Direct burial depth

[bthielen]

I found it interesting that residential direct burial depth is only 18? under gravel driveways and parking areas while across my lawn requires 24? of depth. This sounds backwards to me. It seems to me that under a driveway there is a higher chance of gravel that could cause damage to the wire and in my case that is precisely what I ran into when I dug the ditch to my new pole building. I went 24? even under my driveway and still found gravel. I decided to encase the wire in PVC conduit to add a little abrasion resistance under the driveway. Areas that are not under my driveway are clay soil with very little if any sand or gravel. It would also seem too that living in Minnesota where frost heaving is common there is a greater chance of movement within the soil throughout the year, especially under a roadway, driveway, or parking area. So, my first questions is why the difference in depth requirement?

I also ran into another situation. I was aware of a 12awg run supplying my well pump that was installed by a contractor in 1991. When I got near the area where I expected the wire to be, I began digging my trench by hand in anticipation of locating the wire. There was a tree nearby and I had been cutting roots all along. Yes, I found the wire when I thought I was cutting through a tree root. It was only buried about 10?-12? deep. This was upsetting because here I am being so careful to bury my wires at a depth of 24? and find that when I hire someone to do it they only bury it about 12?. I inquired of our master electrician where I work and he suggested that circuits less than 30amp only require 12? burial depth. I can?t find any reference to this in the 2002 code and to my knowledge Minnesota does incorporate the NEC. Is this correct or perhaps was this was correct in 1991 but is no longer? If it is or was correct then I would be in the wrong with my criticism of the contractor's work. I would be interested in knowing where this is in the code.

Thanks,

Bob

 

[ryan_618]

Re: Direct burial depth

Column 4 of table 300.5 gives some allowances for residential applications, provided that the circuit is 120 volts, 20 amps or less and is GFCI protected. Perhaps this is what is was speaking of?

 

[charlie]

Re: Direct burial depth

Table 300.5, Column 4 Residential Branch Circuits Rated 120 Volts or Less with GFCI Protection and Maximum Overcurrent Protection of 20 Amperes, you are permitted 12 inches. I don't recall when it changed, nor do I have the inclination to look it up but the Code used to permit 12 inches on a 30 ampere branch circuit for residential use.

Now to address your first question. If a cable is direct buried in your yard, the chances are greater that you will dig it up than if it were under a driveway. The Code is concerned about safety, not necessarily being practical.

About the rocks under your driveway, see 300.5(F), "Backfill. Backfill that contains large rocks, paving materials, cinders, large or sharply angular substances, or corrosive material shall not be placed in an excavation where materials may damage raceways, cables, or other substructures or prevent adequate compaction of fill or contribute to corrosion of raceways, cables, or other substructures.

Where necessary to prevent physical damage to the raceway or cable, protection shall be provided in the form of granular or selected material, suitable running boards, suitable sleeves, or other approved means."

Don't forget, the Code is a set of minimum standards.

 

[charlie]

Re: Direct burial depth

OK, I got off my lazy b - - - and looked it up. The 1987 NEC permitted a residential circuit of up to 30 amperes to be buried at 12 inches. In the 1990 Code, the permission was changed to GFCI protected 20 ampere, 120 volt circuits.

 

[pierre]

Re: Direct burial depth

Thanks Charlie, I was very interested and you saved my lazy butt from the research

Pierre

 

[bthielen]

Re: Direct burial depth

Thanks for the info.

see 300.5(F), "Backfill. Backfill that contains large rocks, paving materials, cinders, large or sharply angular substances, or corrosive material shall not be placed in an excavation where materials may damage raceways, cables, or other substructures or prevent adequate compaction of fill or contribute to corrosion of raceways, cables, or other substructures.

Where necessary to prevent physical damage to the raceway or cable, protection shall be provided in the form of granular or selected material, suitable running boards, suitable sleeves, or other approved means."

If the purpose of the code is only for safety and therefore the issue of depth is as you suggest, which sounds reasonable and logical, why then does the code address the backfill material? I agree that it is good to protect the cable from abrasive backfill material but isn't this more of a practical practice than a safety issue? After all it is buried and movement from frost and heavy equipment could be just as damaging as cinders or rocks,etc. I guess it sounds a little contradictory to your explanation.

Bob

 

[charlie]

Re: Direct burial depth

I said, "The Code is concerned about safety, not necessarily being practical." I did not say the Code was interested only in safety, it is also interested in property and the UG run is property.

From the Code:
90.1 Purpose. (A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.

From the NEC NFPA Handbook:
The NEC is prepared by the National Electrical Code Committee, which consists of a Technical Correlating Committee and 20 code-making panels. The code-making panels have specific subject responsibility within the Code. The scope of the National Electrical Code Committee follows:
This committee shall have primary responsibility for documents on minimizing the risk of electricity as a source of electric shock and as a potential ignition source of fires and explosions. It shall also be responsible for text to minimize the propagation of fire and explosions due to electrical installations.
In addition to its overall responsibility for the National Electrical Code, the Technical Correlating Committee is responsible for the Electrical Code for One- and Two-Family Dwellings (NFPA 70A) and for the correlation of Recommended Practice for Electrical Equipment Maintenance (NFPA 70B), Standard for Electrical Safety Requirements for Employee Workplaces (NFPA 70E), Electrical Inspection Code for Existing Dwellings (NFPA 73), and Electrical Standard for Industrial Machinery (NFPA 79).

Code again:
(B) Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

FPN:Hazards often occur because of overloading of wiring systems by methods or usage not in conformity with this Code. This occurs because initial wiring did not provide for increases in the use of electricity. An initial adequate installation and reasonable provisions for system changes will provide for future increases in the use of electricity.