18inches for direct burial, driveways and parking areas

[Twoskinsoneman]

Had an interesting conversation with a very eager Lowe's employee today. As he walked past me he noticed I had some USE-2 cable and some rigid NM conduit on my cart. He stopped me and said "Hey, you know you don't need that conduit if you dig your trench down to 18in." Normally I try to steer clear of the helper and their "advice". I was definitely intrigued by it though. " Don't you mean 24in?" I asked. He was pretty adimate about his figures. Anyway when I got home I pulled the old NEC and read 300.5 and looked at the table. Anyway I saw "One- and two-family dwelling driveways and outdoor parking areas, and used only for dwelling-related purposes" -18in burial depth.

I was hoping to strike up some responses as to an explanation of this requirement. Why would a shallower depth be allowed for "dwelling driveways and outdoor parking areas"? Seems odd. Thanks.

 

[Psychojohn]

Under a solid covering (concrete/Blacktop) without a large volume of heavy trafic, maybe?

 

[lowryder88h]

18 inches

18 inches

I'd have to agree with John. the type of covering and the lack of high use by large trucks and the sort.

 

[dnem]

I'd have to agree with John. the type of covering and the lack of high use by large trucks and the sort.

That's probably the reason. . There's nothing in that whole 1 + 2 family dwelling column that has a depth greater than 18". . And 120v 20a GFCI circuits can even go 12" under the driveway.

But that Lowe's guy is screwing up by spitting out 18" without knowing the details. . The max requirement on that chart is 24". . He should always say 24" unless he knows the details.

Another strange thing about Table 300.5
Under streets, highways, etc is 24" but Under a building is 0" even if its running thru a warehouse building that will have truck traffic driving thru the building.

We try to reason out various code rules whenever possible so we can understand how to apply general rules to the variety of installations. . Sometimes it's like jamming a square peg into a round hole. . And sometimes there's no way to make sense of it at all. . You just comply and move on.

David

 

[Twoskinsoneman]

Under a solid covering (concrete/Blacktop) without a large volume of heavy trafic, maybe?

That's why I scratch my noggin a little. It doesn't specify concrete or black top as it does in the other sections of the column. It even specifies how much concrete is required, 4inches or 2inches (respectively). I know a large portion of residential driveways and "parking areas" are dirt, gravel, etc.

 

[jetlag]

whats nice is muliwire branch circuit

whats nice is muliwire branch circuit

The way they word 300.5 has always confused me because it never really says about the areas for residence yard that are not in driveway or parking areas. If you have to go to locations not specified the cover is 24 in. Where it says and used only for dwelling related purposes, it should say and other "AREAS" used only for dwelling related purpoes. Whats nice though is if they allow multiwire branch circuit in your area, you can have two hot wires a neut. and GEC on 20 amp double pole GFCI and it only counts as 120 v circuit and only require 12 " burial

 

[dnem]

The way they word 300.5 has always confused me because it never really says about the areas for residence yard that are not in driveway or parking areas. If you have to go to locations not specified the cover is 24 in. Where it says and used only for dwelling related purposes, it should say and other "AREAS" used only for dwelling related purpoes.

"it should say and other "AREAS" "

That depends on what the code panels intention was. . Maybe they said what they meant.

Stone driveways are pretty hard packed and don't commonly have truck traffic. . Wiring run under the drive is less likely to be chopped by the home owners shovel than wiring in the yard. . Are you sure yard depth for direct burial isn't 24" ?

There's nothing on that chart that specifies allowing the dwelling yard to have a 18" burial depth. . Our areas rocky soil dictates conduit most of the time which allows a column3 depth of 18" everywhere. . But where column1 direct burial is used, we enforce 24" in the yard.

Whats nice though is if they allow multiwire branch circuit in your area, you can have two hot wires a neut. and GEC on 20 amp double pole GFCI and it only counts as 120 v circuit and only require 12 " burial

Why would column4 be restricted to one circuit or one multi ? . I know you run into restrictions in 225.30 + 250.32(A)x when you run more than a multi to a detached building, but the wording of Table300.5 column4 doesn't have a circuit limit. . You could supply more than 2 circuits of site lighting direct burial or in conduit at 12" if you follow the requirements of column4.

David

 

[jetlag]

"it should say and other "AREAS" "

That depends on what the code panels intention was. . Maybe they said what they meant.

Stone driveways are pretty hard packed and don't commonly have truck traffic. . Wiring run under the drive is less likely to be chopped by the home owners shovel than wiring in the yard. . Are you sure yard depth for direct burial isn't 24" ?

There's nothing on that chart that specifies allowing the dwelling yard to have a 18" burial depth. . Our areas rocky soil dictates conduit most of the time which allows a column3 depth of 18" everywhere. . But where column1 direct burial is used, we enforce 24" in the yard.



Why would column4 be restricted to one circuit or one multi ? . I know you run into restrictions in 225.30 + 250.32(A)x when you run more than a multi to a detached building, but the wording of Table300.5 column4 doesn't have a circuit limit. . You could supply more than 2 circuits of site lighting direct burial or in conduit at 12" if you follow the requirements of column4.

David

Yes I didnt mean you could have only one multiwire branch circuit , you are not suppose to connect any 240 v equip. even though the power is there. Maybe the code did say what it means. That would mean other yard areas require 24" but no one seems to think RESIDENCE has to be more than 18" any where. Thats my question are areas other than drives and parking in residence 24 or 18" . The conduit doesnt matter it requires the same as direct but that doesnt seem right either. The conduit offers more protection

 

[dnem]

Thats my question are areas other than drives and parking in residence 24 or 18" . The conduit doesnt matter it requires the same as direct but that doesnt seem right either. The conduit offers more protection

And my answer for residential is:

column1 yard direct burial 24"
column1 drive direct burial 18"

column3 yard PVC 18"
column3 drive PVC 18"

column4 yard 120v 20a GFCI 12"
column4 drive 120v 20a GFCI 12"

We'll see if somebody else has a different answer

 

[jetlag]

Thanks Dnem

Thanks Dnem

And my answer for residential is:

column1 yard direct burial 24"
column1 drive direct burial 18"

column3 yard PVC 18"
column3 drive PVC 18"

column4 yard 120v 20a GFCI 12"
column4 drive 120v 20a GFCI 12"

We'll see if somebody else has a different answer

You are first one Ive ever found to agree that yard areas other than drive and parking are not listed and have to go by "all locations not specified" at top of 300.5 . even the inspectors in my area believe this to be 18". Look at another interesting item, the other yard areas call for more burial but for RMC , it is only 6 " which is just the opposite an is much less than the drive and parking area. Why do you think that would be?

 

[dnem]

You are first one Ive ever found to agree that yard areas other than drive and parking are not listed and have to go by "all locations not specified" at top of 300.5 . even the inspectors in my area believe this to be 18". Look at another interesting item, the other yard areas call for more burial but for RMC , it is only 6 " which is just the opposite an is much less than the drive and parking area. Why do you think that would be?

Think about what is the primary threat for each wiring method.

For direct burial, it's a homeowners shovel in the yard. . So the yard depth has to be low enough to stay away from the shovel. . The driveway isn't accessible to the shovel so it can be shallower. . There's no way to prevent backhoe damage [ other than 300.5(D)(3) ribbon ] because direct burial gives almost no operator feedback resistance.

For pvc, the shovel isn't the threat and for rigid, the shovel is nothing. . The biggest threat is the backhoe bucket, but that threat is based on feedback to the operator. . Depth is less critical where there's more feedback. . And there's more bucket feedback in dirt than in gravel. . So depth is less critical in the yard.

When you think direct burial, think manual shovel threat.
When you think conduit, think backhoe bucket threat and then the numbers make sense.

A shovel easily pushes down into the dirt for the first foot and a half before the foot pressure top edge is recessed and digging becomes more arm than foot. . Your foot can easily send a shovel thru a direct burial cable insulation.

The homeowners shovel doesn't penetrate conduit. . And even a backhoe bucket would give the operator feedback if it he hit rigid in the yard. . But that same operator wouldn't get that same feedback if digging thru the large gravel of a driveway. . Every scoop would feel like you were cutting thru rigid.

I don't think T300.5 has any mistakes.

I believe it's all based on the logic of homeowners shovel foot pressure and feedback to backhoe operators.

David

 

[jetlag]

thanks again dnem

thanks again dnem

Think about what is the primary threat for each wiring method.

For direct burial, it's a homeowners shovel in the yard. . So the yard depth has to be low enough to stay away from the shovel. . The driveway isn't accessible to the shovel so it can be shallower. . There's no way to prevent backhoe damage [ other than 300.5(D)(3) ribbon ] because direct burial gives almost no operator feedback resistance.

For pvc, the shovel isn't the threat and for rigid, the shovel is nothing. . The biggest threat is the backhoe bucket, but that threat is based on feedback to the operator. . Depth is less critical where there's more feedback. . And there's more bucket feedback in dirt than in gravel. . So depth is less critical in the yard.

When you think direct burial, think manual shovel threat.
When you think conduit, think backhoe bucket threat and then the numbers make sense.

A shovel easily pushes down into the dirt for the first foot and a half before the foot pressure top edge is recessed and digging becomes more arm than foot. . Your foot can easily send a shovel thru a direct burial cable insulation.

The homeowners shovel doesn't penetrate conduit. . And even a backhoe bucket would give the operator feedback if it he hit rigid in the yard. . But that same operator wouldn't get that same feedback if digging thru the large gravel of a driveway. . Every scoop would feel like you were cutting thru rigid.

I don't think T300.5 has any mistakes.

I believe it's all based on the logic of homeowners shovel foot pressure and feedback to backhoe operators.

David

I own a backhoe myself and like you say I never try to sink the bucket deep in a yard but rather rake back very slowly and would take only 3 0r four inches first time and would feel the claws hit the rigid because they would be a little deeper. In the drive way its so hard would never it hit any thing. Thans for your help , it bothers me if I cant understand the reason for a code, I just think its a better electrician that can give a customer the reason for a code instead of just saying whats against the code .