Electrical plan shows exterior receptacles on 4 sides of house. House is 250 linear feet.

Considering the voltage drop calculations here: What is the maximum 12/2 wire length from a 20A breaker to a baseboard heater?

What is the best approach for wiring these 4 exterior receptacles on the same circuit? What about running a 6-2 gauge wire to the center attic in a junction-box, then branching from that using 10-2 to all 4 exterior receptacles? Will that meet NEC code if i drop it in conduit from lintel-level to receptacle box?

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  • Again it depends on your location. You need to check the local codes.
    – Trevor_G
    Commented Mar 7, 2017 at 16:04
  • Seems like modifying your first question would've been a better move.
    – isherwood
    Commented Mar 7, 2017 at 18:42

2 Answers 2


The drawing is cut off and we can't see most of the exterior receptacles, but it looks like they're mostly on the far side of the building from the panel (lower left and I really hope the panel isn't outside, that's asking for trouble).

Your building is EDIT: 54x58, that's 76' on diagonal, plus 10' twice to get to the attic; 96'. That would be about 5.6% voltage drop at full 20A, or half that at 10A. If you strictly follow the 3% voltage drop on max current rule, you can only get 51' out of 12AWG... but that's not the rule. The actual rule is try to stay around 3% for the actual ampacity you expect to use, so for a 12A electric lawnmower you're a tick over 3% for a 96' run. No big.

Seriously, one guy strictly following the 3% fake-rule was about to install 600kcmil for his 1600' run to a couple of 10-watt post lights. I showed him how to do it with 14AWG.

Nothing says you must wire in a string all the way around the circumference of the building. You can wire a separate homerun for each receptacle. Or more likely, you can wire in a star configuration, or even more than one star.

For instance as you suggest, I would establish two "stars", one at the panel and one, about I don't know, the kitchen. I'd take each receptacle to whichever "star" is nearest. In some cases that might end up being a home run to the panel.

If you have 2 or more homeruns, you can either make them separate circuits or the same circuit, up to you.

I know some places like the Philippines use 2 breakers per circuit, and panels get pretty crowded pretty fast. You can get panels with as many as 84 spaces - panel spaces are cheap, buy LOTS of extra. You can also do sub-panels - that might be a good idea given the size of the house. If you had a sub-panel in, say, the laundry room, a lot of these concerns would go away. You'd have to do a load calculation to see how large the subpanel should be, too large is not bad. It can't exceed the size of the main breaker obviously.

  • My home is 54'x58'. I like the idea of putting the exterior receptacles on 2 different circuits, i might consider it vs. stringing it all the way around the perimeter. Now, i do really like the idea of a sub-panel in the laundry room. I can imagine running all the wiring to my dining, kitchen, gallery, patio, living room on it. But what amperage should the panel be and what wire size would i use to connect from it to the main breaker. Are there any downsides to this layout?
    – true. lion
    Commented Mar 7, 2017 at 19:51
  • 1
    Subpanel(s) seem like a much better idea. That way you're running four long fat aluminum wires instead of dozens of long thin copper ones at much greater expense, not all wires are active at the same time, but the ones that are have a lot of loss. Not with a subpanel. Especially if a subpanel can be near your kitchen, there is a lot of load there. Subpanels also give you more spaces, and you always can use more spaces. Most electricians want to leave you 2-spares like they're saving you money. My goal is 50% spares. Commented Mar 7, 2017 at 20:29
  • I've never installed a sub-panel nor really knew the concept of it really. What would be a good amperage for a sub-panel? What conductor size would connect the sub-panel to the breaker panel? Is it 4 gauge?
    – true. lion
    Commented Mar 8, 2017 at 2:58
  • You have to do a load calculation same as you would for a main panel, but I would aim for 1/2 to 2/3 of the service you have coming into the main panel. Most of your loads could come off it. Commented Mar 8, 2017 at 5:02

What is the max length for a base board heater? There is no max length but there is a max voltage drop. Since you did not provide the load there is no way to help other than point you to the web voltage drop calculator. Easy to use input the info and it will provide wire size. Use 3% for max voltage drop. You are way over thinking the voltage drop, even with huge homes I have never used more than #10 and that is because the owner wanted it. You will have a hard time finding a 20 amp breaker that is listed for #6 wire.

  • Ed, I don't see how you can run more than 60' of 12-2 without getting more than a 3% voltage drop. I just ran a calc and got this: "A maximum distance of 51.839 feet will limit the voltage drop to 3% or less with a #12 Copper conductor delivering 20.0 amps on a 120 volt system." How for example could I run power to my Kitchen which is further than 51 feet away without voltage drop?
    – true. lion
    Commented Mar 7, 2017 at 19:46
  • Using 20 amp outlets in my state the load li limited to 16 amps using 15 amp outlets the load is limited to 12 in my area. The owner did not like the look of 20 amp outlets. Local code here we also have to comply to the industrial / commercial 180 va per yoke but I think that is gone with the 2017 code adoption. So local codes can adjust what can be done. It comes down to what needs to be done. Personally I would rather have 2 circuits with 12 gauge than 1 with 10 in a residential setting, some because of the cost and some for the easier to installation with common staples, clamps and boxes.
    – Ed Beal
    Commented Mar 7, 2017 at 20:33

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