How-To: Splice Wire to NASA Standards

Some commenters on my post about using a washer as a soldering aid noticed my sloppy splicing technique and were kind enough to educate me about the so-called “Western Union splice,” aka the “Lineman’s splice,” which is the preferred method for twisting solid-core wire leads together for inline electrical connections.

Developed during the heyday of the telegraph, the Lineman’s splice is designed for connections that will be under tension. It is commonly claimed that, properly made, a Lineman’s splice is stronger than the wires of which it is composed. In any case, it is a time-proven method, and, coolest of all, one of NASA’s Required Workmanship Standards. To wit, in a NASA-approved Lineman’s splice:

1. The conductors shall be pre-tinned.
2. There shall be at least 3 turns around each conductor and the wraps shall be tight with no gaps between adjacent turns.
3. The wraps shall not overlap and the ends of the wrap shall be trimmed flush prior to soldering to prevent protruding ends.
4. Conductors shall not overlap the insulation of the other wire.

Though the Lineman’s splice was originally used without solder, today soldering is common. And NASA insists on it:

1. Solder shall wet all elements of the connection.
2. The solder shall fillet between connection elements over the complete periphery
   of the connection.

This material comes from page 84 of NASA-STD 8739.4 (PDF), which is a great reference if you’re interested in best practices for interconnecting cables and wires. [Thanks, Alex Barclay!]