burial depth under concrete driveway

[wwhitney]

So if it is not in a trench you can't use that line.

Are you actually proposing that if I (a) dig a trench of sufficient depth, install the wiring, cover it up with the spoils, and then add 2" of concrete on top, that complies, while if I (b) have overexcavated a large area, install the wiring, add the necessary depth of fill over the large area, and then add 2" of concrete, that doesn't? While I agree that's a literalist reading, it's too nonsensical even for me.

Regardless, if it's being installed under an existing driveway from above, then that will likely involve cutting a trench through the driveway and it will end up in a trench.

Cheers, Wayne

 

[petersonra]

Are you actually proposing that if I (a) dig a trench of sufficient depth, install the wiring, cover it up with the spoils, and then add 2" of concrete on top, that complies, while if I (b) have overexcavated a large area, install the wiring, add the necessary depth of fill over the large area, and then add 2" of concrete, that doesn't? While I agree that's a literalist reading, it's too nonsensical even for me.

Regardless, if it's being installed under an existing driveway from above, then that will likely involve cutting a trench through the driveway and it will end up in a trench.

Cheers, Wayne

There are a lot of similar code requirements that don't make much sense but we have to follow them. If it's not in a trench it's not in a trench.

 

[Stevenfyeager]

I'm planning to trench across the gravel base and install pvc (as the sub contractor is starting to install and form) before the 4" concrete driveway is poured, several places crossing it for future lighting. I'm anticipating lots of lighting at turn arounds etc., but the owner hasn't decided it all yet. Some 120v and maybe low voltage. It's a large house and long driveway with turn arounds at the house and detached building. Trying to determine the necessary depth. Thank you.

 

[infinity]

The first line in that table says "All locations not specified below". If there is a specific burial depth for a one and two family driveway then that is what you use to determine the burial depth.

 

[wwhitney]

The first line in that table says "All locations not specified below". If there is a specific burial depth for a one and two family driveway then that is what you use to determine the burial depth.

If the second line also said "all locations not specified below," I'd agree with you. But since it doesn't, there's no particular reason to pick line 6 over line 2.

That said, pick line 6 and you satisfy both requirements.

Cheers, Wayne

 

[wwhitney]

If it's not in a trench it's not in a trench.

I'm planning to trench across the gravel base

Sounds like in this case it's a trench, so we don't have to resolve our disagreement.

Cheers, Wyne

 

[infinity]

The wording isn't great but the implied meaning is that the concrete is within the trench not above it as in a driveway. Dig a trench, install the raceway, pour 2" of concrete over it then install the driveway above the trench. Now you can use the lesser depth of line #2.

 

[wwhitney]

The wording isn't great but the implied meaning is that the concrete is within the trench not above it as in a driveway. Dig a trench, install the raceway, pour 2" of concrete over it then install the driveway above the trench. Now you can use the lesser depth of line #2.

That totally makes sense, it's just not what the words convey.

Sounds like the following would comply with your description: If the driveway is going to be 4" thick above the gravel, then trench 2" + conduit diameter deep in the gravel, put some conduit in the bottom of the trench, and just let the driveway be 2" thicker there. Then you've got your 2" of concrete, plus your driveway on top.

Personally, I don't think the wording chosen means that the trench has to have the 2" of concrete in it.

Cheers, Wayne

 

[Little Bill]

My reading is that it is a driveway for a dwelling, so you have to use the location of dwelling driveway. It doesn't mention the surface type or trench.
It's very clear, by the wording "one and two family driveways" that is the location to use. Concrete doesn't affect anything.
Then depending on the column, you use the depth of that column. If the OP is using GFCI protection, then 12" is correct. That would be 12" from the top of the raceway to the top surface.

 

[petersonra]

That totally makes sense, it's just not what the words convey.

Sounds like the following would comply with your description: If the driveway is going to be 4" thick above the gravel, then trench 2" + conduit diameter deep in the gravel, put some conduit in the bottom of the trench, and just let the driveway be 2" thicker there. Then you've got your 2" of concrete, plus your driveway on top.

Personally, I don't think the wording chosen means that the trench has to have the 2" of concrete in it.

Cheers, Wayne

Yeah nowhere does it require that you have 2 in of concrete in the trench. Just a trench. It's just very poorly worded but like a lot of other poorly worded parts of the code That's what it says.

 

[infinity]

Personally, I don't think the wording chosen means that the trench has to have the 2" of concrete in it.

That is because the wording is poor. Although not code there is a good graphic in the NECH depicting the concrete in the trench.

Also if your argument is true then the under a driveway requirement needs to be changed to say driveways less than 2" thick since line #2 would cover most driveways (except for stone) 2" and above in thickness.

 

[wwhitney]

Also if your argument is true then the under a driveway requirement needs to be changed to say driveways less than 2" thick since line #2 would cover most driveways (except for stone) 2" and above in thickness.

If the driveway is concrete. Lots of residential driveways with no concrete.

Cheers, Wayne

 

[infinity]

If the driveway is concrete. Lots of residential driveways with no concrete.

Cheers, Wayne

Line #2 uses the word concrete. Driveways are made with different types of concrete. Poured concrete or pavers made from concrete are more than 2". Also asphalt driveways are typically 2.5" thick. Asphalt is a type of concrete. So by your interpretation about the only time you would be required to follow the driveway burial depth in the table is when the driveway is gravel.

 

[wwhitney]

Asphalt is a type of concrete.

Since the NEC doesn't define concrete, I'm happy to go with the common usage where asphalt is not "concrete."

Your point about 2" thick concrete pavers is an interesting one. So if I dig a trench and throw a layer of those in the trench above some conduit, that qualifies for line 2?

Anyway, I think we agree the line 2 description is vague and open to interpretation. And I hope we can agree that means a particular install could fit the descriptions of more than one line, and that the NEC doesn't give any explicit guidance about what to do in that case.

Cheers, Wayne

 

[gene6]

I keep table 300.5 as a closely guarded secret and only go to it in times of desperation and I advise other sparkies to also.
The reason is I work around allot of residential contractors and the excavator/driveway type contractors I have found (by trial and error) they need very simple instructions sometimes they are not native English speakers, so a while ago I just told them all electrical trenches need to be two feet deep and I put a piece of electrical tape at the two foot mark on their shovels.
You got to figure an electricians time as a ratio to a laborers time has got to be 1:5 even a owner/operator running a excavator per hour is less than me per hour, therefore if I have to mess around with a code book and an inspector for 1 hour that ditch digger could have dug for 5 hours.
Now every time I work with them they don't need to ask, they just make all trenches 2 feet deep.

 

[wwhitney]

Now every time I work with them they don't need to ask, they just make all trenches 2 feet deep.

Which is sufficient except when you need 24" of cover.

Cheers, Wayne

 

[Little Bill]

I keep table 300.5 as a closely guarded secret and only go to it in times of desperation and I advise other sparkies to also.
The reason is I work around allot of residential contractors and the excavator/driveway type contractors I have found (by trial and error) they need very simple instructions sometimes they are not native English speakers, so a while ago I just told them all electrical trenches need to be two feet deep and I put a piece of electrical tape at the two foot mark on their shovels.
You got to figure an electricians time as a ratio to a laborers time has got to be 1:5 even a owner/operator running a excavator per hour is less than me per hour, therefore if I have to mess around with a code book and an inspector for 1 hour that ditch digger could have dug for 5 hours.
Now every time I work with them they don't need to ask, they just make all trenches 2 feet deep.

You have to remember, and lots of people don't, the depth has to be measured from the top of the conduit, not the actual trench depth. So if you were using 2" conduit, in your 2 foot deep trench, the trench would need to be more than 26" deep if the depth requirement for the trench was 24".

 

[gene6]

Which is sufficient except when you need 24" of cover.

Cheers, Wayne

Yeah good reminder fella's. I am just talking residential type lighting and yard work, where its the builder, landscaper or homeowners kids digging a trench for a shed, hot tub or lighting post like the OP described.

 

[kwired]

Is a trench still a trench after it has been backfilled or is only a trench while still open?

 

[petersonra]

Is a trench still a trench after it has been backfilled or is only a trench while still open?

the code seems to suggest that. perhaps it is more of a technique.

keep in mind what the code says about rod and pipe electrodes.

250.53
(G) Rod and Pipe Electrodes. The electrode shall be installed
such that at least 2.44 m (8 ft) of length is in contact with the
soil. It shall be driven to a depth of not less than 2.44 m (8 ft)
except that, where rock bottom is encountered, the electrode
shall be driven at an oblique angle not to exceed 45 degrees
from the vertical or, where rock bottom is encountered at an
angle up to 45 degrees, the electrode shall be permitted to be
buried in a trench that is at least 750 mm (30 in.) deep. The
upper end of the electrode shall be flush with or below ground
level unless the aboveground end and the grounding electrode
conductor attachment are protected against physical damage as
specified in 250.10.

There are other places where similar language is used. It seems like they mean you start with a trench and end up with whatever you end up with.