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Old 02-23-2019, 03:01 PM   #1
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Default PVC depth For Barn With RV

I’m looking for some clarification on the 300.5 table for my situation. I have a job for the electrical in a barn. There will be an RV parked inside and I’d like to run pvc underground for the different outlet locations.

My question is does the 0” depth for PVC still count for under a building if they will be driving an RV or other machinery inside or would that put me at 18” for the 6th column down (one and two family dwelling driveways and outdoor parking)?

Thanks!
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Old 02-23-2019, 03:58 PM   #2
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Will there be a concrete floor in the building or just the dirt/gravel?

If it's just dirt/gravel, I'd consider it the same as under a laneway/driveway/roadway/subject to vehicular traffic.

I know nothing about NEC code so that's all the help I can offer.
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Old 02-23-2019, 04:58 PM   #3
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0" counts in my opinion.

but remember this isn't a design manual, if there is a decent expectation of settling of the ground it should be buried deeper to reduce likelihood of damage. and if it is exposed to severe physical damage it would need to be sched 80
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Old 02-23-2019, 05:11 PM   #4
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The reason for the burial depths is for protection from excavation. If there is not going to be digging in the barn, and it is going to have a concrete floor, you are probably fine just scratching it in.

Is it obvious to the inspector that this barn is being used for heavy vehicles? Going by the letter of the code, it doesn’t use words like parking lots “or similar locations”. If this was an attached residential garage, I don’t see a reason to go 18” or 24”. My reason would be that it’s under a slab of 4” of concrete. 300.5 does not apply to under buildings or under slabs 4” thick. Commercial garage and explosion proof would be different.

Here’s a excerpt from an old code book.

Quote:
300.5 only applies to “underground installations” and is not applicable if the conduit is laid directly on the ground. No Code rule prohibits conduit laid on the ground, provided the conduit is “securely fastened in place” (300.11) and is not exposed to physical damage such as vehicular traffic, and many such instal- lations have been made for years. But, when conduit is installed in the ground, there is serious concern about damage due to digging in the ground, which 300.5 addresses.
As shown in Fig. 300-11, Table 300.5 recognizes that raceways run under concrete slabs at least 4-in. (102 mm) thick or under buildings have sufficient protection against digging and are not required to be subject to the burial-depth requirements given in the top line of Table 300.5. Where raceways are so installed, the rule requires that the slab extend at least 6-in. (150 mm) beyond the underground raceway, as follows:
1. Any direct burial cable run under a building must be installed in a race- way, as required by 300.5(C), and the raceway may be installed in the earth, immediately under the bottom of the building, without any earth cover.
2. Any direct buried cable under a slab at least 4-in. (102 mm) thick and not subject to vehicles is subject to the 18-in. (450-mm) minimum burial- depth requirement of Table 300-5. The reason for the equivalence in cover between the 2-in. (50 mm) concrete-in-trench rule and the 4-in. (102 mm) concrete slab rule is that when the concrete is buried in the trench an excavator will recognize it as a protective structure. The surface slab may not be easily recognized as performing an additional protective function.
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Old 02-23-2019, 05:28 PM   #5
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I would expect a BARN (with or without an RV) to have a dirt floor.

But, you'd have to ask.

4" of concrete is not cheap -- not that cheap.

Animals are bio-engineered for dirt. Does this guy have horses?

Raceway depth is spec'd deeper for traffic because of injuries caused by settling and for the high dynamic loads that trucks and buses impose.

Out my way, our 'soils' are so bad that we have to use sand as back-fill. Local stones would drive through PVC as if they were arrowheads. ( The natives chipped many an arrowhead in my front yard -- so I have quite a collection of shards and brokens.)
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Old 02-23-2019, 05:44 PM   #6
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Quote:
Originally Posted by telsa View Post

Out my way, our 'soils' are so bad that we have to use sand as back-fill. Local stones would drive through PVC as if they were arrowheads.
is that because of ground movement or the type of stone or combination?

i like sand to smooth off the bottom of the trench, and i'm just working in clay for an average of 30' deep
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Old 02-23-2019, 06:37 PM   #7
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I don’t think there’s much difference in pressure on conduit by varying few inches of depth, or the weight above. If the soil is rocky, then yes it needs something to protect PVC. Quarry dust was the big thing when I lived/worked in a more rocky area. I suppose it was dirt cheap.

The reason is because roads and parking lots will be re-milled and dug up. Having the extra depth I guess helps? Remember it doesn’t matter if it’s rigid or direct burial, it’s all the same depth.
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Old 02-23-2019, 06:39 PM   #8
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We have micro-quakes ALL the time. Only the bigger stuff makes the news.

Micro-quakes cause settlement.

The local stones are brutal on equipment. You'd have to see it to believe what they can destroy.

It's such a problem that our inspectors jump all over bedding and backfill.

After you've got 8" of cover, you're usually able to go the rest of the way with native backfill -- after its been sorted to get rid of the Big Stuff.

The last big underground I worked we brought in sand by the tandem truck load and hauled out over-sized spoils by the truck load -- many truck loads. It's so bad that many sites just plan on bringing in topsoil by the 'train load' -- lifting the entire site by two feet!

Earthquakes are the reason California construction is so expensive. Our framing is often eight-times as robust as that of the Midwest. We've got shear walls going forever, too.

Lucky us!
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Old 02-23-2019, 06:52 PM   #9
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Quote:
Originally Posted by HertzHound View Post
I don’t think there’s much difference in pressure on conduit by varying few inches of depth, or the weight above. If the soil is rocky, then yes it needs something to protect PVC. Quarry dust was the big thing when I lived/worked in a more rocky area. I suppose it was dirt cheap.

The reason is because roads and parking lots will be re-milled and dug up. Having the extra depth I guess helps? Remember it doesn’t matter if it’s rigid or direct burial, it’s all the same depth.
The NEC is written to this standard because -- in most situations -- compaction is nowhere near 95%. Re-compacting soils (to 95%+) quickly gets EXPENSIVE. I've only ever seen it attempted under transformer pads and other similar loads.

Our micro-quakes expose poor compaction as the years roll by. Boy do those trenches show! And they WERE compacted. Just not enough to stand the test of time.

BTW, you'd be surprised at how many times guys fail their first or second compaction test. What feels firm underfoot proves to be still too soft for dynamic loads or heavy static loads.



When streets are trenched -- typically for plumbing // sewer -- the extra 6 inches is a joke. The same stats hold true for road rebuilding.

By specifying a horizon, the NEC gives the other trades something predictable to hunt for. ( The vacuum excavation crew, in particular. ) The typical back-hoe bucket can go down through 18" during the first pass... just pealing off the asphalt. Yeah, they're cross-eyed goofs.
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Old 02-23-2019, 07:30 PM   #10
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Quote:
Originally Posted by telsa View Post
We have micro-quakes ALL the time. Only the bigger stuff makes the news.

Micro-quakes cause settlement.

The local stones are brutal on equipment. You'd have to see it to believe what they can destroy.

It's such a problem that our inspectors jump all over bedding and backfill.
is concrete encasement of duct-banks not a good idea out there either, because of the lack of flexibility? we encase a lot around here, but mainly for excavator protection. concrete may be cheaper here to($60/yd), you can dig a limestone quarry anywhere here.
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Old 02-23-2019, 08:40 PM   #11
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Quote:
Originally Posted by Sparkzman22 View Post
I’m looking for some clarification on the 300.5 table for my situation. I have a job for the electrical in a barn. There will be an RV parked inside and I’d like to run pvc underground for the different outlet locations.

My question is does the 0” depth for PVC still count for under a building if they will be driving an RV or other machinery inside or would that put me at 18” for the 6th column down (one and two family dwelling driveways and outdoor parking)?

Thanks!
Do you consider a barn, a building?

We do. So does our AHJ's.

We scratch them in below the slabs a little so they don't end up in the concrete.
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Old 02-23-2019, 10:21 PM   #12
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Quote:
Originally Posted by Cow View Post
Do you consider a barn, a building?

We do. So does our AHJ's.

We scratch them in below the slabs a little so they don't end up in the concrete.
Same for us.
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Old 02-23-2019, 10:50 PM   #13
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@Wiresmith

Where are you buying concrete for $60/yd?

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Old 02-24-2019, 10:41 AM   #14
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Quote:
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@Wiresmith

Where are you buying concrete for $60/yd?

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That's the lowest rate I know around here that the large contractors get a low rate on, it's from locally owned mixers. That's not the average rate, just the absolute lowest I have seen, if you go buy it on your own just a private individual it can be around 120

ETA: and that's the lowest grade concrete, for duct bank encasement
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