rat-tail splice
A simple method of joining two ends of metal wire by twisting them together.
rat-tail splice: the twisted-wire join that shouldn't stay permanent
A rat-tail splice is formed by twisting the bared ends of two wires together without solder, flux, or crimping. The two conductors are stripped of insulation for roughly half an inch, overlapped, and then twisted around each other in a helical pattern until they grip mechanically. The result resembles a rat's tail, hence the name. This method has been used since the early days of electrical work, when it was sometimes acceptable for temporary connections or low-current circuits.
Despite its simplicity and zero tool requirement, the rat-tail splice has serious drawbacks that have made it obsolete in professional work. The mechanical grip degrades over time as vibration, thermal cycling, and corrosion weaken the twisted joint. The contact resistance between the wires is unpredictable and often high, causing localized heating under load. There is no strain relief; any pulling or bending of the wires can untwist the joint entirely. In damp environments, moisture migrates along the twisted strands and causes oxidation deep within the connection.
Where it persists and why it fails
Rat-tail splices appear most often in old installations that predate modern code, in emergency field repairs by untrained personnel, and occasionally in hobby electronics work. Building codes and electricians' standards have prohibited them for decades. The National Electrical Code treats them as unsafe for any permanent installation. When a rat-tail splice fails, it typically does so by gradual loosening rather than sudden breakage, making the failure mode insidious: the connection becomes intermittent, resistance rises, and heat generation increases until the insulation melts or a fire starts.
Modern practice replaces rat-tail splicing with soldered joints (which still require flux and cleanup), crimp connectors (faster and stronger), or wire nuts for low-voltage AC work in residential circuits. Each method maintains consistent contact resistance and provides mechanical strain relief. The term rat-tail splice survives in electrical vocabulary primarily as a cautionary reference, teaching point, and historical marker of pre-code practice.