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  • Wiring Question

    I've got 2-2-2-4 aluminum SER cable running from my main breaker panel to a 125 amp sub panel in the garage. (The cable actually comes off of a 100 amp breaker at the main panel). Before going to the subpanel in the garage, I'd like to tap into this line with another subpanel - i.e. this one cable would run the garage box and a 40 amp breaker box for a rotary phase converter. These two panels would never be on at the same time, so I'm not concerned by the current draw - my question is about the physical wiring. What is my best bet for tapping into the cable? Is there a minimum box size required by code? I'd like to do this as cheap as possible and I've got some heavy gauge copper wire I was planning on feeding the RPC with, but I need the propper hardware to make the connection between aluminum and copper.

  • #2
    If you can run the 40amps off a 40 amp breaker in the subpanel to a disconnect I think would be the way to go. Or if it's in the house use a 40 amp breaker in the main panel.
    "Let me recommend the best medicine in the
    world: a long journey, at a mild season, through a pleasant
    country, in easy stages."
    ~ James Madison

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    • #3
      Tin-plated copper split-bolt connectors will do the job, but you may have to go to an industrial supply house to find them. They must be marked Cu/Al or something equivalent.

      The minimum ampacity of your proposed copper tap conductors can be determined in the NEC , art. 240-21. It is far too complicated to post the requirements here.

      RWO

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      • #4
        I suggest that you simply install a 50 amp dryer/stove breaker in your sub-panel in the garage, and then feed the second panel main from there. Gives you a lot of protection and can simply turn off the breaker in the garage panel when you want to kill the secondary panel.

        Be sure to use some of that nice aluminum ant-seize gook on your aluminum wire connections at all locations. It will really keep the long time corrosion down. My favorite one is "Penetrox A Anti-Oxidant", in an 8 oz Squeeze Bottle.

        George
        My Web Site
        www.mrrace.com
        Builder & Test Pilot N73EX

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        • #5
          The trouble is, I don't have enough spaces in the subpanel to run the RPC, too. It would save $$ on wire costs if I could locate the rpc near the sub panel (I actually wanted to place it on the other side of a concrete wall from where the sub-panel is located).

          Unless ... can I hook two conductors to one breaker if I twist the conductors together first? One would go to a 230 volt outlet and the other to another breaker box for the RPC. Seems like that would be a violation of electrical code.

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          • #6
            Originally posted by Fasttrack
            Unless ... can I hook two conductors to one breaker if I twist the conductors together first? One would go to a 230 volt outlet and the other to another breaker box for the RPC. Seems like that would be a violation of electrical code.
            No, don't do that. You essentially want to wire two sub-panels off of one feed using a pull box or junction box. Split bolt connectors should work along with anti-corrosion paste. Should it be a 6x6 or 9x9" pull box? Can't tell you, this calculation for box capacity and wire fill goes beyond any of the simple calculators found on the 'net. I'd ask an electrician or dig into the NEC article 314 C for more info.
            Last edited by Rosco-P; 06-22-2012, 04:04 PM.

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            • #7
              Thanks for the help so far!

              So can I connect two 2 awg and one 6 awg with a split bolt connector? How do I insulate/seperate the connectors once I have made the connection? E-tape?

              EDIT: I guess I don't need to connect two 2 awg conductors - I'd just strip a section of insulation away from the 2 awg cable and use it as a "tap" rather than a "connector". I was thinking I'd need a connector for three conductors - sort of like a giant wire nut.
              Last edited by Fasttrack; 06-22-2012, 04:07 PM.

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              • #8
                Are you planning on having this installation inspected? You might avoid some rework by sketching out your planned wiring, including wire sizes, length of run and panel capacities and talking this over with you electrical inspector. He might have a very different idea in mind in order to get your installation approved. Like feeding your RPC panel from a set of lugs installed in your first panel.

                Just saying....
                Last edited by Rosco-P; 06-22-2012, 04:56 PM.

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                • #9
                  My RPC sits on an anti-fatique mat on the floor under my panel. When not in use I flip off it's breaker & when I need it I open the panel, flip the breaker,reach down & turn it on. Iis very smooth & quiet & the pad keeps it off the concrete.
                  "Let me recommend the best medicine in the
                  world: a long journey, at a mild season, through a pleasant
                  country, in easy stages."
                  ~ James Madison

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                  • #10
                    How about using a couple tandem breakers to make room in the garage panel for the 40A breaker?

                    That's assuming your panel is approved for tandem breakers.

                    This would be the cheapest and easiest solution, I think.

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                    • #11
                      HI Fasttrack,


                      There is definitely a minimum box required by code. To do this from NEC "first principles" you need to do the box fill calculation for the conductors entering and leaving the box and verify that the box is large enough to handle all of those conductors.

                      If I remember correctly, the "tap rule" in the NEC allows the wire you are talking about to be protected by the main breaker in the sub panel rather than needing a breaker at the point where it is tapped if the wires are big enough. This should allow you to make the tap in a good sized electrical box rather than a panel. This also means that you don't have to use a full scale box for the RPC and can use a single breaker box like the kind that normal people would use for a hot-tub. If it is done properly, you should be able to leave both the RPC breaker and the garage breaker on as the chances of having enough stuff running to actually to blow the breakers probably will be pretty low.

                      Don't forget to trun off the power before cutting into the wire

                      --Cameron

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                      • #12
                        Just an additional note, no connection between neutral and ground in the subpanel. They only get tied in the main panel.

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                        • #13
                          Hsm'er is on the track I'd use. Put tandem or 1/2" breakers in for 4 - 20 or 15 amp circuits which would make room for your 40 amp breaker.

                          Steve

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                          • #14
                            Originally posted by ckelloug
                            If I remember correctly, the "tap rule" in the NEC allows the wire you are talking about to be protected by the main breaker in the sub panel rather than needing a breaker at the point where it is tapped if the wires are big enough.
                            --Cameron
                            That's backward. The sub-panel is protected by a breaker in the main panel which it is fed from. The sub-panel(s) may not have a main breaker, only main-lugs.

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                            • #15
                              Another possibility would be to remove one or two of your circuits from your existing panel, feed your sub-panel from a double breaker installed in those slots, and feed the removed circuits from new breakers in your new sub-panel.
                              Don Young

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