Wiring sub panels for out buildings from main

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Old 03-29-10, 06:23 PM
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Wiring sub panels for out buildings from main

My name is Jeff, 3 years ago I wired a lock off basement of some 1600sf w/ bdrm, kitchen, bath, theatre room, lots of lighting and more (all 120v circuits). I passed all inspections with no problems and even a few cudos from the inspector. So I have some experience.

I am in the planning stages for rewiring some out buildings on a property I recently purchased. I am having trouble understanding voltage drops and I really want to do this right. So here goes. From the main panel to the nearest corner to out buildings is 100’. Currently at that corner there is a 3wire #6 from the main panel to a fused disconnect to run a well pump, in Ύ” emt for the metal ground. From that corner to the middle out building is approx. 130’ and then 40’ to each bldg from there. Each out building is approx. 22’ by 100’ greenhouse, with approx. 8’ between for a total of 270’ to farthest building from main panel. Metal frame with thick plastic (pvc) type covering, plywood ends w/ garage doors. In each building will be a propane space heater 2.8amp motor, 1500 watts of lighting, 2 receptacles for occasional power tools or fans or whatever (1-20amp circuit and 2-15amp circuits).

Here is my plan (all copper):
Replace well circuit w/ 3wire #2 + grnd, to Sub panel #1 off 100amp breaker @ main panel (100’ run). This panel will run the well and 2-20amp receptacle circuits for now. Go from this panel to Sub panel #2 w/ 3wire #2 + grnd off 100amp breaker @ Sub panel #1 (130’ run). This panel will feed the 3 out bldgs described above by placing 4circuit Sub panels #3 and #4 in the other bldgs. Both runs are 40’ of 3wire #6 +grnd. I was planning a grnd rod at panel #1 and #2, do I need them at #3 and #4 as well? Also is the fused disconnect for the well still required if the breaker will be right next to the motor controls?

My calculation is that I can run 32-40 amps out of Sub panel #2 with this Plan. The actual would probably never be more than 20-23amp draw at any given time.

Does this seem ok and to code before I pull permits, and I don’t know yet what size the grnd wires should be? I welcome your input please.
 
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Old 03-29-10, 07:53 PM
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For your feeder runs you need to use the full length of wire, not just the lateral distance between buildings, when calculating voltage drop. This means that if the service (main) panel is located five feet above grade and you run conduit 24 inches below grade that you have seven feet plus the distance to the outbuilding plus another seven feet rising up to the sub-panel. Always err on the side of too long rather than too short.

Note that voltage drop is a function of BOTH the distance and the current flow. If the well pump is fed from the first sub-panel you would NOT include that load when calculating the voltage drop on the feeder from the first sub-panel to any subsequent sub-panel.

Rather than have additional sub-panels at each additional building you may want to use "multi-wire branch circuits" to give you two circuits in each of the additional buildings. Since the heater load is fairly small you could also include the receptacles on this circuit.

Each detached building that has a sub-panel MUST have its own ground rod and it must be connected to the sub-panel with a #6 copper conductor. If you enclose the grounding conductor in conduit you may use a smaller size but the trade off is slight so just use the #6.

The equipment ground that runs between the service panel and the various sub-panels must be sized to the circuit breaker supplying the feeder conductors. A 100 ampere feeder requires a #8 copper equipment grounding conductor, a 30, 40, 50 or 60 ampere feeder requires a #10 copper conductor, a 20 ampere feeder requires a #12 copper conductor and a 15 ampere feeder requires a #14 copper conductor. As always, if using aluminum the sizes increase by one size.
 
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Old 03-30-10, 07:46 AM
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Furd, thank you for the info, it is sincerely appreciated. All distances are wire run distances and not lateral. I have a couple of follow up questions, please. The "copper equipment grounding conductor" must be solid core, right? And forgive my confusion please, but you say #8 with the 3wire #2 feed and then #6 to the smaller end sub panels. As I read your last paragraph the end sub panels only require a #10 equip ground, right?

My reason for the smaller sub panels was just to keep the wire as big as possible as far as possible, and I will have #6 left behind from the deleted well circuit. Also because "bigger is better" lol. I also found 6ea 8' grnd rods behind the main bldg yesterday (so no extra expense there).

Lastly, I still would like to know if I can eliminate the fused disconnect to the well? Thank you again!
 

Last edited by jefferoo; 03-30-10 at 09:10 AM.
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Old 03-30-10, 08:06 AM
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Its a commendable idea to want to use copper but they make a quad alum, number 2 at the box stores just for this.
 
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Old 03-30-10, 10:17 AM
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Originally Posted by jefferoo View Post
The "copper equipment grounding conductor" must be solid core, right?
Stranded or solid is okay.

And forgive my confusion please, but you say #8 with the 3wire #2 feed and then #6 to the smaller end sub panels. As I read your last paragraph the end sub panels only require a #10 equip ground, right?
The #6 is for the grounding electrode conductor (panel to ground rod). The #8 or #10 is the equipment ground from panel to panel. Both are required.

Lastly, I still would like to know if I can eliminate the fused disconnect to the well? Thank you again!
A breaker within line-of-sight is an acceptable disconnect. You only need to have the fuses if the pump installation instructions specifically require fuses.
 
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Old 04-01-10, 07:04 AM
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Thanks to all for your input, really helped clear some of the fog.

Jeff
 
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