18" depth rule for PVC.

[Strife]

Being that I lived all my immigrant life in south florida I never figured the 18" depth rule for PVC vs 6" for GRC.
Recently, I think I've figured the reason.
I just found out (I think) the reason behind it. From what I understood, the freeze table up north is 18". So is that the reason PVC has to be 18" deep?
If that's the case that's one silly rule to apply here in Florida. Even up north (Florida "up north") we might get close to freezing, and sometimes even freezing, but I seriously doubt it freezes 18" below ground, EVER.
I mean, we're allowed to run PVC above ground here, which is not allowed up north, but I have to install it 18" bellow ground because that's the freezing table?????????

 

[tryinghard]

Not for freezing but protection from damage. Check out 300.5

 

[tom baker]

I mean, we're allowed to run PVC above ground here, which is not allowed up north, but I have to install it 18" bellow ground because that's the freezing table?????????

There are thousands if not millions of installations of PVC above ground in my state.
The NEC, is well, a national electrical code, and it allows PVC per the uses allowed listed in the article. No mention of climate.

 

[tom baker]

Being that I lived all my immigrant life in south florida I never figured the 18" depth rule for PVC vs 6" for GRC.
Recently, I think I've figured the reason.
I just found out (I think) the reason behind it. From what I understood, the freeze table up north is 18". So is that the reason PVC has to be 18" deep?
If that's the case that's one silly rule to apply here in Florida. Even up north (Florida "up north") we might get close to freezing, and sometimes even freezing, but I seriously doubt it freezes 18" below ground, EVER.
I mean, we're allowed to run PVC above ground here, which is not allowed up north, but I have to install it 18" bellow ground because that's the freezing table?????????

And in Michigan where I grew up the frost depth is 4 feet.

 

[petersonra]

I have never quite understood it either. It is often OK to lay it on the ground, but if you bury it, it has to be 18" deep.

 

[Dennis Alwon]

I have never quite understood it either. It is often OK to lay it on the ground, but if you bury it, it has to be 18" deep.

If it is visible you can avoid it but if it's buried they want to protect it from "The Dig". But you knew this.

 

[kwired]

Look at T300.5 column 3 for NM raceways.

buried under 2" concrete - 12 inches

buried under 4" concrete - 4 inches

under a building - 0 inches (some buildings are not heated and the ground freezes there just like outside.

It is not about frost in the ground it is about physical protection.

Look at column 1 for direct burial cables or conductors - they are generally deeper than all other methods - if you are digging at least the raceway in the other columns provides some protection of the conductors within but a direct buried conductor is immediately exposed to the excavation method.

 

[Steviechia2]

Homeowners and landscapers would have a lot of surprises when they put there spade shovel into the ground to plant, if it was only 6" down

 

[steve66]

Yes, I don't think freeze depth has anything to do with it. Here in Illinois to freeze depth is about 3'.

I think it has a lot to do with it being easier to knock a hole in PVC with a shovel than it is for GRC.

 

[renosteinke]

I can't claim to know what anyone else thinks - let alone the code panels of years gone by. Yet, here's how I look at conduit burial:

It's not all that obvious, but the deeper you go, the more any loads from above are distributed to the entire area. I suspect that you reach a 'point of diminishing returns,' though, where forces are pretty much as distributed as they will ever be. I also expect that point, in nearly all soils, to be at about 6". So, there's one possible starting point.

The next question is: what else is in the ground? With lawn sprinklers also at that 6" level, you want to be under them enough that your pipe isn't disturbed by sprinkler maintenance.

Just what do you use for digging? Why, a shovel, of course. There's some sense in putting your pipe about 'one shovel' deep. The blades of most shovels are just about 12" long.

If you've got something 'weaker' than a steel pipe, maybe you want to go "shovel blade and tang" deep. That's about 18"

Once it's buried, it stays put, right? Well, if you're in a freezing climate, the soil actually heaves and shifts with every weather change. You really want to go below this shifting soil. Depending on where you are, this level is likely 24-30" deep. The colder the winter, the deeper you want to go. That's why foundation footings have to go so deep.

 

[Strife]

Have you ever tried to put a shovel through SCH 80 PVC? That think must be stronger that IMC.

Homeowners and landscapers would have a lot of surprises when they put there spade shovel into the ground to plant, if it was only 6" down

 

[kwired]

Have you ever tried to put a shovel through SCH 80 PVC? That think must be stronger that IMC.

Have you ever tried to cut IMC with a PVC cutter?

I have put backhoes and trenchers through sch80 but never IMC. If anything it just rips the IMC out of the ground, maybe breaks a fitting someplace, bends the hell out of the pipe but never goes through.