Follow 8 rules for safe and long-lasting underground conduit installation. These rules apply to both engineering and at-home projects.
In my discussion, I’ll reference the National Electrical Code (NEC). You can then follow up on these references, to gain more insight into the subjects. But also, check out my electrical conduit guide for added information and tips. And most importantly, hire a professional for your electrical work, if you don’t have experience.
Important Note: before any underground work, contact your local utility. They’ll give you the location of any buried pipes. This is a required safety precaution before you ever pick up a shovel.
Also, use 811 “Call Before You Dig.” You can look up your state to learn about your state-specific laws and processes.
#1 Bottom of trenches
After you dig your trench for your conduits, there are 3 things you need to do.
1) Remove all hard and sharp materials
First, be sure your trench is free from rocks, clods, and foreign material. Any hard objects can damage your conduits. It only takes a small micro-fracture in a conduit for water to penetrate.
Important Note: an underground conduit is prone to water leaks. The hydrostatic pressure plus water surface tension leads water into small conduit holes. In other words, water travels to the lowest pressure areas, which would be pierced conduits.
Say you have a coaxial instrument cable in your underground conduit. If water enters your conduit, it’ll then seep in the very tiny pinholes in the cable jacket. The cable shield will then saturate causing corrosion. In return, the resistance of the shielded cable increases. This then increases the total cable losses and you’ll have a poor signal.
What’s more, if water gets into your coaxial cable, it’ll soak the conductor’s insulator. This will further increase the cable losses, and now you can lose your entire signal.
Important Note: corrosion has a high electrical resistance. This leads to a voltage drop and a drop in current. I go over a voltage drop calculation showing the effects of wire resistance on current.
If you have power conductors inside your conduit, the electrolysis effect is much faster. This is because of the increased current. You can combat such damage though by using cathodic protection.
2) Compact and level soil
First, make your trench sides as nearly vertical as possible. You don’t want sloping sides. But more importantly, make sure the bottom of your trench is compact and level. You want uniform bearing and support for each conduit section you install.
Because differing soil elevations can add stress to a conduit when you backfill the trench. The result is then conduit damage, and you’ll have the water penetration issues we discussed.
Important Note: the bottom of a trench can be soft or rocky. In such cases, remove the unsuitable material to a depth of at least 6-inches. Then replace with crushed rock, sand, or gravel as appropriate.
3) Dewatering
Keep your excavation zone free from water during construction. You want the static water level drawn down to a minimum of 2 feet below the bottom of excavations. This will maintain the undisturbed state of natural soils. Also, it’ll allow for the proper placement of any stable fill.
For more information on dewatering, read here.
Important Note: control groundwater to prevent softening of the bottom of excavations. At the same time, be sure dewatering systems don’t remove natural soils.
#2 Freezing locations
In very cold regions, install your conduit below the frost depth. This will help prevent the freezing of any water, which enters your conduit. Also, this prevents conduit damage from thermal expansion and contraction.
If your frost depth is 8-feet below grade though, the installation may not be practical. In these instances, first, check with your local jurisdiction in charge. If you live in a very cold region, I’m sure they’ll have guidelines for best installation practices. But if you don’t receive helpful feedback, all hope isn’t lost. There’s a great hack I like to use.
Install a layer of rigid insulation above your conduit in the area you’ve excavated. This will help prevent the frost from penetrating any deeper into the soil. For example, use 1-inch rigid insulation. This insulation is equal to roughly one foot of regular soil depth. So if your frost line is 5-feet below grade, install the 1-inch thick insulation at the 4-foot mark.
Next, a segment of your conduit will eventually rise above grade through the frost. And this conduit segment will be prone to damage. This is why you find the most damage in the riser section of installations. So add expansion fittings on the vertical riser portions, to allow for settling.
Important Note: temperature swings typically are much less underground. You may have a 10°F swing underground versus 100°F above ground. Also, the temperature swings underground are much slower.
Extreme temperature swings cause conduit damage from expansion and contraction. This is one reason to add expansion fittings on vertical risers, to allow for settling.
#3 Conduit spacers
When you have two or more conduits, install molded plastic spacers every six feet. Also, separate the conduits by 3 inches. This helps prevent the sagging and deforming of conduits.
Plus, for power conduits, you’ll maintain the current carrying capacity of your conductors. Refer to NEC section 310.15(B)(2)(a).
What’s more, you’ll maintain a constant impedance across your wires and avoid hot spots. Hot spots will over time break down your wire’s insulation. This can then cause ground fault issues.
Finally, use spacers if you’re vertically stacking your conduits while using flowable fill. Flowable fill is a self-compacting, cement-type material used instead of compact fill. The spacers allow you to get the compaction you need around your conduits. And, you’ll prevent your conduits from floating during concreting.
#4 Backfill material
Proper backfill is critical to conduit installation work. Because in your native excavated soil, you’ll typically find small and large rocks. As we discussed, you don’t want any damage inflicting material around your conduit.
To better explain, we’ll go over the three backfill material layers.
Bedding material
This is the material used to refill a trench from the trench bottom to your conduit. You want to use granular material free from organic matter. The material should have a sand equivalent value of not less than 20. And the material should conform to the following table:
| U.S. standard sieve size | Percent by weight passing |
|---|---|
| 3/4 inch | 100 |
| No. 4 | 35 - 100 |
| No. 30 | 20 - 100 |
| No. 40 | 20 - 100 |
Also, the coefficient of uniformity should be 3 or greater. So clean natural sand or gravel is usable. But trench excavation is usable too, or a mixture thereof, with a sieve no greater than 200.
Important Note: don’t use crushed concrete or reclaimed asphalt. Use material consisting of either sand-sized particles or sand-sized particles mixed with gravel, crushed gravel, or crushed stone.
Intermediate backfill
This is the backfill from the conduit to 12 inches above the conduit. This material is the same as the bedding material.
Backfill material
This is the material used to refill the remainder of the trench. The maximum size of stone in this backfill shouldn’t exceed 3 inches. Also, the backfill material should meet the following requirements:
- Non-expansive in nature
- Have a plasticity index of less than 8
- A liquid limit less than 30
- An organic content less than 3.5 percent
If the material excavated from the site meets these requirements, you can use it as backfill material.
Important Note: dumping dirt over conduits while using spacers can cause damage. Because the poured dirt will create air gaps. Then the entire weight of your fill and the above traffic will fall on your conduits.
This is why you use fine granular material around conduits in trenches. Also, you can use concrete as it’s fluid, and it’ll flow around conduits filling almost all air gaps. Then tamping the poured concrete will fill any remaining air gaps.
#5 Backfill compaction
After you backfill the soil into your trench, you’ll need to compact the soil. This makes the soil denser and thus reduces air gaps and the rate of water infiltration. Use one of the following mechanical methods to compact the loose backfill material:
- Compactor
- Excavator
- Jumping jack type compactor
You want to compact to the minimum required percentage of maximum dry density. This is determined by the Proctor test ASTM D698 Method A. Also, the backfilling process happens in the following layered steps:
- Backfill in 4 to 6-inch layers
- Compact with the appropriate compactor (e.g. 1,200-pound compactor)
- Add water to help with the compaction
Important Note: the backfill needs to be mechanically compacted. While the level of compaction needs to match the density of surrounding undisturbed soil. Also, mechanical compaction shouldn’t be within twelve inches of the conduits.
#6 Conduit installation depth
Conduit installation depth to meet the following guidelines:
- The top of conduits will be a minimum of 24 inches below finish grade for circuits 600 volts and below
- The top of conduits will be a minimum of 36 inches below finish grade for circuits above 600 volts
This will protect the public from exposed conduits. But also, it’ll prevent damage to the conduits themselves from routine shoveling. Now, of course, you can install deeper. But if it’s not required, don’t go overly deep. Because the deeper you go, the harder it becomes to later find the conduits.
Refer to NEC Table 300.5 for the minimum coverage requirements for buried conduits. You’ll find many other underground conduit installation case scenarios listed too.
Important Note: typically the worst corrosion area is near the soil surface. More specifically, the first few inches below grade where a conduit turns up. This is the point where a conduit is in contact with a corrosive agent in the soil. But also, it’s exposed to oxygen.
This is why the segment of a pipe entering the ground shows much greater corrosion than other areas. Because the lower you travel underground, the soil generally becomes more tightly packed. This keeps air away from a pipe.
As an added tip, wrapping pipe with plastic tape in this rising segment provides corrosion protection.
#7 Concrete encasement
Encase conduit in concrete in the following scenarios:
- Great chance for soil dig-in
- Likelihood of soil caving in
- Presence of vehicular traffic
For added information, check out the below table. It’s a general overview of how to approach different installations, including concrete encasement. Keep in mind though, much of the table is subjective but best practice.
| Direct burial rated cable | Direct buried non-metallic conduit (PVC) | Concrete encased rigid galvanized steel conduit | Concrete encased non-metallic conduit (PVC) | Concrete encased cable | |
|---|---|---|---|---|---|
| No vehicle traffic | Bad design, but allowable per NEC | Acceptable design & allowable per NEC | Acceptable design allowable by NEC, but not NEC requirement | Acceptable design allowable by NEC, but not NEC requirement | Bad design & not allowable per NEC |
| Regular city vehicle traffic | Bad design, but allowable per NEC | Acceptable design & allowable per NEC | Acceptable design allowable by NEC, but not NEC requirement | Acceptable design allowable by NEC, but not NEC requirement | Bad design & not allowable per NEC |
| Heavy oversized equipment traffic | Bad design, but allowable per NEC | Not recommended design, but allowable per NEC | Highly recommended design allowable by NEC, but not NEC requirement | Highly recommended design allowable by NEC, but not NEC requirement | Bad design & not allowable per NEC |
Also, conduit in concrete in contact with earth or water needs adequate separation. Maintain at least 4 inches of concrete separation from the earth or water to the conduit(s).
The concrete is to be three-sack, ‘lean mix.’ Also, it should contain 10-pounds of red oxide pigment per cubic yard.
Important Note: provide reinforcing steel in concrete encased conduit runs of three or more conduits. Reinforcing steel to be #4 bar, continuous, run linearly with the conduits. Place reinforcing steel at all four corners with three inches of concrete cover.
Additionally, install reinforcing steel as required to assure a maximum spacing of 12 inches, on center, between adjacent reinforcing steel lengths. And place number 4 bar circumferentially around the duct cross-section. Maintain three inches of concrete cover, at intervals not exceeding 24 inches along the length of your duct run.
To point out, concrete is somewhat alkaline, and it’s a good chemical buffer too. So, it won’t cause much damage to galvanized steel, but the zinc coating of a steel pipe may not look too good. What you do need to realize is, concrete is porous like a sponge. If it’s overly wetted with a compound bad for steel, the concrete will over time damage your pipe.
#8 Additional underground conduit installation guidelines
The following are more best practice rules to follow:
A) Slope all outdoor underground conduits uniformly away from buildings and underground structures, toward pull boxes or manholes.
B) Avoid and/or prevent damage to existing underground piping, conduits, and cables.
C) Some installations require you to remove existing pavement or concrete slabs. At the end of your work, repair them as required.
D) Take the proper precautions to prevent dirt, concrete, or trash from entering conduits. This includes entering inside fittings and boxes.
E) Install a pull rope in each empty conduit run. The rope needs to be the polypropylene type at least 3/16 inch in diameter.
F) Identify all conduits with conduit identification plates at all terminations, enclosures, and wall and floor penetrations.
G) Provide the minimum separation for instrumentation conduits from power conduits as follows:
| Voltage | Separation (inches) |
|---|---|
| 120 Volts | 12 |
| 240 Volts | 24 |
| 480 Volts | 36 |
| Above 600 Volts | 72 |
H) Place caution tape at least 12-inches above underground installations, not concrete encased. Refer to NEC section 300.5(D)(3).
The tape will typically read:
“CAUTION BURIED ELECTRIC LINE BELOW”
Important Note: for buried conduits, use special warning tape. Use polyethylene plastic and metallic core or metallic-faced, acid-and alkali-resistant tape. The tape should also meet the following specs:
- Minimum width of 3 inches
- Warning tape placed on a roll
- Color-coded for the intended utility
- Warning and identification imprinted in bold black letters continuously over the entire tape length
- Color and printing are to be permanent, unaffected by moisture or soil
Underground conduit installation wrap up
With underground conduit installation, there’s much more than meets the eye. Just always remember to follow any local rules and guidelines. And if you ever get stuck, even as a professional, always ask for help. You should never do something you’re uncomfortable with.
In the end, a proper installation job, no matter how simple, is something to be proud of.
What other best practice rules do you use in underground conduit installation? What do you find to be the biggest challenge with an underground conduit installation?
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Koosha started Engineer Calcs in 2020 to help people better understand the engineering and construction industry, and to discuss various science and engineering-related topics to make people think. He has been working in the engineering and tech industry in California for over 15 years now and is a licensed professional electrical engineer, and also has various entrepreneurial pursuits.
Koosha has an extensive background in the design and specification of electrical systems with areas of expertise including power generation, transmission, distribution, instrumentation and controls, and water distribution and pumping as well as alternative energy (wind, solar, geothermal, and storage).
Koosha is most interested in engineering innovations, the cosmos, our history and future, sports, and fitness.
This useful information provides clarity to what’s needed, regarding a section of UF cable that exits & enters the earth, unprotected by conduit. The cable provides 120 volts to a condo buildings exterior lighting in a common area & becomes energized between dusk & dawn, via a photocell control. The power cable is surrounded by combustible pine straw & is against a wood sided, occupied, multi-unit residential structure. Hopefully code enforcement will see the potential risk of this.
Great point. The designer needs to make a prudent and pragmatic design decision, as the code is not black and white (e.g. stating not permitted where subject to physical damage). In most instances, I find it to be a liability concern given the potential risks involved.
Anyone familiar with conduit relining as used in plumbing being used in electric conduit? Is this NEC approved?
If you’re referencing to sandblasting followed by epoxy, you can potentially alter the conduit’s cross-sectional area and in return affect the fill. Also, you can create sharp parts in the conduit.
Instead, use a camera on a snake to check for damage/obstructions. Then use a variable pressure water jet or dry air, to remove any possible mud/dirt (assuming this is the problem).