Is it OK to splice underground wire?

Splicing underground wire is generally considered acceptable in certain circumstances. There are a few key factors to consider when determining if splicing underground wire is the right choice for your project.

What is splicing?

Splicing refers to joining two wires together, often using some type of connector or splice kit. This allows you to extend the length of a wire run or repair a damaged section of wire without having to replace the entire run.

When is splicing underground wire allowed?

Most electrical codes allow splicing underground wire in certain situations, such as:

• Extending an existing circuit
• Repairing damaged underground wire
• Connecting an underground wire to an above-ground wire
• Tapping into an existing underground circuit to add an outlet

However, there are restrictions on where and how splices can be made:

• Splices must be made in approved enclosures like junction boxes or splice kits. Burying bare splices is prohibited.
• Only power wires, not service entrance wires, can be spliced underground.
• For power circuits, only splice as many wires as will fit into the approved splice enclosure.
• Splices should be avoided in wet locations when possible.

So in summary, splicing is allowed for extending circuits, making repairs, adding taps, etc., as long as proper enclosures are used.

Is it better to splice or replace the full run?

As a general rule, it is better to replace the entire wire run rather than splicing if possible. Splicing underground wires introduces potential failure points in the circuit. However, there are cases where splicing makes more sense than replacing the full run:

• If only a small section of wire is damaged, splicing can repair it without excavating the entire run.
• For extending an existing circuit, splicing avoids the major work of installing a completely new wire run.
• In conduit, pulling new wire may be impossible without major conduit modifications.

So if the choice is between splicing a short damaged section or replacing hundreds of feet of wire, splicing may be the better option. But avoid excessive splicing over long wire runs if possible.

Is there a limit to how many splices are allowed?

Most electrical codes do not specify a maximum number of splices allowed in an underground wire run. However, there are recommendations for limiting splices to avoid reliability issues:

• NEC recommends no more than 4 splices for any power circuit up to 400A.
• No more than 6 splices are recommended for feeders over 400A.
• For service entrance wires, only 1 splice is recommended and should be avoided if possible.

The key is to use prudence – splicing in moderation as needed for repairs and short extensions helps balance cost and reliability. But avoid splicing excessively over very long runs.

What are the approved methods for splicing?

Any splices in underground wiring must use approved connectors and enclosures. Common options include:

• Wire nuts – Approved for copper wire splices but wire nut twist-ons alone do not provide a watertight seal.
• Crimp connectors – Must be moisture-resistant crimp connectors suitable for direct burial.
• Compression splices – Mechanical splice joins wires together and provides watertight seal.
• Resin splices – Uses special resin that is injected around the bare wire splice for insulation and moisture protection.
• Junction boxes – splice is made inside waterproof enclosure buried underground.

Burying bare pigtail splices without an approved enclosure is not permitted. The right type of connector and enclosure ensures a solid, well-insulated, watertight underground splice.

Can you splice wires of different sizes?

Yes, it is possible to splice copper wires of different sizes together underground as long as proper connectors are used to join the varying wire gauges. There are a few ways to approach splicing different wire sizes:

• Use crimp connectors or compression splices rated for the smallest wire size.
• Build up the smaller wire to match the larger size using insulating tape.
• Use a junction box with terminal blocks to transition between wire sizes.

The key limitations are that the splice must fully insulate and seal both wires, and the larger wire should not exceed the ampacity of the smaller gauge. With the right connectors and technique, different size splices can meet code.

What size wire is needed for an underground splice?

For power circuits, the wire used for an underground splice should always match the size of the existing circuit wires. For example:

• Splicing #12 AWG wires – use #12 AWG for splices
• Splicing #6 AWG wires – use #6 AWG for splices
• Splicing #2 AWG wires – use #2 AWG for splices

The only exception is if you are extending the circuit and transitioning to a larger wire size for the new portion of the run. The splice connecting old and new would use the appropriate transition connectors.

Can you use wire nuts for underground splices?

Wire nuts alone are generally not suitable for underground wire splices, since the twisted connection does not provide a watertight seal. However, you can use wire nuts for underground splicing if they are combined with other approved products to fully seal and insulate the connection:

• Wire nut + electrical tape – not recommended, tape can deteriorate over time
• Wire nut + heat shrink tubing – provides a better long-term seal
• Wire nut + waterproof splicing kit – encapsulates connection in epoxy
• Wire nut + junction box – wire nut connects wires inside waterproof box

So wire nuts are fine for the actual copper-to-copper splice connection as long as the overall splice is sealed for direct burial using approved methods.

Is there a recommended depth for underground splices?

Yes, the National Electrical Code (NEC) specifies required burial depths for electrical wiring and splices:

Location	Minimum Cover Depth
Below ground level	24 inches (610 mm)
Under concrete slab	4 inches (102 mm)
Under dwelling unit	6 inches (152 mm)

So according to NEC 300.5, any underground splices must be buried at least 24 inches below grade. More depth provides even better protection.

How can you locate or access underground wire splices in the future?

Finding and accessing underground splice points after backfilling can be challenging. Here are some tips to allow locating splices in the future:

• Ensure junction box covers are flush with grade for easy access
• Mark splice locations on the ground with spray paint during backfill
• Create an as-built drawing noting GPS coordinates of splices
• Attach a wire coil above the splice box to allow metal detection
• Use wire tracer tools that can detect splice signals along the wire path

Proper planning and documentation of underground splice locations can save a lot of headaches down the road if repairs are ever needed.

Is it better to use splice kits or junction boxes?

Both splice kits and junction boxes can make compliant underground splices. Here’s how they compare:

Splice Kits	Junction Boxes
Self-contained epoxy encapsulates splice	Box provides enclosed space for splice
Used for in-line splices	Allows access to splice in future
Limited to number of conductors	Accommodates more wires
Direct burial	Must be buried flush with grade

For most in-line splices, splice kits provide a simpler watertight sealing solution. But junction boxes allow access and more splicing capacity in one location.

Should you use splice kits in wet areas?

Direct-burial splice kits can be used in damp areas, but are not suitable for continually wet underground locations. The epoxy seal provides water resistance but is not designed for submersion. For routinely wet areas like irrigation boxes:

• Use extra waterproof sealing putty with splice kits.
• Opt for a junction box where splice is protected inside.
• Consider running wires in wet-rated conduit rather than direct burial.

In general, minimize splicing in areas prone to standing water or moisture saturation. But if needed, take extra precautions to waterproof connections.

Do you need a ground wire for underground splices?

Yes – any splice made in a grounding or bonding conductor underground requires those grounding connections to be maintained:

• Use splice method suitable for grounding wire (usually copper)
• Connect ground wire pigtails to continue ground path
• Maintain grounding integrity with proper connectors

This ensures any ground fault current can still safely pass among spliced sections. Do not interrupt the grounding path during splicing or rely solely on conduit for grounding.

Can you use underground rated wire above ground?

Underground feeder (UF) cable and underground service entrance (USE) cable can be run above ground in certain circumstances:

• UF cable may be run above ground up to 10 feet to transition from underground.
• USE cable may be run above ground up to 10 feet on exterior of structures.
• Cables must be protected from physical damage when run overhead.

However, direct burial cables cannot substitute for standard NM, MC, or AC cable used for permanent above ground wiring. The insulation is not designed for long exposure to sunlight and weather.

Is there an easy way to splice large gauge wires?

Splicing large gauge wires like 4/0 or 250 kcmil can be challenging due to their thickness and inflexibility. Some tips for easier large wire splices:

• Use leverage splicing tools to push stiff wires together.
• Select splice types allowing wires to be laid in straight rather than twisted.
• Use junction boxes with terminal blocks instead of in-line connectors.
• Reduce wire radius by scoring insulation slightly back from the cut end.
• Lengthen junction boxes to provide more working space.

Taking the time to use the right techniques and tools for the wire size will produce better, more reliable large gauge splices.

Should you use shielded cable for underground splices?

Shielded power cable provides noise resistance and data security for sensitive circuits. But it has limited relevance for most standard household electrical splices:

• Added shielding not needed for basic 120V circuits.
• Difficult to effectively bond shielding when splicing.
• Shielded cable not required by NEC for underground circuits.
• Added cost of shielded cable harder to justify for basic power.

However, for circuits feeding sensitive data/communications systems, shielded cable and proper shielding practices provide better signal and EMI protection.

Do you need to waterproof underground Romex splices?

NM (Romex) cable is designed for indoor, dry locations and should not be used for underground circuits. But if NM cable is already buried, any splices require suitable waterproofing:

• Use junction box with gasketed cover.
• Fill junction box with silicone sealant after splicing.
• Encapsulate splice in epoxy underground splice kit.
• Cover splice in electrical tape AND self-fusing rubber tape.

These methods compensate for the lack of water-resistant sheathing on NM cable. But avoid burying NM cable for new underground wiring.

Do splices affect nearby CATV or network cabling?

Electrical interference from splices is very unlikely to disrupt CATV, antenna, or low-voltage network wiring, assuming proper installation:

• Use shielded coax for CATV to minimize RF interference risk.
• Keep low-voltage cables well separated from power splices.
• Avoid routing LV and line voltage wires in same conduit.
• Use surge suppressors to improve connected devices’ noise tolerance.

Follow basic RF separation best practices and you can safely install power splices near other cabling with minimal interference concerns.

Conclusion

Splicing underground wires is permitted by code in many situations with certain guidelines:

• Use only approved, watertight splice methods for underground connections.
• Limit splices to repair damage, add taps, and extend short distances rather than splicing excessively.
• Bury underground splices at the proper depth and mark locations for future access.
• Match wire size and maintain equipment grounding when splicing.
• For wet areas, junction boxes or extra waterproofing precautions are preferred.

By following basic precautions, underground splices can provide reliable electrical connections and avoid the major work of re-pulling full wire lengths. Take the time to make splices properly, and they can perform well underground for years to come.