Is it Legal to Splice Romex Wire in a Wall? The Complete Guide

Hey there! I‘m Lillie, a home renovation expert here to provide you with a comprehensive guide on splicing Romex wiring in walls. Whether you‘re a DIYer taking on electrical projects or a professional electrician, splicing Romex properly and safely is crucial.

In this guide, I‘ll cover everything you need to know about splicing NM cable based on expertise from my years of experience and knowledge of the National Electrical Code. My goal is to ensure you fully understand the legality, methods, materials, and best practices when it comes to this common task.

Is Splicing Romex in Walls Up to Code?

This is one of the most common questions I get asked. The short answer is yes, you can legally splice Romex wiring in walls as long as you follow the requirements explained in NEC articles 300, 334, and 358.

The key things the National Electrical Code says about splicing NM cables in walls:

• All connections must be made inside an approved electrical box, conduit body, or other enclosure. Junction boxes specifically designed for NM cable are required.

• The box must be securely mounted to framing or other structural members and provide protection against physical damage.

• You must use approved splicing devices like wire nuts or crimp connectors. Simply twisting bare wires together is prohibited.

• All conductors must be suitably insulated and protected from abrasion within the box.

• Only certain box fill thresholds are permitted based on box size and wire gauge.

Trying to splice wires loosely behind drywall or other permanent construction is extremely hazardous and 100% against code. All connections absolutely must be contained in proper electrical boxes.

Data Point: According to the Consumer Product Safety Commission, over 40,000 home fires each year are caused by electrical failures or malfunctions. Proper splicing is essential to safety.

As long as you follow NEC guidelines for boxes, fill limits, secure splices, and insulation methods, splicing Romex in walls is perfectly legal and up to code. But specific requirements must be met, which I‘ll cover next.

Step-by-Step Guide to Splicing Romex in Walls

Splicing Romex or NM wiring in walls is done by electricians and homeowners every day for extending circuits, adding outlets, installing lighting, and many other purposes. Here is a step-by-step guide to do it properly:

1. De-energize the Circuit

Any time you work on wiring, the very first step is to turn off power to the circuit you‘re splicing into. Locate the right breaker in the panelboard and switch it to OFF.

Pro Tip: Shutting off the wrong breaker can lead to accidental shocks. Label your breakers clearly.

Make sure to double check wires are de-energized using a non-contact voltage tester before touching anything. Trying to splice live wires can result in deadly shocks. Safety first!

2. Select a Suitable Junction Box

Choosing the correct electrical box is important for proper cable support and containment of splices. The NEC requires boxes to be made from durable material able to withstand heat and physical damage. Common options include:

• Metal or PVC rectangular boxes
• Round boxes – 4′′ or 4-11/16′′ sizes
• Lighting fixture boxes with built-in clamps
• Handy boxes for flexibility

Make sure your box is clearly listed and labeled as NM approved. Also pay close attention to box fill calculations covered in the next section.

3. Calculate Box Fill

The National Electrical Code limits how many wires and connectors can be inside a junction box, known as box fill:

• For boxes up to 100 cubic inches = Max 50% fill
• Boxes between 100-200 cubic inches = Max 40% fill
• Over 200 cubic inches = Even lower fill %

Here‘s how to calculate fill for your splice box:

1. Look up cubic inch fill volume for each cable based on gauge (see NEC Chapter 9, Table 5)

2. Add up volumes of all splicing connectors

3. Add conductor volume + connector volume

4. Verify total fill does not exceed NEC threshold for box size

Proper box size ensures wires have room to dissipate heat. Keeping junction boxes from being overfilled is critical.

4. Mount the Box Securely

Install the box in the desired wall location, leaving at least 6" of free wire to make splices and connections. Secure the box to framing members according to manufacturer specifications.

Use proper fasteners like drywall anchors or directly screw into studs. This ensures the box remains fixed and fully supported.

5. Clamp Cables Properly

Clamping cables that enter the box provides important strain relief so splices don‘t pull loose.

For metal boxes, use approved NM cable clamps to affix the sheath. In plastic boxes, tighten down built-in cable clamps fully.

Leave no openings unfilled after wires are inserted.

6. Strip Wires Appropriately

Use a sharp cable ripper tool to remove the outer NM sheath about 3" back from the end. Strip the inner insulated conductors so approx. 1⁄2′′ of bare copper shows.

Take care not to nick the copper strands which could lead to breaks over time. Properly stripped wires make better splices.

7. Join Wires Securely

Use twist-on wire connectors (wire nuts) to join the bare copper ends together. Turn a small section of each wire clockwise before capping to better mesh strands.

Screw the wire nut down firmly onto the wires with a final 1/4 turn but take care not to overtighten. Inspect that no copper is exposed.

Optional: Cover each nut with insulating splice caps for additional protection.

8. Carefully Position Wires

With all splices complete, neatly fold wires into the box to take up minimal space. Maintain at least 1⁄4” spacing between wires and walls for reduced friction and heat dissipation.

No loose wire loops should stick out. Position grounds and neutrals together on one side, hot wires to the other.

9. Close It Up

Install a cable clamp into the last open box knockout and affix the cover plate. This seals up the junction box so all wiring is protected within the wall cavity.

Double check no bare conductor strands are exposed before closing it up.

And that covers the full process! The key is taking care with every step. Rushed or improper NM cable splices lead to issues down the road.

Where Can You Legally Splice Romex?

Assuming proper methods are used, common locations where it is generally permitted to splice Romex wiring include:

• Inside wall junction boxes to extend circuits or add outlets and switches
• Ceiling electrical boxes tying fixtures to wiring from below
• Inside conduit bodies as a code-approved wiring enclosure
• Inside panelboard cabinets to connect hot bus bars to breakers
• Split-wired receptacles using pigtail splices
• Surface mounted raceways like Legrand Wiremold which open to access the interior

Some locations where NEC prohibits splicing Romex:

• Open wall cavities or exposed areas without an approved box
• Most appliances where heat can degrade connections
• In contact with plumbing pipes or other systems that may damage wiring

The key is any splices made must always be:

• In an approved enclosed space like a junction box
• Properly fastened with secure cable clamps
• Contained neatly using tie-wraps or other organizers
• Made up with rated splicing devices like wire nuts

This ensures maximum safety by eliminating exposed live connections. Review your specific application if ever unsure about where NM cable splices are permitted.

Choosing the Right Wire Splice Connectors

There are a few different options when it comes to suitable devices used for splicing Romex style NM cables.

Twist-On Wire Nuts

Wire nuts are the go-to choice of most electricians for splicing Romex. To use properly:

• Select the correct size nut for number of conductors. Use nut sizing charts.
• Twist individual wires together before capping the nut.
• Tighten nut firmly onto wires with a final 1/4 turn but avoid over-torqueing.
• Optionally cover with splice caps for extra insulation.

Using the right size wire nuts for the gauge / quantity of wires is critical for proper electrical and mechanical connection.

Crimp Connectors

Insert wires into an enclosed metal sleeve and use a special tool to permanently crimp the connector onto the wires. Very secure mechanical connection.

Split Bolt Connectors

Split bolts have a tapered opening to insert wires into. Tightening the bolt clamps them together firmly. Useful for heavier gauge cables.

Terminal Blocks

Terminal blocks allow easy connecting and disconnecting of wires without re-stripping. Helpful for troubleshooting or swapping out wires down the road.

Solder Sleeves

Rarely used since improper heating can damage Romex. Appropriate for small 18-22 gauge wires in low voltage applications only.

No matter what type you use, always ensure splicing devices are UL listed and rated specifically for the size and type of NM cable you are working with.

Key Precautions When Splicing Romex

Splicing NM style cables comes with some important safety considerations:

• De-energize fully – Working on live circuits risks electrocution. Double check wires are dead using a voltmeter.

• Use proper boxes – Only use junction boxes approved for NM cable to contain splices. Verify correct volume capacity.

• Avoid overheating – Excessive heat damages insulation and causes hazards. Take care when stripping or joining conductors.

• Neat organization – Containing wires neatly makes maintenance easier and reduces abrasion issues.

• Tight connections – Improperly installed wire nuts or loose connectors lead to arcing, shorts, and fires. Verify tightness.

• Proper wire prep – Cleanly strip wires without nicking copper strands. This allows better splice connections.

• Insulate fully – No bare copper should be exposed. Check insulation with an ohmmeter if unsure.

Taking the time to splice carefully avoids safety hazards and future problems down the road.

FAQs about Romex Splices in Walls

Here are answers to some of the most common questions I get about splicing NM style cable properly:

Can I splice Romex without a junction box?

Absolutely not! The NEC requires all connections be made inside approved boxes. Uncontained splices pose a severe fire and shock risk.

What size junction box do I need?

Consult NEC fill thresholds. For 4-6 NM cables, a standard 4” square box is often suitable. Larger wires may require upsizing to a 4-11/16” box.

Where should I install boxes to splice Romex?

Typical spots include above ceilings, near existing outlets and switches, and in open stud bays to extend wiring runs. Group splices together when possible.

Can I use wire nuts outside a box?

No, wire nuts alone would leave a hazardous exposed splice. All connectors must be fully enclosed inside a junction box.

Is electrical tape enough to insulate splices?

Tape alone does not provide a secure electrical splice. Approved connectors like wire nuts must be used first before tape as supplemental protection.

Can I add new boxes in finished drywall?

Yes, you can install old work boxes to retrofit splices after drywall is finished. Carefully cut the correct opening size and patch.

How many wires can I put through one Romex knockout?

Typically no more than 4 NM cables through one knockout depending on gauge. Limiting knockouts prevents box crowding.

I hope these common questions help provide more insight into properly splicing NM cable in walls! Remember the key steps we covered:

• Turn off power
• Choose the right box
• Watch fill capacity
• Secure cables tightly
• Strip wires correctly
• Make tight splices
• Position wires neatly
• Close it up safely

Taking the time to follow best practices ensures you make safe, long-lasting Romex connections.

Final Thoughts

That wraps up my complete guide on splicing Romex or NM wiring in walls. To summarize the key points:

• Splicing NM cable is permitted by code as long as all requirements for junction boxes, cable entry, box fill, and splice protection are followed.

• Always turn off power first and splice inside suitable junction boxes using approved connectors like wire nuts.

• Mind the fill capacity of boxes based on cubic inch limits and watch wire prep, organization, and insulation.

• Common splice locations include walls, ceilings, panels, receptacles, and conduit bodies as long as enclosed properly.

I hope this gives you increased confidence when it comes time to make your own Romex splices. Just take it slow and double check your work. Proper splicing prevents electrical fires, shock hazards, and other dangerous situations.

And as always, don’t hesitate to reach out with any questions!