Supplying ground to old subpanel

Discussion in 'Electrical Forum discussion & Blog' started by rerickson, Apr 18, 2010.

  1. rerickson

    rerickson experienced amateur

    Messages:
    14
    Location:
    Puget Sound
    I discovered our old garage/rec-room subpanel is fed with 3-wire 220V (no ground), with N-G bonded locally. :eek: (This is an attached garage, soon to have a laundry.) If I'm reading the tables correctly, this 50A feed requires a #10 Cu grounding conductor to the service. Before talking to the inspector, I'd like to run my plans past you for reasonableness.

    1) The existing feed is buried in walls, so the new ground won't be able to run beside it. (Code requires the 4 wires to run together, right?) I'll instead take a fairly-direct route, and label it well. Is this likely to be acceptable as the best fix for a bad situation?

    2) This grounding wire needs to be either protected (cable, conduit, etc) or #6 or larger. Instead of running 75' of #10 Romex, it appears that two #12's would cover the current rating of one #10, so I wonder about substituting both conductors of 12/2 Romex. There's a grounding bar at each end, so no splicing would be required. (It'd be necessary to mark with green tape at both ends, and probably green on the outer sheath wouldn't be a bad idea.) Is this reasonable and safe? (No point asking the inspector something that'll offend him if I can avoid it.)

    Of course I'll isolate N from G, adding a ground bar to this subpanel.

    Thanks much for your advice!
    Last edited: Apr 18, 2010
  2. drick

    drick In the Trades

    Messages:
    392
    Parallel conductors is not a good idea and I doubt the inspector would approve it. In theory it sounds good because you would think that 50% of the power would flow in each conductor. In practice this is not guaranteed because if one conductor has slightly less resistance than the other it can end up carrying 80% of the load.

    I doubt the inspector will care if the new ground wire follows the existing conductors. Sometimes you have to work with what is already there.

    -rick
  3. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    Your old panel is probably fed with SE cable that has a bare neutral/ground.

    There isn't a legal fix other than running a new 4 wire feed. Anything you try to do to fix it will just cause other code issues.

    The reason we separate grounds from neutrals in a sub panel is to keep the bonded/grounded wires/things plugged in from carrying current in case of a neutral failure.

    I'd just make sure the ground/neutral feeder connections are secure and leave it alone.
    Last edited by a moderator: Apr 18, 2010
  4. rerickson

    rerickson experienced amateur

    Messages:
    14
    Location:
    Puget Sound
    not SE

    Fortunately the subpanel's feeder is not SE; neutral is white, and similar to the other two: individually marked as TW, though in a cable. (Probably it's whatever they used in 1955.)
  5. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    I might run a ground then, seperate from the cable. It probably wouldn't pass an inspection but if the ground wire was run cleanly and secured, it would do the trick.

    I would still probably leave it.

    Again, the only time there would be a problem is if the feed neutral somehow got loose, broken or somehow "open". The current would then be flowing on any ground wires trying to find a path "to ground".
  6. rerickson

    rerickson experienced amateur

    Messages:
    14
    Location:
    Puget Sound
    Checking, I discover the old 50A feed starts out as SE (red, black, wire braid) and then splices (probably in the crawl space) to the cable with R/Bk/Wh type TW that's found at the subpanel.
  7. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    Great.

    I'd worry more about the splice than the neutral/ground thing.

    Consider replacing the cable(s)
  8. Furd

    Furd Engineer

    Messages:
    446
    Location:
    Wet side of Washington State
    To be compliant with current code you need to replace the entire run of cable from the sub-panel back to the service panel. In my opinion this would be something that you seriously consider as the existing condition does present some hazard.
  9. TWEAK

    TWEAK New Member

    Messages:
    86
    Location:
    Bay Area CA
    I would run a new 4-wire feed to the sub. You currently haven't got an effective safety ground, which is no big deal until you have a fault condition. But then it becomes *very* important. Since it's a garage you can put GFI protection in as well, always a good idea. And since there is some question about the splice, all the more reason to get in there and do it right.

    I used to try to avoid opening walls. Now, I don't hesitate... sheetrock is so fast and easy. Do it right, you will have a safer installation. Plus, no worries about insurance giving you grief over poor wiring in the event (but let's hope not) that you ever have a claim.
  10. rerickson

    rerickson experienced amateur

    Messages:
    14
    Location:
    Puget Sound
    Thought I'd give you an update: we're replacing the entire feed with four-wire SE-R (Al #2 with #4 ground). Wasn't as expensive as I'd feared, tho it's work to get it into place.

    I didn't trust the (location-unknown) splice; there's no avoiding inspection, and our inspector wouldn't approve adding a ground outside of the existing cable; and the old 3-wire SE segment turned out to be #8. So all signs pointed to replacement.

    It's my first time working with these wire sizes; found a flower pot of the right diameter to help get the 5x minimum turn radius.
  11. Norcal01

    Norcal01 New Member

    Messages:
    18
    Is your existing breaker going to accept the #2 AWG AL conductors?
  12. rerickson

    rerickson experienced amateur

    Messages:
    14
    Location:
    Puget Sound
    Edit - Yes, it turns out the existing square-D bkr lists a torque for #2 Al, and I was able to get the conductors to fit comfortably.
    Last edited: May 21, 2010
  13. Norcal01

    Norcal01 New Member

    Messages:
    18
    There are a number of issues when oversizing conductors, 1) the lugs ability to accept larger ones. 2) NEC art. 250.122(B), here is a copy & paste from the 2008 edition.

    (B) Increased in Size. Where ungrounded conductors are
    increased in size, equipment grounding conductors, where
    installed, shall be increased in size proportionately according
    to the circular mil area of the ungrounded conductors.
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