How to Splice Wire (Code-Legal Methods)
The entire code on splicing fits in one sentence: every splice lives in a box, and the box stays accessible. Everything else is technique.
Every splice in a box, every box accessible
The NEC says a box goes at every point where conductors are spliced (NEC 300.15), and every box has to stay accessible without tearing the building apart (NEC 314.29). That's the whole framework. A splice done right is a non-event for 50 years. A splice done wrong — twisted together, taped, and drywalled over — is one of the most common things inspectors flag and one of the most common causes of electrical fires.
Wire nuts vs. lever connectors
Twist-on wire nuts
The classic. Strip about 3/4 inch of insulation, hold the stripped ends side by side (for solid wire, many electricians pre-twist with lineman's pliers; the nut itself does the twisting on stranded), and screw the nut on until it's tight and no bare copper shows below the skirt. Size the nut to the wires — the package lists how many conductors of what gauge each color fits. An oversized nut on two 14s won't bite; an undersized nut on four 12s won't hold.
Lever connectors (Wago-style)
Strip to the marked length, push the conductor fully into the port, flip the lever down. The internal spring clamp holds with constant pressure — no twisting skill required, and they handle mixed solid/stranded combinations better than wire nuts. They cost more per connection, but on a big job the speed pays for itself. Use listed connectors only, installed per the manufacturer's instructions.
Both methods are equally legal when done right. The connector choice matters less than the box: a perfect splice outside a box still fails inspection, and a mediocre splice in a box can be opened and fixed.
Why buried splices fail inspection
Every connection is a point of resistance, and resistance makes heat. In an accessible box, a failing splice shows symptoms — a warm cover plate, a tripped breaker — and someone can open the box and fix it. Buried inside a wall, a failing splice has no symptoms until it has a fire.
That's why inspectors treat hidden splices as automatic failures: a joint twisted and taped inside a wall cavity, a splice buried behind drywall with no box, a "temporary" extension that became permanent five years ago. If you're opening walls and finding these, cut them out and put proper boxes in. A blank cover plate on the wall is a small price for a splice you can actually reach.
Box fill matters too: a box can only hold so many conductors (NEC 314.16 gives every box a conductor count based on its volume). Cramming ten splices into a box sized for four overheats the same way a buried splice does. When in doubt, go one box size up — boxes are cheap, rework isn't.
Splicing is a small skill; knowing what the wire itself is made of and what it's worth is the bigger one. The Wire Guide covers it all — jacket markings, gauges, and old wire types.
Rewired and sitting on wire you no longer need? We buy usable leftover rolls — anywhere in the country. Fast, free quote.
Splicing questions, answered.
Can you splice wire without a junction box?
Not legally, in general. The NEC requires a box at every splice point (300.15), and the box must remain accessible (314.29). There are a few listed in-wall splice devices for specific repair situations, but they're the exception — the rule is box and access.
Are lever connectors better than wire nuts?
They're faster, more forgiving of mixed solid/stranded combinations, and don't depend on twisting technique. Wire nuts are cheaper and have a century of track record. Both are code-legal when listed and installed per instructions — pick whichever you install correctly.
Can you splice Romex inside a wall without a box?
No. A Romex splice must be in an accessible junction box. The old practice of twisting conductors together, taping them, and burying them in the wall fails inspection and is a genuine fire hazard.
How many wires can you put in a junction box?
It depends on the box's volume — NEC 314.16 assigns each box a maximum conductor count (a standard 4-inch square box, 1-1/2 inches deep, holds nine 14 AWG conductors, for example). Count every conductor, the clamps, and the device. When in doubt, use a bigger box.
Do you have to pre-twist wires before a wire nut?
For solid conductors, most electricians pre-twist with pliers for a mechanically sound joint before the nut goes on — manufacturers' instructions vary, so follow the one on your package. For stranded wire, the nut's spring does the work; pre-twisting stranded can actually make a worse joint.
Can you splice different gauge wires together?
You can physically, with a connector rated for both sizes — but the circuit's breaker must protect the smallest conductor in it. Splicing 14 AWG onto a 20-amp 12 AWG circuit creates a 14 AWG weak point; the breaker has to drop to 15 amps.
What do you do with the ground wires in a splice?
Join all the equipment grounding conductors together with a wire nut (or crimp) and add a pigtail bonding the box if it's metal. Grounds never get left unconnected — an ungrounded box is a shock hazard waiting for a fault.
I know wire. That's how I know what it's worth.
Reading a jacket is the first thing we do when a roll comes in — the letters tell us the type, the gauge, the rating, and whether it's resale inventory or scrap. That's also how we know when usable wire is worth more than the yard pays for it.