 24 circuit Square D QO Subpanel as that is what is used in my 200Amp Main Entrance Panel and I just as soon keep them the same brand. The 24 Circuit Panel is probably over kill as I will only be using about 8 circuits but my main panel is full, as I have electric baseboard heat, and I wanted extra circuits if I should ever need something else wired for the house or garage.

    Now the problem, what wire to use? Through my research, and info from electricians and friends etc. I have about four possible scenarios for the feeder wire for this 100 amp Panel.

    SCENARIO #1; Use SER 4-4-4-6 Copper across the 42ft Crawlspace and thru the approximate 37ft of schedule 40 conduit. Use a 90 Amp Circuit breaker in the 200 amp Panel to supply the feeder. Leave the 100 amp breaker in place on the 100 amp Panel.

    SCENARIO #2; Use SER 3-3-3-5 Copper across the 42ft Crawlspace and thru the approximate 37ft of schedule 40 conduit. Use a 100 Amp Circuit breaker in the 200 amp Panel to supply the feeder. Leave the 100 amp breaker in place on the 100 amp Panel.

    SCENARIO #3; Use SER 2-2-2-4 Copper across the 42ft Crawlspace and thru the approximate 37ft of schedule 40 conduit. Use a 100 Amp Circuit breaker in the 200 amp Panel to supply the feeder. Leave the 100 amp breaker in place on the 100 amp Panel.

    SCENARIO #4; Use four Single THHN Wire's for the feed. I really want to avoid this as I believe I will need conduit across the entire crawlspace and the pull thru the 37ft of Conduit under the garage floor has me concerned already due to the number of 90's.

    As far as the panel goes, my research tells me that I would not want a bonded neutral and ground since the workshop is attached to the house. I also should not need rebar in the ground for a grounded box since I am using 4 wires from the main 200 Amp panel which is already grounded. I believe Scenario #1 is for a light load type situation, Scenario #2 would be for heavier loads and Scenario #3 for a mega load situation like heavier machinery and such. I am thinking Scenario #2 would fit my situation since my shop would have a welder and air compressor on one hand but since I am the only one going to be in the shop, it would rarely have many things in use at once. Ok, I know this is a bit long winded but can anyone give me an idea weather I am on the right track here. Is any of these scenarios correct? Which, if any would you do in my situation? If none, How would you do it? Thanks in advance for any help anyone can offer.

  2. #2
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    your installation poses a bunch of questions.
    you say you have access to the garage underfloor. does this mean you have a spare manufactured "cell" in the slab? Does this mean you had an extra cell poured big enough to clear a 100 amp service entrance?
    floor re-bar is generally 4 inch by 4 inch using 3/8 inch metal laid horizontally in the center of the pour, so I don't see how your contractors could get away saying this woukd pass for an equipment grounding conductor because a minimum depth is a full 8 foor depth.
    WIRING: for wire pulls you say you need 2 - 45 degree pulls - same pull or part of the 9-=90=9- pulls. you are alrealy in violation as far as number of pulls in any ONE run. Make a drawing using microsoft "paint" and subimit it here.

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    Ok here is the story for the floor and conduit. I replaced the original garage floor in 2009 and had three 1-1/2 inch conduit pipes laid in parallel under this floor. At the time, I new I was going to build a workshop but did not know if it would be attached or a separate building. When I finally knew it would be attached and had it built, several years later, the contractors attached conduit to these existing conduit and brought them up outside the wall where the 100 Amp panel will be. So there is three parallel 1-1/2 inch schedule 40 conduits running from inside the crawlspace to where the panel will be. There is no direct access here, the conduit is buried under the cement floor of the garage. The building inspector would have seen the conduit. He did not say anything about it having to many bends. Sounds like I may have a problem here.

    I think there is a bit of miscommunication here. I do not have access to the conduit under the floor. The only points accessible to the conduit are the ends in the crawlspace wall and the ends where they come up outside the wall where the panel will be mounted inside the garage. The conduit is buried under the cement floor. Also. regarding the rebar; What I was trying to say is that I did not think I needed a rebar grounding rod for the subpanel because this was a subpanel in the attached new garage and I am using 4 wires (ground and neutral included) from main entrance panel. The main entrance 200 Amp Panel has a ground Wire installed from the panel to the underground metallic conduit feeding the meter. Sorry for the convoluted explanations but I am trying to offer as much detail as possible.

    Also, I have tried to attach a bmp file but I cannot seem to get it to work. It will upload the file but then says the file is not a valid image file. I think the picture would clear up a lot of misunderstanding! Do you have an email address that I can email it to you or any other way for me to get a file to you?
    Last edited by MRCLUNKER; 11-28-2015 at 09:31 PM.

  4. #4
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    tmierzwa@twcny.rr.com

    a cable assembly like the SER you're planning, cannot by code be instaled in any kind of conduit. you'll be forced to use thw thhn type wire.

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