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Residental mains wiring questions (USA)

Discussion in 'Misc Electronics' started by Bob E., Jun 30, 2010.

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  1. Bob E.

    Bob E. Guest

    I am installing a new meter & load panel. The old panel is 50 feet from the
    new location. I'm going to install a junction box at the old location and
    splice new wires to run in conduit to the new panel. The existing circuits
    are a mix of 120v 15A & 20A circuits, plus a 220v 20A (weird, I know), a
    total of 10 circuits.

    Can I use a single neutral wire between these 2 boxes? How do I size it?

    The mast for the service conductors must be threaded at the bottom end (where
    it mates with the meter box). How about the top end (where the weatherhead
    mounts to the mast)? Must it also be threaded, or can the weatherhead be a
    non-threaded type?

    This is in N. California.

    Thanks.
     
  2. Bob E.

    Bob E. Guest

    Thanks, Tom.
    OK: size for the greater of the 2 phases used for those circuits. Since this
    depends how one runs those to the new panel, it's important how those are
    terminated. I was going to balance the loads, overall, but now I see how this
    impacts the neutral conductor size.
    I can find all kinds of charts on-line that show me how many 10 or 12 or 14
    ga conductors I can run in a 1-1/4 inch EMT, but not conductor cross-section.
    Do these exist? Or do I just convert the chart myself (ie, 25 conductors x 12
    ga cross-section)?
    EMT, so a grounding conductor is a must. By "the largest circuit in the
    conduit" you mean that if the biggest is 10 ga / 30 A, I should run a ground
    conductor sized for that one circuit?
    Right! Thanks!
     
  3. Glen Walpert

    Glen Walpert Guest

    In accordance with your local codes. Not all jurisdictions use the same
    version of the NEC, and some have additional requirements.
    Your local Authority Having Jurisdiction (building inspector) has the
    final say, but any local licensed electrical inspector should be able to
    answer your questions. I suggest you discuss the installation in advance
    with the inspector you plan to hire for the final inspection, and also
    that you consider leaving the old panel intact as a sub-panel with its
    main fed from a branch breaker in the new panel. It would help a lot if
    you get whichever code applies in your area and read it carefully before
    talking to the inspector, so you can ask reasonable questions or better
    yet have reasonable drawings for review. This is generally cheaper than
    doing the job twice.

    If you don't have the required tools such as torque screwdrivers and
    wrenches and some training in electrical work, or a willingness to obtain
    both, then you will be a lot better off just hiring an electrician for
    this sort of job.
     
  4. Rich.

    Rich. Guest

    Save yourself a world of headaches and don't use conduit. Instead just match
    the size and type of each cable there and run a matching romex cable over to
    the new location. Better yet, for any cable runs that are exposed, unstaple
    them and run them towards the new location to help keep the length of the
    run to a minimum. You will still need to put the splices in a j-box, but
    you're going to avoid the neutral issue, conduit bending and fitting, plus
    you won't have to figure and make allowances for derating of the conductors.
     
  5. Bob E.

    Bob E. Guest

    Can you keep the old panel where it is? If so, the easiest way to feed
    The old box probably won't pass inspection. It's an old Zinsco(sp?) box with
    the ratty breakers and the cover's missing. I plan to replace it with a
    proper J-box. The romex will terminate in the box without a problem, I think.
    These are all ungrounded circuits, but a separate ground wire will be run
    from each outlet over the roof (it's a flat roof that's being overhauled) to
    the new main panel. The ground conductor doesn't have to run along side the
    power conductors, does it?
    10 existing circuits: 6x15A, 3x20A, 1x30A (220v). Can't I just oversize the
    conduit and extend the 14 ga (for 15A circuits), 12 ga (for 20A circuts), and
    10 ga (for 30A circuits)? My understanding is that the issue was heating in
    the conduit and that if you oversize the conduit (EMT) that you will avoid
    approaching the heating limit. No?
    Service feed is arial, from the pole to a mast on the roof.
    There are no existing grounds, but nonetheless, I will separate the box
    ground and neutral in the old panel (now sub panel).
    Thanks. I just wanted to know if an unthreaded could be used at the top.
    Looks straightforward.
    The utility's reference manual (that they gladly hand out) states the max,
    min, and other parameters. Seems pretty clear and they provide a phone number
    for answers.
    It has to go through the roof, so 2" threaded according to the utility
    (PG&E).
    I'm a big boy. Just ask my GF. ;-) No worries, mate. Thanks.
     
  6. PeterD

    PeterD Guest

    Huh? Your outlet wires go over the roof? Uh, no you don't take
    seperate runs for the ground wire, all need to go together. You
    mention 'passing inspection'. That won't...
    Yes, they do.
    You can get and use conduit as large as you want.
    Consider that the main breaker must be within a given distance of
    where the service entry wire comes into the building. A long run of SE
    to a 'remote' box will likely also raise some questions with the
    inspector.
    I'd strongly recommend a session with the building inspector in your
    area, describe what you want to do, and ask him/her if that will pass.
    If they say "no", then look for other alternatives. If they say it is
    "OK", ask if they have any suggestions or things to watch for.

    Their advice, without any doubt, will be worth more than all the
    advice you will ever get on the Internet.
     
  7. JosephKK

    JosephKK Guest

    No. The NEC rule is one neutral per breaker.

    Where to you find non-threaded weatherheads?
     
  8. PeterD

    PeterD Guest

    a 15 amp wire (there is no such thing, you are referring to a 14 AWG
    wire), derated by 50% would be: 15 * 0.5, or 7.5 amps max. Not 30
    amps!
    Again, totally flawed math. The 14 AWG would be (roughly) the equal to
    18 AWG. Not 10 AWG!
     
  9. JosephKK

    JosephKK Guest

    That is just asking for multiple code violations.
     
  10. Rich.

    Rich. Guest

    Um, it's completely legal and approved. Why are you suggesting it's not?
     
  11. JosephKK

    JosephKK Guest

    How about that there is not appropriate bus transfer? Nor is the
    occupancy and some other special applications properly addressed. There
    were big changes between the 2005 and the 2008 NEC for all classes of
    backup and alternative power systems.
     
  12. JosephKK

    JosephKK Guest

    Yes it does. 2005 NEC 250.24(C)(1) This [grounding] conductor shall be
    routed with the phase conductors...
    Give me each of the run lengths and i will calculate it up for you. Also,
    i will need to know how the conduit is mounted and against what material.
    I also will want to know what conductor insulation you are considering, i
    may ask you to change it.
    Bonding the neutral at the service entrance is an NEC requirement.
    Bonding the rest of the neutrals is a separate and somewhat twitchy
    matter. I was party to a 3 hour meeting in the workplace trying to
    determine correct policy on this a month or two back.
    You likely will have to install a grounding electrode. If you just
    follow NEC you certainly will.
    Don't be afraid to use that phone number. They would much rather that
    you get it right. It is definitely in their interest that you do.
     
  13. Rich.

    Rich. Guest

    What the heck are you talking about. All he's doing is relocating the panel
    and extending the homeruns over to the new location. There's no back-up or
    alternate power sources being used here.
     
  14. JosephKK

    JosephKK Guest

    Wire ampacity is not derated due to conduit fill. The derating for
    operating temperature is usually minor. (pretty much for small
    conductors #6 AWG and smaller)
     
  15. JosephKK

    JosephKK Guest

    For the level and kind of change contemplated it is near certainty that
    upgrading to current code can be required. It is an Authority Having
    Jurisdiction thing and if they say upgrade you upgrade.
    I third that.
     
  16. JosephKK

    JosephKK Guest

    If i am fully informed about what is existing and what is contemplated i
    can give "official" advice. Use the local electric company and
    inspectors as much as you can first, though.
    The NEC has cut that back pretty sharply recently. A single neutral for
    a single multipole breaker is allowed and very common. It may be allowed
    in some other cases, provided none of the load current is presented to
    the neutral conductor.
     
  17. Rich.

    Rich. Guest

    Multiwire circuits are now required to have a common trip (multi-pole)
    breaker feeding all circuits that share a neutral. In your example of 3 (20
    amp) outlet circuits in an office, yes you can run 3 hots and one neutral
    back. But now, instead of 3 1-pole 20 amp breakers you are required to
    install one 3-pole 20 amp breaker. The reasoning behind this is if you turn
    off one of the 1-pole 20 amp breakers to work on the circuit. Yes that one
    hot wire is dead, but if you were to break the neutral splice, the other two
    circuits sharing the neutral can backfeed the white wire and kill you.

    BTW, your load carrying neutral example is incorrect. The neutral carries
    current anytime there exists an imbalanced load between any two hot wires
    sharing a neutral. If you have two hots and one neutral with one hot
    carrying 10 amp and the other carrying 13 amp, then the neutral carries the
    difference of 3 amps. Assuming the 2 hots are correctly install to not be on
    the same phases. If they were on the same phases, then the neutral carries
    the combined load of 10 and 13 amps, or 23 amps. It's basically the same
    with a 3-phase setup except you throw the third hot into the mix and the
    unbalance load calculations are a bit more complex. If all hot carried the
    same current load, then the neutral is carrying nothing.
     
  18. All neutrals are white, but not all whites are neutrals. You have to
    Ah, it's beginning to make some sense to me. Thanks!
    Your example is for 3-phase, right? For residential split-phase, the vectors
    would be 180 degrees apart, right?
     
  19. JosephKK

    JosephKK Guest

    So i check the thread and i can't find any such either. I must have
    wigged over he new panel and j-box.
     
  20. JosephKK

    JosephKK Guest

    Thanks, i was about to try expressing that clearly. You already did it
    well.
     
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