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I have dug my trench. I want to use FMC (flexible metal conduit) since some of my trench is only 8-12 inches deep. I need to know what wires I need inside my conduit. Do I just need a black, white and a green ground wire? FMC can be used as the ground from what I have read. I need to run this about 85 feet from my electrical box. I believe I need 8 AWG THWN wire for that length. But I want to make sure I put the right wires inside the conduit. Oh and I want a 120 volt receptacle. Thanks!!

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Wrong conduit for the job

First off, you are looking at the wrong kind of conduit for this task. NEC 348.12 point 6 prohibits the use of FMC underground:

(6) Underground or embedded in poured concrete or aggregate

Given your desired trench depth, you want to use rigid metal conduit (RMC) instead, as even liquidtight flexible metal conduit (LFMC), which can be buried, requires 18" of topcover.

You don't need wires that fat, either

Second, your 8AWG wires are quite a bit too large; for a general-use receptacle at a mere 85' distance, 12AWG will suffice. (Rule of thumb: if the one-way length of the circuit is longer than the nominal voltage, then you start need to worrying about voltage drop.)

Other tip: put the GFCI at the house

Another tip here is to put the GFCI inside, at the house, where it is better sheltered from the weather. While a good weatherproof box and while-in-use cover will do their darnest to keep rain out, they're still not as good as the roof of your house at it. Also, don't forget to fit the bottom of your receptacle box with a weep hole (1/8" is fine) to both let water out and keep air pressure inside even.

  • Okay so I’ll do 12 gauge wire. I can get to 18 inches so i’ll use the flexible PVC. Any advice for fishing the wires through the PVC? – Jason Good Sep 7 at 18:06
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    @JasonGood -- why the insistence on flexible conduit? – ThreePhaseEel Sep 7 at 18:08
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Wrong pipe

You misheard. The things you are saying about FMC actually apply to IMC, or Intermediate Metal Conduit. This is a heavy, stiff pipe, similar to Rigid conduit.

FMC is wholly unsuitable for direct burial.

IMC and Rigid only require 6" of burial depth, that is to say, 6" above the top of the pipe.

Now let's talk GFCI

It's common, in technology changes, for people to look for unit substitution, a new thing that "drops in" as a substitute for the old thing, to avoid thinking about how the technology actually works. Like putting an on-demand water heater where the old tank was, instead of near the loads. Growing organic but refusing to rotate crops, and using tons of manure instead.

So you have a GFCI requirement in this garden. The "unit substitution" mentality is to fit a GFCI+receptacle combo device there, regardless of weather effects. However, GFCI devices can actually protect electric loads that are downline of it. So as ThreePhaseEel discusses, the smart play is to fit the GFCI device inside the house - for weather-damage reasons alone. However, this also has an effect on the landscape.

A single 120V 15A or 20A circuit only needs 12" burial depth if it is GFCI protected (meaning, back at the house). Further, direct burial is permitted, which means, so is any kind of conduit allowed outdoors (e.g. PVC, IMC or Rigid). Again not FMC; that's not allowed in any case.

If you use a conduit other than Rigid/IMC, you might as well use 1/2" pipe because nothing else will be allowed in this pipe given the 12" burial depth. PVC conduit this small is fairly bendable and can handle limited wide-radius curves. Total curvature end-to-end cannot exceed 360 degrees.

To be clear, you can use PVC conduit at 12" cover atop it, if it contains a single 15/20A, 120V circuit that is GFCI proteced at the origin.

Wire type

In conduit, your best bet is THWN individual wires, W for Wet locations, which all outdoor conduit is. For a 120V circuit, you need

  • a hot wire that is black, brown, red, orange, yellow, pink, blue or violet.
  • a neutral wire that is white or gray.
  • A ground wire that is green, green/yellow, or bare and copper. This can be omitted if you are using Rigid or IMC pipe, use the pipe as the ground path.

With only 2 wires in Rigid or IMC, beware the conduit fill rules - they are extremely restrictive on pipes with only 2 wires. (Lazy way: Compute for 3 wires).

If you want to direct-bury cable, you would use something like UF cable. You cannot use NM, it is not rated for outdoor/wet.

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At 65' you are not going to have significant enough voltage drop to justify up-sizing wires. Most of your loads are 12A max. Run the numbers with 12A and 5% voltage drop (3% is a myth, not a Code requirement), and your worst voltage drop will be in the low 3’s. (never use the default 3%, it will force a cable upsize at 3.001% drop, and that's super silly). So you can get away with 14 AWG wire on 15A circuit or 12 AWG on a 20A.

If you were speccing 8 AWG because of a future plan to raise this circuit to 50A, that won't work because it'll blow the conditions of the 12" exception. It will work with with Rigid or IMC conduit. Presumably you want 50A@240V (since 50A@120V is silly) so you'll need to add another hot wire later. Remember to size the conduit for that extra wire.

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What country are you in? I'm pretty sure the FMC cannot be used unless it's the sealtite and it would still have to be buried at 18", same as regular PVC, which would be a better choice because the sealtite will be expensive and hard to pull a fish through. Rigid can be buried with 6" cover. You'd need a black, white and green wire. #12 cu TWHN will work with a 20 amp breaker at the panel, Don't forget the waterproof outlet covers. I'm assuming just lights for the garden.

You can also use direct-bury UF-B (underground feeder) cable at only 12" deep, provided it meets three criteria: It has GFCI protection before it enters the ground, is limited to 120 volts, and is protected by no more than a 20-amp fuse or breaker. good luck.

  • I am in the US. Virginia to be more precise. I have read that FMC does not have to be buried as deeply. But I hear you on fishing the wire through PVC vs FMC. – Jason Good Sep 7 at 16:06
  • I'm reading conflicting info on the FMC but there are some really smart people who will jump in and clarify it. I've just always put conduit at 18". – JACK Sep 7 at 16:43
  • @JACK FMC is not usable outdoors. Rigid/IMC is allowed at only 6" burial depth, so you are trading very expensive pipe for the ability to trench it with a garden trowel. The principle is a hand shovel can't penetrate it, and the backhoe will destroy it but the metal shell will assure breaker trip if it does. – Harper - Reinstate Monica Sep 7 at 19:00
  • @Harper I was pretty sure it wasn't allowed but that the LFMC would be, but impracticable. Thanks for the explanation. We all basically gave the same answer so I consider it a win. – JACK Sep 7 at 20:44

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