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Thread: Minimum wire gauge required for sub-panel

  1. #1

    Default Minimum wire gauge required for sub-panel

    I am running wire underground to a guest cabin in PVC conduit that is 150ft from the main 100amp panel. I will be using a 30amp double breaker in main panel. The sub-panel is 70amp(because it was free), but will have two 15amp breakers. One breaker will be for 7 outlets, the other for ceiling fan/light, and approx. 5 can lights(75 watt each). I will have a small cabin fridge and lamps basically plugged in to the outlets. No A/C or large electrical equipment. Coffee pot and micro possibly as well.

    My question is: What is the minimum gauge 3 wire plus ground that I could get away with? Could I get away with 10/3???? This is a remote cabin and inspection/code isn't an issue. Obviously, I'd like to be safe and will have two ground rods coming from the sub-panel's ground bar.

    Thanks

  2. #2
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    Since you only have 2 circuits, this feeder will not be able to draw more than 15A. So a 15A double pole breaker in the main panel would be the minimum. For the 150' distance, I'd go up at least 1 wire size and preferably 2, so 12-2 minimum wire size. If you keep the main panel breaker to 20A double pole or smaller, then you don't even need a panel at the remote building. If you put in a panel you'll have growth potential, but also need the ground electrode system.

    If you use a 30A double pole breaker, the wire size cannot be smaller than 10-3.

    For your stated application, I think 10-3 will be fine. You could go to 8-3 if you want to prevent voltage drop. I'd consider making a receptacle only circuit that is 20A to accommodate the microwave and coffee pot. Or, give the microwave its own 15A circuit (on the same leg as the lights/fan). A panel with 3 circuits or more requires a 30A or larger feeder. If you have any plans to grow the equipment, I'd go larger on the wire (6-3). Will you ever have heating or cooling requirements (even just space heaters)? An electric range?
    Last edited by suemarkp; March 20th, 2013 at 07:41 PM.
    Mark
    Kent, WA

  3. #3

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    could I get by using a 20amp breaker in the sub-panel for the outlets and be fine plugging in micro and coffee pot? The other a 15amp breaker for lights? 30amp double breaker from the main panel 150' away using 10/3 or 8/3? I do want the sub-panel, so would I need a ground bar with 8 gauge copper out to 2 8ft ground rods 6ft apart, buried as well?

    I don't forsee any cooling equipment. I will have a direct-vent LP wall furnace, with a blower however. Don't think blower pulls much. I guess if the micro and blower come on at same time, may trip breaker?

  4. #4
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    Maybe. A microwave generally draws about 12 amps and a coffee pot about 10. Since they both cycle on and off, it will most likely work. But put nothing else on that circuit. I'd put any other receptacles on the lighting circuit (for smaller things like a TV or radio), so kind of have a kitchen area where only the 20A circuit is. Since you have a panel, adding another circuit is easy an inexpensive.

    Code wants "central heating" to have its own circuit. So you may want to put the wall furnace on its own (but that seems like a waste) or also put it on the lighting circuit. Or again, make a dedicated coffee/microwave circuit and than another receptacle circuit for all other outlets and this heater. Put this circuit and the lights in the same row of the breaker panel, and the micro/coffee circuit in the adjacent row (opposite phase leg from the other two circuits).

    I'd use 8-3 if you can afford the wire. It is just a small incremental cost compared to ever having to do it again. With 8-3, then it may not matter much which breaker row you use for which circuit. But in a feeder this small, an unbalanced panel can quickly make lights that flicker or circuits that "brown out", especially when the wire sizes are smaller.

    For the ground rods to the subpanel, I'd use #6 copper. Although code allows #8, a #8 GEC must be in conduit. #6 doesn't have to be as long as you staple it to a surface or keep it otherwise protected. Yes, two ground rods at least 6' apart and 8' to 10' would be better.
    Last edited by suemarkp; March 23rd, 2013 at 01:32 PM.
    Mark
    Kent, WA

  5. #5

    Default

    Thanks for your great knowledge. One more question just out of curiosity.

    I have already connected all the recepticle neutrals and grounds together with wirenuts(three each) and pigtailed leads to the neutral and ground bars. and have 3 hots to the breaker. I didn't pig tail those, as I could get all 3 blacks under the breaker screw with ease. One black could easily be switched to the other breaker that feeds the lights, as you suggested, and putting micro/coffee pot receptical on same circuit as lights, leaving he other recepticals on their own circuit.
    My question is:
    Can I leave the neutral and grounds together and just switch the hot to the other breaker, or should I undo wirenuts and remove that neutral and ground and connect directly to the bars? I don't know if it makes a difference or not that they are connected to the other neutral/grounds? It isn't a major deal if I have to, just easier to switch the black and be done.

    Also, why are the ground bars necessary? For lightning?

    Thanks again
    Dan, Mpls

  6. #6
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    If the neutral bar is long enough, put all your neutrals there (one neutral per hole). If you don't have enough holes, then you need to interconnect ONLY the neutrals on the same circuit. If you move the hot to another breaker, you could end up doubling the current in the neutral (so the pigtail to the neutral bar would need to be #8 ). I don't think a larger pigtail is allowed anymore. If the breakers happen to be opposite phase, then the neutral currents will subtract instead of add. But it best not to leave this up to the careful eye -- need to group your circuits together.

    Next, most breakers are only listed for one wire (some for two). I don't think any are listed for three. So you should pigtail these or just add more breakers to your panel.

    The ground bar primary function is a path to carry fault current (if a hot wire shorts to a metal box or metal appliance, the bare ground to that box will carry a lot of current -- so much that the breaker will trip very very quickly). The ground electrode system (ground rods) purpose is to shunt lightning surges to earth. The dirt won't allow enough current to flow at 120V to trip a circuit breaker. Lightning is 10's of thousands of volts, so a ground rod gives it a decent path.
    Last edited by suemarkp; April 2nd, 2013 at 12:11 AM.
    Mark
    Kent, WA

  7. #7

    Default

    Thanks!
    The sub-panel I'm using is the square D homeline 70amp(main lug I think), two spaces, 4 circuits.

    How can there only be 2 breakers and 4 circuits? I just realized that I have 4 circuits. Thought I only have 2. How does one turn two breakers into 4 circuits?

    http://www.fleetfarm.com/detail/squa...Fcw7MgodsxgAyg

  8. #8

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    with the use of a tandem breaker Brilliant!

  9. #9
    Join Date
    Dec 2003
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    Not a very big box. Some things to consider:

    • To be code legal, this box must say inside it somewhere "Suitable for use as Service Equipment". But you said codes aren't an issue, and if you really have no codes then I wouldn't worry about this rating.
    • You're going to be low on neutral holes because you lose one for the ground electrode conductor and you probably have no 240V or shared neurtal circuits.
    • You'll never be allow to have more than 4 circuits.

    You may want to spring for a larger box (perhaps an 8 to 12 slot box), and you may have to jump to the 100A or 125A sizes to get one Suitable for use as Service Equipment. They still don't cost much -- probably about $25. Not much to spend for more potential, and easier working space.
    Mark
    Kent, WA

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