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Old 03-19-2009, 01:51 PM   #1
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Underground wiring job while landscaping


Question : I need to know what gauge of wire to bury and how deep I need to bury it for an outdoor electrical run. My guess at this point is to run underground 12/3 for a line with a 30 Amp breaker but I'm not sure, and haven't been able to find resources on this. I will have an electrician connect it to the breaker box when the time comes to actually use it (that's beyond my comfort level) but I'd just like to get the buried wire down now while it's most convenient.

Details:
I am in the process of re-sodding my front lawn and thought this would be a great time to drop any wire that I need or may need for future electrical projects (so I don't have to dig up the yard ... it's already dug up).

The purpose of the electrical run would be for exterior lighting and christmas light displays (no electrical cord spiderwebs going across the lawn for 2 months) and to provide electricity for a future back yard lighting project (where the cable, in the future would be run through the fence to supply power to low voltage light such as LED's).

I've read in some of the forums where an exterior outlet can come off an interior outlet as well and don't know if this information would be helpful in providing an answer.

Diagram with measurements:


Any advise would be greatly appreciated. Thank You,
TJ
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Old 03-19-2009, 02:21 PM   #2
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Underground wiring job while landscaping


well, in a residence....15 amp breaker for #14 and 20 amp for #12...depth 18 inches should do it and 12 inches if its gfi protected ...and you don't need 12-3, unless you want to 2 circuits...also, you cant bury nm-b, you need either uf or pipe it with thwn/thhn

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Old 03-19-2009, 02:29 PM   #3
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Underground wiring job while landscaping


For a 30 amp drop you would need to go with 10 Ga. not 12 but the distances your are running may need a larger size depending on load to stay within the 3% voltage drop required by NEC standards.
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Old 03-19-2009, 02:36 PM   #4
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Underground wiring job while landscaping


Quote:
Originally Posted by NolaTigaBait View Post
well, in a residence....15 amp breaker for #14 and 20 amp for #12...depth 18 inches should do it and 12 inches if its gfi protected ...and you don't need 12-3, unless you want to 2 circuits...also, you cant bury nm-b, you need either uf or pipe it with thwn/thhn
So if I run 12/3 vs 12/2 then I can run 2 - 20Amp circuits instead of a singe 30 amp? My concern is that I don't overload the line and now is the time to bury too large instead of too small. My assumption was that 12/2 was ungrounded and 12/3 was grounded but after looking at the wiring I see 12/3 has 3 wires and a ground so that's why you're saying I can run 2 circuits off a 12/3 (just clarifying so I know that I'm understanding it correctly) ... How ... that's something that I'll actually hire out when it's time to connect it. I'm also assuming, because I'm not familiar with all of the wiring abbreviations, that nm-b is interior wire and uf is underground wire but I'm not sure what thwn/thhn is but I think I remember it being heat and water resistant wiring (is that different from what is found in the Lowe's & Home Depot as "Underground Wiring"?). Thanks for your advice on the burying depths, I can start digging (wish I had a well trained dog right now ).

As far as running 12/2 vs 12/3 ... It's going to be outside electrical, so doesn't that make it necessary to use GFCI (and then don't I have to use a single 12/2 because, if I'm not mistaken in my readings, GFCI can't use shared wiring?). My plan was for the first outlet to be inside and be the GFCI and all other outside recepticles being GFCI and in a weatherproofed enclosure so as not to get wet from a sprinkler system.

Thanks again for the quick response ... I'm off to dig for a while because the sod comes tomorrow,
TJ

Last edited by tjfitzgerald; 03-19-2009 at 02:45 PM.
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Old 03-19-2009, 02:44 PM   #5
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Underground wiring job while landscaping


Quote:
Originally Posted by hayewe farm View Post
For a 30 amp drop you would need to go with 10 Ga. not 12 but the distances your are running may need a larger size depending on load to stay within the 3% voltage drop required by NEC standards.

Well my run is basically 45' indoors before going to a 95' outdoors with 2-20' branches going off (so my longest cable run is 140' from the breaker box). If I keep it down to 20Amp, would the 12/2 or 12/3 do the trick for a run of about 100'? ... I've been rethinking where I'm going to run and I may be able to cut about 30' out of the run based on my diagram in the original post (by keeping the long run right up against the front of the house rather than going out and coming back in ... that would shorten my run down to 100-110' from the breaker panel). Still not clear on whether I should be doing 12/2 vs 12/3 (trying to not have to use the larger 10 gauge if I don't have to, but safety first!)

Thanks for the quick response,
TJ
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Old 03-19-2009, 02:57 PM   #6
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Underground wiring job while landscaping


If you have a 140 ft. run of 12/2 and you have a load that requires 20 amp (2400 VA)you would have a voltage drop at the end of abot 9 volts which is beyond NEC allowable 3%, however you would still be above 110 V with a 120 v supply which is in the operating tolerances of lighting fixtures. It probably wouldn't be a problem unless you have an inspector that is a real stickler for details.

If you want 2 circuits you could run 4 wire plus ground with each set on a seperate GFCI.

UF is direct bury cable it should have gray insulation.
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Old 03-19-2009, 03:10 PM   #7
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NEC mentions 3% drop in the FPN (Fine Print Notes) it is not a requirement only a suggestion for operating within the limits of the fixture. You will NOT have any issues with this except some of the last lights may be dimmer.
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Old 03-19-2009, 03:23 PM   #8
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Underground wiring job while landscaping


Using 12-3you can run 2 circuits
I use a 20a 240v breaker to supply 2 circuits to my timers
Then from my timers I use 12-2 to each outlet
Each run is a seperate GFCI
I seem to recall someone explaining how to connect 12-3 & 2 seperate GFCI's ?? After the GFCI outlet you can't use 12-3 - it woudl need to be seperate 12-2 runs

I have 8 dedicated circuits for my Christmas display
I ran conduit in mygardens, based on thefact someone at some point may dig down & I prefer to not have direct burial wire

I ran my runs in the basement & only went outside where required
If you have a basement that may be an option
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Old 03-19-2009, 03:24 PM   #9
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Underground wiring job while landscaping


You could go real simple and simply buy and bury conduit (put a heavy duty string inside.) That way when it comes time to put the circuits in, the electrician can select the right size wire for what you want and pull the wire through the existing conduit.
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Old 03-19-2009, 03:53 PM   #10
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Underground wiring job while landscaping


Quote:
Originally Posted by tjfitzgerald View Post
So if I run 12/3 vs 12/2 then I can run 2 - 20Amp circuits instead of a singe 30 amp? My concern is that I don't overload the line and now is the time to bury too large instead of too small. My assumption was that 12/2 was ungrounded and 12/3 was grounded but after looking at the wiring I see 12/3 has 3 wires and a ground so that's why you're saying I can run 2 circuits off a 12/3 (just clarifying so I know that I'm understanding it correctly) ... How ... that's something that I'll actually hire out when it's time to connect it. I'm also assuming, because I'm not familiar with all of the wiring abbreviations, that nm-b is interior wire and uf is underground wire but I'm not sure what thwn/thhn is but I think I remember it being heat and water resistant wiring (is that different from what is found in the Lowe's & Home Depot as "Underground Wiring"?). Thanks for your advice on the burying depths, I can start digging (wish I had a well trained dog right now ).

As far as running 12/2 vs 12/3 ... It's going to be outside electrical, so doesn't that make it necessary to use GFCI (and then don't I have to use a single 12/2 because, if I'm not mistaken in my readings, GFCI can't use shared wiring?). My plan was for the first outlet to be inside and be the GFCI and all other outside recepticles being GFCI and in a weatherproofed enclosure so as not to get wet from a sprinkler system.

Thanks again for the quick response ... I'm off to dig for a while because the sod comes tomorrow,
TJ
well, you can forget about the 30 amp circuit, the devices aren't rated for that...they need gfi protection...and as far as voltage drop, honestly thats a nice run, but i wouldnt worry about that for landscape lights...i'd pipe it in pvc and pull 4 #12's or 5 if you want them to have their own neutrals...good luck ...theres several ways to tackle this issue, just find which is the cheapest and easiest
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Old 03-19-2009, 04:32 PM   #11
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Because the OP mentioned low voltage lights such as LED, voltage drop should be considered to make sure you are within operating tolerances for power supplies.
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Old 03-19-2009, 04:40 PM   #12
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Quote:
Originally Posted by hayewe farm View Post
Because the OP mentioned low voltage lights such as LED, voltage drop should be considered to make sure you are within operating tolerances for power supplies.
thats great but you gave conflicting statements, firsy you said he should worry and then you said it wasn't a big deal, which it isn't...and the inspector can't fail him b/c he didn't consider voltage drop
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Old 03-19-2009, 04:42 PM   #13
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i just did a vd calc and i came up with a 3.6% drop...thats at a 16 amp load!...dont worry about voltage drop
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Old 03-19-2009, 04:53 PM   #14
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thats great but you gave conflicting statements, firsy you said he should worry and then you said it wasn't a big deal, which it isn't...and the inspector can't fail him b/c he didn't consider voltage drop
You are correct about me making an error, I didn't remember it was an FPN. On your other post I get 6.1% drop for 140 ft. one way run of 12/2 with @ 16 amp load and as I said still within voltage tolerances for most devices.
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Old 03-19-2009, 05:03 PM   #15
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well, i dont know...i did it on 3 different calc. and got the same 3.6% drop...i still don't think it will be a prob either way

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