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I am in the process of putting down some outdoor conduit at one of my properties. I am replacing some old UF wire that wasn't buried properly (or came up over the years) and has shorted.

I chose to go with metal conduit, because from what I've read, the NEC only requires a min. depth of 6 inches.

The conduit will hold the 40 amp 'line' for my A/C unit; if possible I would like to have it also hold another 20 amp 'line' for an outdoor outlet.

My questions are:

  1. What size conduit should I use? (I was thinking 3/4")
  2. Can I snake all wires for the AC and outlet in one conduit (2 conductor 1 ground for ac [8awg] / 1 conductor 1 neutral 1 ground for outlet [12awg]) - not sure the fill on this size
  3. When i stub up, I need to wrap around the foundation a little bit; any issue going above a hose spigot?
  4. Am I crazy for using rigid metal conduit? A lot of people I know say just go 'as deep as I can' and use PVC. But that's against code and even though this won't be inspected I try to stick to code as much as possible.

Thank you for the help

justinw
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1 Answers1

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  1. What size conduit should I use? (I was thinking 3/4")

Since you're pulling more than 2 wires through the conduit, you're only going to be able to fill the conduit to 40%. Since you didn't specifically mention what type of conduit you're using, I'll list all metallic and PVC conduit.

  • 3/4" Intermediate Metal Conduit (IMC) @40% = 0.235 in.²
  • 3/4" Rigid Metal Conduit (RMC) @40% = 0.220 in.²
  • 3/4" Schedule 80 PVC @40% = 0.164 in.²
  • 3/4" Schedule 40 PVC @40% = 0.203 in.²

I'll assume you're using THHN/THWN/THWN-2 conductors, so you'll need to know the area of these conductors in the various sizes.

  • 12 AWG = 0.0133 in.²
  • 8 AWG = 0.0366 in.²

To figure out if the conductors will fit, you just have to add them up.

3 #8 x 0.0366 in.² = 0.1098 in²
3 #12 x 0.0133 in.² = 0.0399 in.²
0.1098 in.² + 0.0399 in.² = 0.1497 in.²

You end up with a conductor fill value of 0.1497 in.², which means you'll be able to use 3/4" conduit no matter what type you choose.

NOTES: For a look at some of the tables where these numbers came from, see this answer.

  1. Can I snake all wires for the AC and outlet in one conduit (2 conductor 1 ground for ac [8awg] / 1 conductor 1 neutral 1 ground for outlet [12awg]) - not sure the fill on this size

No problem, see above.

  1. When I stub up, I need to wrap around the foundation a little bit; any issue going above a hose spigot?

Shouldn't be a problem.

  1. Am I crazy for using rigid metal conduit? A lot of people I know say just go 'as deep as I can' and use PVC. But that's against code and even though this won't be inspected I try to stick to code as much as possible

You can use metallic conduit if you want, but it will likely be cheaper to use PVC (even though you'll have to dig deeper).

I think you may be confusing PVC pipe (used for plumbing), and PVC conduit (used for electrical). There's no problem at all with using PVC conduit, unless there's a local amendment that restricts its use. However, you'll only have to bury metallic conduit 6" deep, whereas PVC will have to be 18" deep. So if you don't like digging, metallic conduit might be worth the extra cost.

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Tester101
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  • thanks as always tester, but just to touch on one point - you said there's no problem at all using PVC conduit, but the 'issue' I was describing (#4) between the rigid metal vs PVC in my question was in regards to depth - people were saying to screw the 18" depth and just go as deep as possible , that's why i brought up the 'code' part in #4 (6 v 18) – justinw Jul 17 '15 at 03:06
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    I guess I misunderstood what "as deep as you can" meant. I'm a hard worker, so I took it to mean 10 - 20'. I guess to lazy folks, it could mean 2 - 3". – Tester101 Jul 17 '15 at 03:14
  • Note: Most inspectors won't pull out a tape measure and measure the depth of the conduit. They've seen a ton of trenches, and don't question is if it looks "deep enough". – Tester101 Jul 17 '15 at 13:27
  • thanks again - yeah I didn't really by the deep as you can thing; I get a lot of slack from people for trying to do everything by the code even if it won't be inspected - but a 120 foot trench 6+ inches by hand sure is hard - probably stick with rmc because the root system is so strong in some areas or might just go the additional 12 - still debating if the blisters are worth the extra $100 bucks – justinw Jul 17 '15 at 18:47
  • Rent yourself a Trencher. – Tester101 Jul 17 '15 at 19:42