Yet Another Garage Subpanel DIscussion

Discussion in 'Electrical Forum discussion & Blog' started by chewmanfoo, Jun 20, 2014.

  1. chewmanfoo

    chewmanfoo New Member

    Messages:
    3
    Location:
    Texas
    You guys are probably completely sick of garage sub-panels. Sorry for that.

    I have a new 220V 30A table saw. It's a monster at 5HP and it is sitting there staring at me and I believe it is talking to me in a growly voice, urging me to wire it up.

    Unfortunately I have no 30A service in my garage - so I have electrical work to do. I have wired all sorts of circuits in my house - but nothing of the 220V variety. But I believe I have some good habits/skills with residential electricity.

    I have purchased a new 200A service panel (Square D QO 40 space) to replace my existing 100A panel, which is also in the garage in the back corner. That old panel is a Square D. It's really old. It looks like it belongs on a model T. Here's a picture, with the amps of the breakers labelled:

    [​IMG]

    I also purchased a new exterior 100Amp Square D QO panel with 8 spaces. I would like to mount the sub-panel in the garage near my woodworking gear. I also have a big air compressor which pulls 220V at 15A and I plan on adding a dust collector which will pull 220V at 20A and a few more 220V tools which should be wired to dedicated breakers (in my mind) and not have a plug.

    I plan on running 4-3 Indoor Non-Metallic Jacket Wire (this stuff: http://www.lowes.com/pd_113001-295-...uct_qty_sales_dollar|1&page=1&facetInfo=4 AWG) across the attic about 20 feet from the main panel to the sub-panel, which will be screwed to plywood on a new wall I have erected in the garage.

    So...

    I would like to avoid spending more than I have to to get this done.

    Q1: Assuming I understand the things I need to do to get this done, can I call the city, get a permit, do the rough work, call for rough inspection, then finish the work and call for finish inspection - all myself? No electrician hired at all? (depends on the city I suspect)

    Q2: My existing WWII-aged panel looks like it's way over loaded. If I add up all those breakers, it adds to, like, 410 amps. Why doesn't my house burn down? I don't have breakers tripping all the time. Since there's no room on the panel for me to add a new breaker to power this new sub-panel, am I forced to upgrade this panel before I can think about the sub-panel?

    Thanks in advance. This is my first post - this is my first sub-panel. So be gentle.
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    Yes, possibly so, but sometimes a homeowner is allowed to do his own stuff simply because it is his own stuff. In my own case, however, I had an electrician pull the permit and do the riser/cables upgrade while I changed the panel so he would be the one calling the power company and getting good response for the upgrade disconnect and reconnect rather than my taking a chance on their just coming whenever it might have been convenient for them and then possibly not coming back again until the next day.

    Each breaker protects its own circuit from overload, then the main breaker protects the incoming line from being overdrawn. So, the total sum of all circuit breakers is not a primary factor unless you might somehow manage to have all circuits running at full capacity all at once. In my case, I had 60-amp feed wire from the pole coming into a very-old 60amp panel (containing breakers totaling 180 amps) plus a couple of sub-panels adding up to an additional 170 amps...and all of that means there were definitely times when my total load was more than 60 amps. That was not good, of course, but the previous homeowner had been allowed to do his own work and the inspector (as I discovered for myself) does barely more than a drive-by to mark his approval.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,019
    Location:
    Cave Creek, Arizona
    I would say that changing the panels and then rewiring them would be VERY ambitious DIY project, unless you have a day or so available to just work on it. you would probably be without power during that time since the meter would usually have to be removed to make the switch.
    Last edited: Jun 21, 2014
  4. bluebinky

    bluebinky Member

    Messages:
    408
    Location:
    Santa Clara, CA
    Where are you in Texas? Downtown or in the middle of God's country (nowhere)? It can make a huge difference in how uptight everyone is...

    Unless there is a separate disconnect at the meter, the process will probably be something like the following:

    1) get permit
    2) have the electric utility pull (take) the meter
    3) wire things up
    4) get inspection
    (if you fail inspection, you will be without power until things are fixed and re-inspected)
    5) have the electric utility replace the meter
    6) do mandated upgrades like CO detector, if any
    7) finish up
    8) get final inspection

    Like hj says, this can take a lot of time and several trips to the store (likely days) -- but not for someone like hj.

    First check online if your local requirements are explained there.
    Then, go downtown and have a chat with the building department and hopefully they will explain the real process.
  5. Vegas_sparky

    Vegas_sparky Digital Billy

    Messages:
    370
    Location:
    LV,NV/ Nowhere,UT
    Is that existing 100A panel your main? If so, what do you have at the meter? You'll need new feeders from the meter if that is your main.
  6. PatrickH

    PatrickH New Member

    Messages:
    17
    Location:
    Virginia
    Just a few thoughts.

    1. May want to check with your county building code compliance department in order to determine their minimum requirements for an electrical service upgrade. Sometimes they require you to do things like upgrade your kitchen and bathroom circuits. Also ask about Arc fault requirements.
    2. You need to check with your utility company to ensure that the service lateral will support your new service, especially if it's underground.
    3. You will likely have to upsize the meter box to support to larger service. Power companies generally provide these when you get an electrical permit. If the service is underground, then you'll likely need to provide a new PVC stub, or if overhead, then you'll likely need to provide a weather head, and attachment point.
    4. You may also need to update the building ground.

    It'll take a good full day to complete the service change part, so pick a cool day. Also, make sure you have a generator as once you pull the meter, you lose the house power.

    Your existing panel has some piggyback breakers in it already. Am not sure what's on the other 20 amp single pole breakers, but you might be able to add two more piggy backs and add a new 2 pole 30 amp breaker to the existing panel. I'd likely install a 220 volt outlet for the circuit. Won't get you where you really want to be, but it might scratch the itch to use your saw. Good Luck! Be careful!
  7. chewmanfoo

    chewmanfoo New Member

    Messages:
    3
    Location:
    Texas
    suppose I just add the sub-panel?

    Suppose I just do the following:

    1.) pull the appropriate wire for a 100amp sub panel.
    2.) install a 100amp breaker inside my existing 100amp panel by doubling up enough breakers in the main panel to make room.
    3.) install one 30 amp breaker in the sub panel to power the saw.

    Would that work? In other words, not doing all the work all at once, but taking little steps which are appropriate to get me there. The only concern I have is in adding a 100amp breaker to a 100amp service panel. Seems like somebody would call bullshit in some inspection.

    TIA,
    Chewmanfoo

  8. PatrickH

    PatrickH New Member

    Messages:
    17
    Location:
    Virginia
    I'm not 100% positive, but I suspect that the 30 amp two pole breaker could be used in the new subpanel when you ready to make the big switch. Recommend you do this top down, beginning with the main electrical service. Be careful of shared neutrals if you add piggybacks. You can't share a neutral on the same phase. Phases alternate top to bottom 1&2 on black, 3&4 on red, 5&6 on black, 7&8 on red, etc. GL
    Last edited: Jun 23, 2014
  9. bluebinky

    bluebinky Member

    Messages:
    408
    Location:
    Santa Clara, CA
    By the way, do you have an electric clothes dryer in the Garage?
  10. chewmanfoo

    chewmanfoo New Member

    Messages:
    3
    Location:
    Texas
    I do. 30amp
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