how deep should cable be


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Old 04-12-04, 10:07 AM
dsbrown
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how deep should cable be

hi - how deep should 6-3 direct-bury cable be from a house to a garage? i am in mass and the cable is about 130 feet long.

thanks

ds
 
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Old 04-12-04, 10:22 AM
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Id check code for where you are. Some time it will call for it in conduit


ED
 
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Old 04-12-04, 01:39 PM
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Check with the authority having jurisdiction

I have never practiced in Massachusetts so I can only answer in terms of the US National Electric Code. If your state has amended it my answer could be wrong. Cable of that size would be buried at least two feet deep unless it is carrying thirty amperes or less and is protected by a rather expensive GFCI breaker.

That does bring up another question though how many amperes are you planning to run through that number six cable. 130 feet is a long run and the wire must be sized to carry the load without excessive voltage drop or your power tools will take a beating.

Remember to drive the two ground rods that are required at the barn through the bottom of the trench. That is not required but it will get your ground rods two feet deeper and thus improve their performance. The code only requires them to be six feet apart but best practice is to drive them at ten and twenty feet from the building. If you make that last twenty feet of the trench at least thirty inches deep and run number two copper instead of the number six that the code permits you will have a better grounding electrode system.
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Old 04-12-04, 05:04 PM
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Check with the local permitting authorities for requirements in your area.

horntd has an excellent suggestion regarding your grounds and I would add that you should either use conduit or cover your cable with boards and some warning tape so it has some level of protection/warning against future work damaging the cable.

When you get it in, make an 'as-built' drawing showing the location of the cable and the depth. Keep the drawing with your other important household papers. Someday, someone will praise your name for the effort.
 
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Old 04-12-04, 06:51 PM
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According to NEC only one grounding electrode is required. Some install two just to be safe. If the ground is soild and not mainly sand one should be fine. Also since the trench will be 24" deep minimum you can just lay the rod in the trench and backfill.

I agree that the cable alone is not the best installatin, but it is the easiest for a DIY'er. If you go conduit you would need to use conductors, not UF. You would also need to make the conduit continuous from point to point.
If you do go with UF you need to protect the wire if it is exposed where it can be damaged. Like up the side of the garage.
 
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Old 04-12-04, 08:44 PM
J
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I agree. Unless modified by local code (unlikely), the answer is 24 inches.

The GFCI exemption only applies to branch circuits, with 20 amp or less protection.
 
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Old 04-13-04, 04:11 AM
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Speedy,
I believe one ground rod is required if you want to verify the 25 ohm max. If you don't have a meter, then you need to install the second rod. 250.56
 
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Old 04-13-04, 11:44 AM
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John is right! It is twenty rather than thirty amperes.

Originally posted by John Nelson
I agree. Unless modified by local code (unlikely), the answer is 24 inches.

The GFCI exemption only applies to branch circuits, with 20 amp or less protection.
Mea Culpa Thanks for catching that John! I must have pulled the thirty ampere figure out of my increasingly defective memory because I could not have gotten it from the code.
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Old 04-13-04, 11:51 AM
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Originally posted by HandyRon
Speedy,
I believe one ground rod is required if you want to verify the 25 ohm max. If you don't have a meter, then you need to install the second rod. 250.56
Handy Ron is correct. Unless you can prove that the single rod is twenty five ohms or less the Inspector is likely to require a second rod. The only proof some AHJs will accept is third party testing. Some will except a witnessed test at the time of inspection but they may require that the instrument used have a certificate of calibration to National Institute of Standards and Technology standard.
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Old 04-13-04, 12:07 PM
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Burying rods horizontally.

Originally posted by Speedy Petey
Snip
Also since the trench will be 24" deep minimum you can just lay the rod in the trench and back fill.
Rods buried horizontally must be buried at least thirty inches deep. This may only be done if a driven rod is not possible due to rock bottom.
I agree that the cable alone is not the best installation, but it is the easiest for a DIY'er. If you go conduit you would need to use conductors, not UF. You would also need to make the conduit continuous from point to point.
If you do go with UF you need to protect the wire if it is exposed where it can be damaged. Like up the side of the garage.
Type UF or USE as individual conductors are readily available and perfectly acceptable in conduit.
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Old 04-13-04, 03:50 PM
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OK, I did not intend for this to get picky. I'll clarify.

I did write "you would need to use conductors, not UF". Meaning no UF cable in the conduit. I know UF or USE conductors are OK in conduit. A DIY will not readily find UF individual conductors available but USE/URD is which is why I did not mention them and did state "conductors". When a DIY'er hears "UF" they think cable IMO.


Have you EVER been called on a ground rod? Have you EVER had one tested? I have not. I use common sense and judgement. If I feel the installation is not sufficient I will add another.

I did get the depth wrong, it is 30". I have not read that section in a while. When doing an underground service the trench depth is usually 30-36" and I typically lay the rod(s) in the trench. I have never been questioned on this and don't feel it is a compromised installation. I will say most of my area is bluestone, shale and ledgerock so the fact that a rod may not be able to be driven is a common occurrence.
 

Last edited by Speedy Petey; 04-13-04 at 07:45 PM.
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Old 04-13-04, 07:23 PM
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It is almost never worth it to test the rod's resistance releative to the earth, so I always drive two rods to begin with. It's not worth the effort to measure.
 
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Old 04-13-04, 09:38 PM
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Originally posted by Speedy Petey
OK, I did not intend for this to get picky. I'll clarify.

I did write "you would need to use conductors, not UF". Meaning no UF cable in the conduit. I know UF or USE conductors are OK in conduit. A DIY will not readily find UF individual conductors available but USE/URD is which is why I did not mention them and did state "conductors". When a DIY'er hears "UF" they think cable IMO.


Have you EVER been called on a ground rod? Have you EVER had one tested? I have not. I use common sense and judgment. If I feel the installation is not sufficient I will add another.
Yes I have been called on single ground rods. One jurisdiction I work in uses the 1996 code the other uses the 1999 code. The 1999 code has been interpreted by the AHJ as requiring a 25 ohm impedance on single rods even when they are driven as a supplemental ground to an underground metallic water pipe.
I did get the depth wrong, it is 30". I have not read that section in a while. When doing an underground service the trench depth is usually 30-36" and I typically lay the rod(s) in the trench. I have never been questioned on this and don't feel it is a compromised installation. I will say most of my area is bluestone, shale and ledge rock so the fact that a rod may not be able to be driven is a common occurrence.
In an area were I know that the rock bottom is too close to the surface to drive rods I will bury them myself. I always try to drive one were possible and I add rod couplers and rods until I'm under twenty five ohms or I bottom out. I use an electric demolition hammer and a ground rod cup to do the driving.
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Last edited by hornetd; 04-14-04 at 07:45 AM.
 

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