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wiring generator

Discussion in 'Electrical Engineering' started by Robert A. Cibiras, Jul 15, 2005.

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  1. I am interested in wiring a generator to my home
    via transfer switch. It is a portable unit, not
    built in. My plan is to run a wire in the attic
    about 80', from the breaker box/transfer switch,
    to an outlet in a waterproof box under a covered
    patio, then use a double male "extension cord"
    from the outlet to the generator's 50 amp plug,
    for about another 45'.

    My question is this. What size wires should be
    used to safely move the power that distance? I
    have been told everything from 4 AWG to 10 AWG,
    mostly by alleged electricians who at least appear
    to make a living at it.

    During the last storm, I was using 100', 12/3
    extension cords to run individual freezers and the
    like, with no problem.

    I am looking to be safe about it. I am not
    interested in the cheapest thing, and I am
    definitely not interested in burning my house
    down. I'd like a little overkill, but on the other
    hand I don't necessarily want to spend a bunch of
    excess money for nothing.

    Understanding that obviously the larger the wire
    the better, without going totally overboard, what
    would be a reasonable and very safe wire size for
    this project?

    Is 6/3 w/ ground SER for the 80' run in the attic,
    and 6/4 SO for the 45' "extension cord" suitable,
    or is 4/3 SER and 4/4 SO needed, or is there
    something totally different out there?

    I am not planning on doing this myself, but I do
    want to be able to make an educated decision
    regarding what materials to use, and who to use to
    put it together. Seems like some people may cut
    corners on materials to get the bid.

    Many thanks.

  2. ehsjr

    ehsjr Guest

    You are unsafe there - double male - regardless of wire size.

    Use a hard wired male attached at the waterproof box on a
    short whip, and make a male/female extension for the run from
    the generator to the box to address that. You'll need to keep
    the male plug out of the weather, too, so that it doesn't sit
    there corroding month after month.

    You should use #4 copper for the attic run, and #4 SER
    for the other run to keep the voltage drop within 5%.
    Here's the math:

    You need to keep the voltage drop to 5% or less, (6 volts
    on a 120 volt circuit). You have a 160 foot run (hot + neutral)
    in the attic and a 90 foot run outside.
    #6 copper is .491 ohms per 1000 feet.
    #4 copper is .308 ohms per 1000 feet.
    #4 alumium is .508 ohms per thousand feet.
    (From Table 8 Conductor Properties in the 2005 National
    Electrical Code)

    Using SER aluminum for the 45 foot run (90 feet total wire):
    #4 aluminum is .508 ohms per 1000 feet. .508/1000*90 = .04572
    ohms. At 50 amps, it will drop 2.286 volts. That leaves a
    maximum of 3.714 (6 - 2.286) volts that can be dropped in the
    160 feet of wire in the attic, which means the total resistance
    of the 160 feet of wire must be 3.714/50 = .07428 ohms or less.
    At 160 feet, that means .07428/160 = .00046425 ohms/foot or
    ..46425 ohms/1000 feet, which makes #6 too small. You'll have
    to go with #4 copper in the attic at .308 ohms/1000 feet. It
    will have .04928 ohms total resistance, (.308/1000*160 = .04928)
    which means it will drop 2.464 volts. The total drop will be
    4.75 volts - about a 4% drop which is within the limit.

    You need to address "physical safety" of the extension cord
    such that cars don't roll over it and damage it, people don't
    trip on it - that kind of thing.

    It would be best if you dug a trench and used permanent wiring
    to the generator site, and had the male plug on its short whip
    inside a weatherproof box there. Then a short male/female
    extension from the generator to the male plug. (My plug is on
    a 2 foot whip in the garage, and I use an 8' extension to the
    generator. I run a 30 amp generator, and the wiring is 80 feet
    (160 in the computation, hot + neutral) of #6 copper.)

    Now *if* you are going to trench it, and *if* there is
    a possibility of using a bigger generator in the future,
    you should go to a bigger wire size. It will yield no
    real benefit now and will cost more for the wire, but
    you *don't* want to dig an 18" deep 45 foot trench more
    than once.

  3. Don't forget the approved transfer switch. Thousands of linemen will thank
    you when you don't accidentally electrocute them.

    Charles Perry P.E.
  4. SQLit

    SQLit Guest

    The others have pointed you in the right direction. Sounds as if you have
    not gotten the transfer switch yet... So look into a 4 pole switch, so you
    can switch the neutral. The reason I suggest this is because of the distance
    between the generator and the service.
    What have you done to ground your generator effectively?

    Running four wires will assure that everything will work correctly. The
    generator creates a new neutral. Hence the reason I like the 4 pole transfer
    switches. Nothing says this as to be an automatic switch you could use a 4
    pole double throw switch as long as it was service rated.

    This one is used and over sized for your application. Offered as a hint what
    to look for. If your generator is only 50 amps you would be able to install
    a 60 amp switch. CH used to make some residential stuff. I never used it or
    installed it.
  5. operator jay

    operator jay Guest |

    I'd suggest sketching a 5-line diagram (as opposed to a single-line).
    Sketch out a couple areas of the distro using five lines representing hot,
    hot, neutral, bond, misc metal connections (such as equipment cases,
    conduit*, structural metal). *Your conduit could be part of your bonding,
    at least here it could. I find this handy to determine what will (and
    won't) happen -- make sure neutral current won't be returning on bonding or
    other connections, check whether a switched neutral or EGC would still leave
    everything safe, allow devices to trip that are supposed to trip, etc.

  6. SQLit

    SQLit Guest

    OOPS,, fat fingered that one I meant 3 pole.

    I am pretty sure that disconnecting the ground execpt in testing situations
    is not allowed.

    Sorry for the confusion |
  7. Mike Lamond

    Mike Lamond Guest

    The transfer switch has to be sized for the larger of the generator capacity
    or the
    utility side circuit feeding it. If the switch is going to be installed on
    the service
    entrance, ahead of the main circuit panel, it will have to be 100 or 200
    A 60 amp switch feeding a subpanel would limit the emergency circuits to a
    amp feeder.


  8. Thanks for all the info. I have obtained 4AWG wire
    for the project. The final piece of the puzzle to
    avoid the double male plug is a replaceable female
    receptacle for one end of the extension cord to
    plug into the inlet box refenenced above. The male
    ends are a dime a dozen at Lowes, Home Depot, etc.
    The female is not. I have located 50 amp extension
    cords for RVs, though generally only with 6AWG
    wire, and no more than 30' long, which have the
    female end on them, so I know they exist. I have
    found replacements for a 30 amp female receptacle at

    , but no 50's.

    Anybody know where I can locate such? I have tried
    the electrical supply houses in town, and have
    found the closet thing on the internet on RV sites.


  9. ehsjr

    ehsjr Guest

    You're looking for something like this?
    Give your electrical supply house the information.
    You want a "50 amp cordset connector, Pass & Seymore model CR6364"
    or equivalent. Perhaps they can order it for you.

    Two good sites:
    and 2003.pdf

    You can order it online for $70.00 here (watch out for line wrap):
    or here for $45.00
    or here for $43.00 (watch the line wrap)

    Check all the sites to make sure of what you are getting.

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