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I want to add some 20 amp patio outlets to the back of my house. The farthest outlet would be about 100 feet from the main panel. Normally you could use 12 gauge wire when dealing with 20 amps, however, because the farthest outlet is about 100 feet away I have to account for voltage drops, which means I have to bump up the wire to 8 gauge. I'm curious about the ground wire though. While there's tons of resources that talk about the main power lines and voltage drops, I'm having a hard time finding information on ground wire size. Obviously since I'm snaking in and out of outlets before I get to the last outlet I would want to use the smallest wire I can get away with for the ground. I know 10 gauge ground wire is typically used for 8 gauge power lines but that's because you're dealing with higher amps (55 amps) over a shorter distance. Since I'm dealing with a lower amp rating (20 amps), but over a larger distance(100 ft), I wasn't sure if the ground wire needs to be the same as the power lines (8 gauge) or if I can get away with 10 or even 12 gauge.

Anyone have any insight on longer runs?

Thanks in advance!

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    What loads are you planning to power from this outlet? Dec 27, 2021 at 5:28
  • Not in the question, but have you considered exterior conduit? The phrase ‘snaking in and out of outlets’ makes me shudder. You might be surprised at how subtle conduit is when it’s painted house color. Dec 27, 2021 at 14:42
  • These would be low level loads. They'll have the sprinkler timer on it, and then maybe some string lights and then if someone needs to plug in a phone or something. They won't be running shop equipment or anything like that. These would be outlets that are behind the patio sofa in case someone wanted to plug something in.
    – Mark
    Dec 28, 2021 at 5:34
  • I should also clarify what I meant by "snaking in and out of outlets". There will be 4 outlets, each one spaced out about 25 feet apart with 3/4" pvc connecting them. So when I said snake, I meant the wire goes through the conduit, up to the outlet, then back down another conduit to the next outlet, and so on. Within each outlet though, I'll have to use a wire connector to connect the 8 gauge wires and then also connect and run a 12 gauge short wire as a pigtail to power the outlet itself.
    – Mark
    Dec 28, 2021 at 5:37

3 Answers 3

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The NEC shows in table 250.122 that #12 is adequate for up to 20A circuits, but the text of 250.122(B) says if the ungrounded (hot) conductors are increased you must also increase the circular mil size proportionally.

For circuits 30A and under the table shows the ground size is the same as the hots so any increase for voltage drop would have to match. For increases wire size of circuits larger than 30A the ground size will vary depending on wire size.

Above 15A your voltage drop at 100' gets a bit questionable, #8 is way beyond need, but I would certainly consider what happens if you ever need to plug a 100' cord into that outlet 100' away and seriously consider pulling #10's for the fixed portion of the circuit.

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  • No, #8. If the hot must be #12 but you use #8, that is a 150??% increase based on circular mil. You must increase ground by same percentage, so if it must be #12, you have to - hold on, multiply by factor, look up AWG sizes... I get #8 right on the button. Gee, what are the chances? Dec 27, 2021 at 23:02
  • @Harper-ReinstateMonica I guess it is pretty easy. Will edit to reflect. Kind of odd to think that if you have a chunk of #8 UF the minimum breaker you could land it on is a 40A. It would seem the NFPA didn't deal with that well when the started requiring full size grounds for 20 and 30 amp circuits even though not needed from conductive property point of view. (I'm remembering it was claimed they increased the ground size for mechanical reasons, not electrical theory.) Dec 28, 2021 at 3:20
  • I was using this calculator to figure out the wire size; calculator.net/voltage-drop-calculator.html
    – Mark
    Dec 28, 2021 at 5:41
  • Using that calculator I increased the wire size till the voltage drop was below 3%. Which for 100 ft and 20 amps, was 8 gauge wire.
    – Mark
    Dec 28, 2021 at 5:42
  • Also, the outlets will never have a 100 ft extension cord connected on them. I have additional outside outlets to serve other areas of the backyard. The longest I would ever see an extension cord being (anywhere in the backyard) would be 10-15 ft. Anything more than that I would use a different closer outlet.
    – Mark
    Dec 28, 2021 at 5:48
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Ground increases in proportion to hot, not based on what's in common cables.

When you increase hot wire sizes, you must increase ground in same proportion, based on circular mils of the wires.

20A circuit requires #12 wire. If you use #8, that is XXX% larger.
20A circuit requires #12 ground. If we multiply that by XXX%, we get #8 as ground size.
Notice we don't need to care what XXX% is, since it cancels out in the math.

So yes, you need #8 ground, and that means using #8 cable is going to be a problem.

You may be miscalculating voltage anyway

Most novices worry excessively about voltage drop. They hear this "3%" figure and really latch onto it. It is a complete and total myth. It's not in Code anywhere. You have to get to 8% voltage drop before Code says anything about voltage drop. Really.

To compound the problem, they calculate based on breaker trip, which you're not even supposed to use - you should never plan to load a circuit beyond 80%.

Even in the People's Republic of Canada, where their Code does call out 3%, it only does that based on actual load, or 80% of breaker trip. So for instance a Canadian 20A circuit, you calculate your voltage drop to be 3% @ 16 amps. Even by those rules, you have 2.80% voltage drop using #10 wire. And as it so happens, #10 cable uses a #10 ground, so that works out nicely!

Everyone else can do what you want based on eliminating irrational fear of voltage drop, and what your loads will actually be. Generally most large appliances max out at 1500W, which is 12.5 amps. At that, #12 wire gives 3.6% voltage drop, not great, not terrible. Sorry I can't resist that joke anytime 3.6 comes up, but really, it's perfectly fine. At 16A it'll be 4.63% and you really shouldn't plan to run higher than that. (that's actually a Code requirement).

If you really see yourself running dual 1500W appliances (which is too much for a 20A circuit), then consider running a Multi-Wire Branch Circuit - though this will complicate GFCI.

To really clobber voltage drop at sane cost, use aluminum to a midpoint

Remember outdoor cable must be outdoor rated, so UF not NM.

If you're convinced you need #8, but don't like the price, here's an idea. #8 and larger won't attach directly to a receptacle anyway, and must be pigtailed. It also requires a fairly large junction box.

Since you're stuck with those problems anyway, I suggest running #6 aluminum for the "home run", going to a midpoint. At that point, splice to #12 copper wire and run in both directions to serve receptacles. With aluminum wire you must use connectors approved for aluminum, which is what they failed to do in the 1970s with small #12 branch circuit wiring. The ILSCO "Mac Block Connector" is a sanely priced splice connector to take #6 aluminum to #12 copper.

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100 feet is not really that bad, unless you are really loading the circuit. In fact, I'd guess that anything short of some serious power tools or space heaters (ugh!) pulling continuous 12A or more really just won't be an issue. Certainly the typical small stuff - lights (almost nothing thanks to LEDs), chargers, TV, stereo, fan, etc. are just not going to be an issue. The 20A rating for the circuit is the maximum the circuit can handle. But the voltage drop is based on the actual usage, and occasional jumps towards the maximum are OK.

Using the Southwire calculator with 120V at 100 feet:

  • 15A = 10 AWG 2.62% or 12 AWG 4.34% - so even at 15A, 12 AWG isn't too bad.
  • 10A = 12 AWG 2.89% - below the typical 3% guideline

So run the normal 12 AWG and don't worry about it.

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