Minimum Burial Depth ? Ductbanks

[charlie b]

Table 300.5 (2008 NEC) does not include the case of PVC conduits that are concrete-encased and that run under a roadway. My particular roadway leads to a loading dock, so it will have 18-wheeler traffic. I told the architect and the civil engineer to give me 24 inches from the top of my concrete encasement to the surface of the roadway. There is a conflict with services running below my ductbank, and they want to know if I can permit a shallower burial.

I can’t find anything specific to my situation. Any ideas?

Edited to add: Voltage level is 13,800 V.

 

[benaround]

Charlie,

For over 600v see table 300.50.

 

[charlie b]

OK, that helps. Thanks Frank.

Now let?s talk about that table?s note 3.

I have four conduits in the ductbank. From the top of the upper row, I have 2 inches of concrete as part of the encasement. Suppose I tell them to add 2 more inches of concrete. I infer from note 3 that the minimum cover, from the top of all the concrete to the surface of the roadway, can be as little as 12 inches. Furthermore, if I add yet another 2 inches of concrete (total of 6 inches from the top of the conduits), then from the top of the concrete to the surface of the roadway can now be as little as 6 inches.

Am I reading that part right?

If it changes the situation, this is a university. They do have conditions of maintenance and supervision to ensure that only qualified persons service the installation. But does a university qualify as an ?industrial establishment??

 

[benaround]

Charlie,

Sorry, I'm not the person to answer your questions. I was looking at 310.60 and saw the

note for 300.50. But it might have possiblities !

 

[pete m.]

There is a conflict with services running below my ductbank, and they want to know if I can permit a shallower burial.

Charlie,

For what it's worth I think you could make a valid argument using note 3 to table 300.50. I'm not sure that the University would qualify as an industrial establishment but I think the "point" of note 3 is that the situation is under control of qualified individuals.

As far as the lesser depth, being that there is a question in the verbiage I would recommend speaking with the AHJ and bring up that note and the conditions involved.

Pete

 

[cycotcskir]

But does a university qualify as an “industrial establishment”?[/SIZE][/FONT]

240.2 says:

Supervised Industrial Installation. For the purposes of
Part VIII, the industrial portions of a facility where all of
the following conditions are met:
(1) Conditions of maintenance and engineering supervision
ensure that only qualified persons monitor and service
the system.
(2) The premises wiring system has 2500 kVA or greater of
load used in industrial process(es), manufacturing activities,
or both, as calculated in accordance with Article
220.
(3) The premises has at least one service or feeder that is
more than 150 volts to ground and more than 300 volts
phase-to-phase.
This definition excludes installations in buildings used
by the industrial facility for offices, warehouses, garages,
machine shops, and recreational facilities that are not an
integral

I made the limitations bold. Although this is a limited definition, I think that you would have a hard time convincing an AHJ that the definition of "industrial" is something other than what is described here, main emphasis being on the latter portion of #2 (industrial process(es), manufacturing activities,
or both)

I think that it's quite a bit of a stretch for a University to be in the same ballpark as "industrial".

In my little ballpark, we require 3' burial pretty much all around and especially at roadways. Many of the road signs that people drive into the ground are driven in to a depth of 3'.

 

[charlie b]

The building being served is a laboratory. There will be bio safety cabinets, fume hoods, and a host of chemical processes involved. I think that may help sell the notion.

I have created a sketch of the proposed ductbank, including dimensions that take advantage of table 300.50, note 3. I'll pass it by the AHJ, and get a ruling, before I give the architect a formal design change document.

 

[cycotcskir]

Good luck!

Let us know (if you don't mind) what the ruling is. This would be good to add to the "gray area" file in the back of my head.

BTW: Is the specific area fenced off in any way or would utility contractors have access to the area without the University's approval?

 

[charlie b]

BTW: Is the specific area fenced off in any way or would utility contractors have access to the area without the University's approval?

Not sure I understand the question. The roadway (under which the ductbank will be installed) is open to anyone. The new building will be card-reader access only.

 

[cycotcskir]

It seems that you could have better luck with the AHJ if the area is not accessible to the general public. It would make it less likely for a "public" accident to happen.

If it is in a gated area, then it follows the logic of (1) Conditions of maintenance and engineering supervision
ensure that only qualified persons monitor and service
the system.


As far as the Utilities part goes, is it near any of their "right-of-ways"? If there is really no chance of the POCO digging without the University's permission.

Around here, there are many things going in all directions and sometimes the right people don't get notified in time.

Maybe another selling point.

I hope it came out more clearly then it sounded in my head. End of the day here on the East coast, brain is winding down.

 

[lakee911]

You can not always design around the minimum code requirement. It's a set of rules, not a design guide. Make sure that you're designing this duct bank for the loads that are required.

If you can not go underneath your obstruction, you most definitely want to ensure that the subgrade both under and around the db is compacted very well and perhaps even add a geotextile fabric for additional support. I would use a high strength concrete with upsized rebar and transition to RMC under the roadway. I'd consult your civil engineer on some of these issues.

 

[benaround]

You can not always design around the minimum code requirement. It's a set of rules, not a design guide. Make sure that you're designing this duct bank for the loads that are required.

If you can not go underneath your obstruction, you most definitely want to ensure that the subgrade both under and around the db is compacted very well and perhaps even add a geotextile fabric for additional support. I would use a high strength concrete with upsized rebar and transition to RMC under the roadway. I'd consult your civil engineer on some of these issues.

--------

 

[charlie b]

Thanks for the comments, everyone.

The area under consideration can be reached without passing any fences. But it is well within the university's property (nowhere near the utility point of service). I don't worry about anyone messing with this ductbank. After all, it will be under a road, so no student will trip over it.

I will be discussing the situation with the assigned civil engineer (turns out, she works in the same building as I do). I like your suggestions, Lakee, and will bring them up with her.