4 Wire cable?

Discussion in 'Electrical Forum discussion & Blog' started by Nate R, Dec 12, 2007.

  1. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    I have a cable that has 2 hots, 1 neutral and 1 ground. Each conductor is AWG 12.

    Can I run 20 amps per Hot leg, or 20 Amps total only since there is 1 neutral?
  2. 480sparky

    480sparky In the Trades

    Messages:
    149
    You can put each 'hot' on a 20a breaker, as long as they are on difference 'phases' or 'legs'.
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Be careful, because if you screw up a multiwire circuit, you can smoke that neutral!
  4. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    20 amps per leg.



    The smoke (heat more likely) will come from the neutral if both circuits are loaded to capacity and are on the same leg.

    The smoke will come from whatever is operated from the circuits if the neutral connection is lost going back to the source.
  5. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Well, it's on a generator, actually.

    Generator came with a 4 wire cord that goes to 2 duplex receptacles on the other end. Manual says 2 recepts are on one leg, and 2 are on another.

    Says not to exceed 20 amps, but then goes on to give an example pulling 4800 watts from the receptacles and states not to exeed the output of the generator.

    I think they meant not to exceed 20 amps per LEG (Breakers are 20 Amps per leg, I think) and total not to exceed the generator output. Just double checking that would make sense.
  6. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The generator probably has its own 20-Amp 2-pole circuit breaker.

    That breaker will trip if EITHER side draws more than 20 Amps for a significant time.

    Since you can theoretically draw 20 Amps from each you can get 20x120=2400 Watts from each or 4800 Watts from both.

    That kind of connection is a nuisance on a modest size generator if you don't need 240 Volts for something like a pump because you have to keep each side at or under 20 Amps. You will almost certainly have a margin on each circuit because your loads won't total 20 Amps on each side.

    I have a similar generator but there were connections and a selector switch to make all of the output 120 Volts with the same polarity. I used #8 for the conductors to my panel where I backfed a breaker interlocked with the main (Yes, it is a UL listed interlock for a QO panel).

    Now I can run any 120 Volt load in the house as long as I manage the total load. Since I don't have a 240 Volt pump I can run any circuit except the dryer, water heater, cooktop, or oven and all I need to worry about for load management is to keep the total below the 40-Amp capacity of the generator.
  7. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Thanks for the input everyone!

    Wife and I were just going to get a generator large enough to power our furnace and a couple little things. We were looking at the 1800 watt Coleman at HD for $400. Decided to get the genny now as we had an ice storm coming in this week. Went in and found that at 5000W Gen w/ a Robin/Subaru engine was on sale for $400 instead of $600. Why pass up 5KW for the price of 1.8KW?

    So, because I got the larger Gen, I'm thinking I'll get a multi-circuit transfer switch instead of the single circ one I was looking at. (I know this will probably take out any savings, but that's fine.)

    Given that we have a gas WH, dryer and stove, I think we'd be able to function pretty darn well on only 5000W in summer or winter.
  8. 480sparky

    480sparky In the Trades

    Messages:
    149
    The best advice I can give someone who is wanting to 'weather the storm' using a genny is to not run it 24hours. Run it long enough to let the furnace bring the house back up to a comfortable temperature, the fridge & freezer down enough to keep things from spoiling, and don't bother with doing laundry.

    The next best advice I can give is to properly install a permanently-mounted transfer switch. Manual or automatic, makes no difference. But when it comes time to use the genny, they make a world of difference! :)
  9. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The 5 kW is much better because it keeps you away from operating at the margin.

    A UL Listed interlock (if available for your panel) instead of a trasfer switch is a lot more convenient. It allows any (but not all at the same time) of the circuits in your main panel to be powered by the generator and you don't have to select and re-wire any circuits through a transfer switch. The interlock also costs less than a transfer switch. Mine was about $80 including shipping from a Square D electrical dealer who listed on that famous auction site.

    It works as follows on my Square D QO panel.
    Installation:
    1. Move the breakers in the top two spaces in the right hand column of the panel to a different location.
    2. Install a 2-pole breaker of the appropriate size in those spaces and fasten it in place with a device (furnished with the kit) to prevent it from being inadvertantly removed.
    3. Connect the generator to the terminals of the new breaker.
    4. Mount the interlock kit on the face of the panel cover using a template and fasteners furnished with the kit.

    Operation when you want to operate with the generator:
    1. Start the generator to let it warm up. (It is not yet powering the house.)
    2. Turn off the main breaker, which will now allow the "generator breaker" to be turned on, but don't do it yet.
    3. Turn off the breakers or switches to any large loads that will overload the generator, such as A/C, dryer, or 3000 Watts of Christmas lights.
    4. Slide the interlock device up and switch on the "generator breaker".

    The switchover process takes about 30 seconds. Reverse the process when power is restored.

    You now have power to lights, computer, TV, refrigerator, and any other circuit in the house as long as the total load doesn't exceed the capacity of the generator. You will typically not be able to operate large 240 Volt circuits.

    You will need to set some rules for the houseold such as no hair dryers, turn out lights when not needed, no toasters, and especially limit multiple uses at the same time. You might not want to operate the microwave and some other big load at the same time.

    I have my 5 kW generator hooked up in 120 Volt configuration as described in an earlier post on this thread and tested it by starting a 1 HP motor while a 14,000 BTU wall A/C, the computer, and some lights were all on, and it just kept going without a blink.
  10. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT
    http://www.interlockkit.com/

    I like to install these, you have to be somewhat smart not to overload your generator, but they give you all the freedom to turn on and off what you wish.... and VERY inexpensive to install.
  11. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The kit is fine but the Square D kit cost about half the price listed at that site. Search QO interlock and check the "Search title and description" block in the "advanced search" screen at that famous auction site.

    See item numbers 160189186389 290188893433 and 290179696076
  12. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    Hmm.

    Well, originally I was going to add a subpanel to the existing main box and put all the new circs in there for the work I'm doing. But then wiring the transfer switch setup would be even more fun.

    Looks like it will be easiest and cheapest to replace the main panel (Only a 12 space panel) with one that will hold more than enough circuits for what i need for the house. Just have to figure out how to remove the conduit, as all the lines in/out of the main box are through conduit currently.

    But 1 box with enough breakers and an interlock switch seems much cheaper and easier for my home's rewire.

    Didn't know about the interlock. Simple, and genius!
  13. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    That panel interlock switch sounds really useful.

    I have something similar, but I came of the 30 amp 120V generator outlet and wired both hots so that the generator powers both legs with the same phase.

    This automatically disables the larger loads like the water heater, Drier, and kitchen oven.
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