Underground Wiring: A Pro Answers Your Key Questions
Here are some important things you need to know about underground wiring so it's safe and provides many years of service.
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As a master electrician and retired chief electrical inspector, I’ve seen my share of underground wiring over the years. Most projects were successful and are still in operation today, something that still brings a smile and makes me proud. But there were a few painful lessons along the way.
Common sense is always a good guide when installing underground wiring. However, the National Electrical Code (NEC) contains some less obvious yet important rules that ensure underground wiring is installed safely and provides years of service.
Here are the answers to the frequently asked question about underground wiring.
What Are The Most Common Wiring Methods?
- The NEC contains close to 50 wiring methods.
- Wiring methods have descriptive names, but they are often referred to by acronyms.
- It’s helpful to be familiar with wiring method acronyms because they’re used widely in home center advertising, on store shelves, in product literature and embossed on the side of conduit, cables and wire.
- The common underground wiring methods are as follows:
- Rigid metal conduit (RMC);
- Rigid polyvinyl chloride conduit (PVC).
- Underground feeder and branch-circuit cable (UF-B).
How Does the NEC Define Burial Depth?
- It’s the amount of cover on top of wiring. Underground wiring must meet certain minimum cover requirements.
- When we talk about burial depths of six-, 12-, 18- or 24-inches, we’re not referring to the depth of the trench, but the depth from finished grade to the top of the buried wiring.
- So if you’re required to have 18 inches of cover over your wiring, your trench will be slightly deeper than 18 inches to account for the cable or conduit.
Are There Options for Reducing the Burial Depth?
- Yes. Underground wiring can be encased in concrete to reduce the burial depth.
- Where conduits are encased in concrete, the concrete envelope around the buried wiring must be at least two inches thick.
- The installation of metal or plastic conduits can also reduce the minimum cover required.
- Type UF cable normally requires 24 inches of cover. But if it’s sleeved with RMC, it only needs six inches of cover, the minimum for RMC.
Can Electrical Metallic Tubing Be Used Underground?
- No. Don’t be tempted to save money and install electrical metallic tubing (EMT), often called “thin-wall” conduit.
- Unless completely coated with an approved corrosion protection material, Type EMT metal tubing is not approved for direct-burial installations and can’t come in contact with the earth.
- Applying some sort of corrosion protection to the conduit and fittings would be a waste of time and money.
- Buried EMT will corrode and disintegrate in no time, creating a serious safety hazard.
Are Underground Electrical Conduits Watertight?
- No. You might think the interior of underground conduits would be watertight and dry. But they’re wet locations, so wires and cables installed in underground conduits must be approved for that.
- Where a conduit transitions between different temperatures — say, from outside a building to inside — the ends of the conduit must be sealed to prevent the free-flow of moist air and condensation buildup inside the conduit.
- Conduit body fittings with removable covers installed at service-entrance and similar locations must be sealed. They also need a drain hole to allow any unwanted, accumulated water or condensation to escape.
- Duct seal putty works great for sealing conduits and conduit bodies because it’s pliable, never hardens and can be removed if necessary.
How Can Underground Wiring Be Protected From Damage?
- Underground cables and wires, once they emerge, must be protected by conduit to a point eight feet above grade or where they enter the building. The conduit must extend at least 18 inches below the finished grade.
- Plastic conduit for physical protection must be heavy-duty Schedule 80 PVC conduit, not the standard Schedule 40 version. Heavy-duty Schedule 80 PVC can better withstand wayward lawnmowers and weed whip tools.
- Standard Schedule 40 PVC conduit may be installed in the horizontal portions of the trench.
- Underground cables and wires are not permitted under a building unless they’re installed in conduit.
What Precautions Are Needed When Backfilling?
- Where a conduit extends underground to protect a cable, the end of the conduit must have a bushing, terminal fitting, adapter or similar fitting. That fitting must have a smooth, rounded surface to protect cables and wires from sharp conduit edges.
- Dirt backfill containing large rocks or other materials that could damage underground wiring can’t be placed over the wiring.
- It may be necessary to install sand or pea gravel or other methods to protect the underground wiring.
- If underground conduits, cables and wires are subject to soil movement, like settlement or frost heave, damage to the wiring must be prevented. Conduit expansion or slip fittings, extra slack in cables and wires and “S” loops in cables or wires will prevent stress, strain and damage and allow for some movement.
Is There a Way To Warn Others of Buried Wiring?
- Warning ribbon tape is only required in trenches for service wires that extend from the electric meter at a utility pole or pedestal to the exterior of your home. It’s cheap insurance to place warning ribbon tape in every trench to avoid potential electrocution or arc-flash accidents.
- The warning ribbon tape must be installed in the trench at least 12 inches above the underground wiring. Then you finish backfilling the trench.
- Warning ribbon tape is only $12 for a 300-foot roll, an inexpensive way to avoid a dangerous situation.
What Else Should I Know About Underground Wiring?
- Splicing direct-buried cables and wires should be avoided if possible. If not, they need to be spliced with a method and means approved for underground use.
- Underground metal conduits are always required to be grounded to metal boxes, or green or bare grounding wires.
- However, short lengths of metal conduits that only provide support or physical protection for cable assemblies are not required to be grounded.
- All underground cables and wires of the same branch circuit must be placed in close proximity in the same trench to ensure proper electrical circuit performance.