Trench Depth For Underground Cable

 
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Old 09-04-2007, 09:55 AM   #1
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Trench Depth For Underground Cable


I'm needing to run both Low voltage and high voltage cable to a few locations in my yard. The cable will be for outdoor lighting, security and sprinkler valve control. I plan to bury 1 X 3/4 Schedule 40 pipe for Low voltage, 1 X 3/4 for 110 Volt AC and 1 X 3/4 for water to supply the sprinkler heads. All will be in the same trench. The water pipe will have a drain so that I can drain all of the water out for the winter. My entire lot is a hill so it will be easy to get all of the water drained out from a low spot. My question is if I have the ability to fully drain the system of water, should the electric be below the frost line. I'm sure this is the prefered way to go, but trenching that deep I'll run into more rock and have to use a larger trencher. I 'm wanting to bury the conduit to a depth of 24 inches. Will this be an issue for the electric?

Thanks
Rob
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Old 09-04-2007, 01:54 PM   #2
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Re: Trench Depth For Underground Cable


Table 300.5

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Old 09-04-2007, 04:55 PM   #3
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Re: Trench Depth For Underground Cable


24" deep is plenty.
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Old 09-05-2007, 09:17 AM   #4
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Re: Trench Depth For Underground Cable


Quote:
Originally Posted by Sparky Joe View Post
Table 300.5
Hey Joe, I don't have a code book, but I should buy one. What does 300.5 have to say? I'm guessing it specifies below the frost line.

Thanks

Rob
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Old 09-05-2007, 05:18 PM   #5
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Re: Trench Depth For Underground Cable


Well here's what the code says on it;

300.5 Underground Installations.
(A) Minimum Cover Requirements. Direct-buried cable
or conduit or other raceways shall be installed to meet the
minimum cover requirements of Table 300.5.

Table 300.5 Minimum Cover Requirements, 0 to 600 Volts, Nominal, Burial in Millimeters (Inches)
Type of Wiring Method or Circuit
Location of Wiring Method or
Circuit
Column 1
Direct Burial
Cables or
Conductors
Column 2
Rigid Metal
Conduit or
Intermediate
Metal Conduit
Column 3
Nonmetallic
Raceways Listed
for Direct Burial
Without Concrete
Encasement or
Other Approved
Raceways
Column 4
Residential
Branch Circuits
Rated 120 Volts
or Less with
GFCI Protection
and Maximum
Overcurrent
Protection of 20
Amperes
Column 5
Circuits for
Control of
Irrigation and
Landscape
Lighting Limited
to Not More
Than 30 Volts
and Installed
with Type UF or
in Other
Identified Cable
or Raceway
mm in. mm in. mm in. mm in. mm in.
All locations not specified
below
600 24 150 6 450 18 300 12 150 6
In trench below 50-mm (2-in.)
thick concrete or equivalent
450 18 150 6 300 12 150 6 150 6
Under a building 0 0 0 0 0 0 0 0 0 0
(in raceway only) (in raceway only) (in raceway only)
Under minimum of 102-mm
(4-in.) thick concrete exterior
slab with no vehicular traffic
and the slab extending not
less than 152 mm (6 in.)
beyond the underground
installation
450 18 100 4 100 4 150 6 150 6
(direct burial)
100 4
(in raceway)
Under streets, highways, roads,
alleys, driveways, and parking
lots
600 24 600 24 600 24 600 24 600 24
One- and two-family dwelling
driveways and outdoor
parking areas, and used only
for dwelling-related purposes
450 18 450 18 450 18 300 12 450 18
In or under airport runways,
including adjacent areas
where trespassing prohibited
450 18 450 18 450 18 450 18 450 18
Notes:
1. Cover is defined as the shortest distance in millimeters (inches) measured between a point on the top surface of any direct-buried conductor,
cable, conduit, or other raceway and the top surface of finished grade, concrete, or similar cover.
2. Raceways approved for burial only where concrete encased shall require concrete envelope not less than 50 mm (2 in.) thick.
3. Lesser depths shall be permitted where cables and conductors rise for terminations or splices or where access is otherwise required.
4. Where one of the wiring method types listed in Columns 1–3 is used for one of the circuit types in Columns 4 and 5, the shallowest depth of
burial shall be permitted.
5. Where solid rock prevents compliance with the cover depths specified in this table, the wiring shall be installed in metal or nonmetallic raceway
permitted for direct burial. The raceways shall be covered by a minimum of 50 mm (2 in.) of concrete extending down to rock.

Sorry the cut and paste didn't work out too well for this application, I'll work on something better(unless someone else beats me too it; hint, hint)
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Old 09-05-2007, 05:24 PM   #6
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Re: Trench Depth For Underground Cable


Code says 18" minimum for your installation
And 6" minimum for your low voltage

And frost line has nothing to do with burying electrical conduit
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Old 09-05-2007, 05:33 PM   #7
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Re: Trench Depth For Underground Cable


text to make post
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Old 09-05-2007, 05:33 PM   #8
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Re: Trench Depth For Underground Cable


Keep in mind also that the depth's Joe gave you (18 and 6) are minimum cover depths. That is to say, if you dig an 18" deep trench, and lay a conduit in it, you won't have 18 inches of cover. You need to overdig to account for the conduit diameter, and any depth of sand or stone dust you might want to make a bed with if you're into rocky loam.
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Old 09-05-2007, 05:45 PM   #9
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Re: Trench Depth For Underground Cable


Can you measure to the top of the grass?

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Old 09-05-2007, 05:48 PM   #10
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Re: Trench Depth For Underground Cable


Quote:
Originally Posted by mickeyco View Post
Can you measure to the top of the grass?
I doubt it. Take a look at Note 1 to the table you pasted in. Not so sure grass height counts as finished grade. It might make the inspector smile if you asked him that, then he'd tell you "nice try".
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Old 09-05-2007, 08:50 PM   #11
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Re: Trench Depth For Underground Cable


Ahh hell. Just bury it 12" inches down in PVC conduit, protect the circuit using a GFCI device (faceless, circuit breaker), and you're good to go!

Good luck!
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Old 09-06-2007, 09:03 AM   #12
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Re: Trench Depth For Underground Cable


Thanks for all of the replies. I'll make sure I cut the grass before measuring.

I'll dig the trench to 24" add a little stone and with my conduit in there, it should still be below 18".

Thanks again guys

Rob

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