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Thread: Discovered a 10 AWG Cable

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    DIY Junior Member GoldMaple's Avatar
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    Default Discovered a 10 AWG Cable

    I live in an old house and I have been doing some renovating. I was looking for a way to get to my electrical panel because I'd like to run a couple of new circuits into my basement. Given the location of the Panel it was not going to be an easy task. While looking for a route I discovered a wire coiled up and stuck in the joists. It was brand new and disappeared so I opened up the panel upstairs and there was the other end coiled up in the Panel box. It was not hooked up. How handy! So, the wire is fairly thick and it has "10 AWG 300 volt" stamped on it. So, I think it's 10 gauge and capable of handling 30 amps? My question is.... Can I attach a very small panel box in my basement to this wire and add one 10 amp and two 15 amp breakers? Or am I constrained to the 30 amps, so only two 15 amp breakers? I'm not sure what this cable is capable of handling. Thanks!

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    Licensed Electrical Contractor Speedy Petey's Avatar
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    30A does NOT mean "two 15a breakers". Also, there are no 10a breakers. If you find one it is an oddball and most likely for industrial use.

    Let's get the basic info first. What conductors are in this cable? If it is black, white, red and GROUND (bare or green), then you can effectively install a sub-panel.
    If it is just black, white and ground then you can install a sub-panel, but it would be a bit hacky as you would be limited to a 120v panel.
    Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Quote Originally Posted by Speedy Petey View Post
    30A does NOT mean "two 15a breakers". Also, there are no 10a breakers. If you find one it is an oddball and most likely for industrial use.
    You can provide any number of breakers provided that you don't exceed the 30A of the cable (which is protected by the panel's main breaker). You'll need to do load calcs to ensure that your loads are within 80% of 30A.

    You'll have a tough time finding a panel with a 30A main. Most likely you'll need to backfeed the panel through a breaker which has some requirements (labeling and hold down, I believe).

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    DIY Junior Member GoldMaple's Avatar
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    Default Wires

    Okay, good. The cable has 4 wires, a red, black, white and a bare wire. I'm hoping that I could add a small panel with 4 breakers. I mentioned a 10 amp breaker because I would like to run a dedicated circuit into the bathroom I'm renovating and use it for the heated floor. All the manufacturers stated that it must be on a dedicated circuit or the GFI will be tripping all the time. The heated floor will take less than 4 amps because the bathroom is so small. Another breaker I'd like to run to my washing machine because someone connected the current circuit to the kitchen upstairs so when my microwave goes on and the washer is on the breaker trips. The third breaker I'd like to save for when I redo my upstairs bathroom and use for the heated floor. Again, it will draw less than 4 amps when running. Both heated floors won't be running all the time. Probably run then on a programmable timer for times we know we'll use it. I would then like to have the option of adding one more circuit for future use. Does this seem reasonable?

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    Moderator & Master Plumber hj's Avatar
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    Default wires

    I would use a small sub panel without the main since the 30 amp breaker in the main panel would provide the "master" breaker function.

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    Senior Robin Hood Guy Ian Gills's Avatar
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    It can't be done with what you are intending to do, but if my needs were more modest (two 15 or 20 amp circuits) I would be tempted to use that cable as a shared neutral and run two circuits directly from the panel using it.

    Of course you cannot very easilly use GFCI or AFCI breakers with a shared neutral and these are very often required with certain types of new wiring work.

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    Does this seem reasonable?
    Sounds reasonable to me.

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    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by Lakee911 View Post

    You'll have a tough time finding a panel with a 30A main. Most likely you'll need to backfeed the panel through a breaker which has some requirements (labeling and hold down, I believe).
    He does NOT need a 30A main.
    He can use any panel. Typically a small sub-panel will have a 60 or 100A rating. They can be fed with anything from 30A up to their rating.
    Also, no main breaker is required since this sub-panel will be within the same structure as the main panel. A main-lug panel would be a typical install.
    Last edited by Speedy Petey; 10-08-2009 at 06:55 PM. Reason: Add missing "to" in ... "up to their rating".
    Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.

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    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by Ian Gills View Post
    It can't be done with what you are intending to do, but if my needs were more modest (two 15 or 20 amp circuits) I would be tempted to use that cable as a shared neutral and run two circuits directly from the panel using it.

    Of course you cannot very easilly use GFCI or AFCI breakers with a shared neutral and these are very often required with certain types of new wiring work.
    Why can't it be done?? Sounds like it will to me.

    Also, two-pole GFCI breakers are very common, and two-pole AFCI breakers are getting more common.
    Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Quote Originally Posted by Speedy Petey View Post
    He does NOT need a 30A main.
    He can use any panel. Typically a small sub-panel will have a 60 or 100A rating. They can be fed with anything from 30A up their rating.
    Also, no main breaker is required since this sub-panel will be within the same structure as the main panel. A main-lug panel would be a typical install.
    You're right.

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    DIY Junior Member GoldMaple's Avatar
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    Thumbs up Another Question

    Thanks for all the replies. I really appreciate it.....

    Here's another question. It seems to me that the simplest way to do this is to add a lug sub panel that uses the breakers in the main panel. I understand that I could power the sub panel with a 3 wire cable but it's better with a 4 wire cable and with the 4 wire cable I would use a double pole breaker. What I don't understand is why is it better to use a 4 wire cable in this configuration? Whether I use a 3 or 4 wire cable it's 30 amps so there must be something else?

    (I have a 4 wire cable and will use a double pole breaker)

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    Licensed Electrical Contractor Speedy Petey's Avatar
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    A 3-wire cable would give you 30A @ 120v, or 3600 watts.
    A 4-wire cable would give you 30A @ 120/240v, or 7200 watts.
    Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.

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    DIY Junior Member GoldMaple's Avatar
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    Okay, so given...

    A 3-wire cable would give you 30A @ 120v, or 3600 watts.
    A 4-wire cable would give you 30A @ 120/240v, or 7200 watts.

    and "Watts = Volts x Amps" and

    3600 = 120 * 30 3 wire
    7200 = 240 * 30 4 wire

    then by using a 4 wire cable we double the volts which doubles the watts. Based on what I see here is that with a four wire cable I draw twice the power before I utilize all the amps. Is this correct? If so, it's a far superior setup.

  14. #14
    Licensed Electrical Contractor Speedy Petey's Avatar
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    A 240v panel is actually a typical setup. A 120v panel is extremely odd.
    Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.

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    Senior Robin Hood Guy Ian Gills's Avatar
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    Also, two-pole GFCI breakers are very common, and two-pole AFCI breakers are getting more common.
    I did not realise you were so rich Petey. I wish I could afford one of those.

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