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NASA wire splicing is governed primarily by NASA-STD-8739.4A, with Change 4, not by one universal splice recipe. The active standard recognizes several soldered and crimped splice configurations. Which one is acceptable depends on the approved design, materials, wire, environment, and project requirements. An unplanned splice is generally treated as a repair, so a neat-looking joint alone does not make it NASA-compliant or flight-ready.
This guide explains the NASA lap splice as a practical example, compares other recognized methods, and covers the inspection, insulation, testing, and approval controls that matter. It is for understanding the requirements—not a substitute for an approved work instruction or qualified training.
Which NASA document governs wire splicing?
The principal document is NASA-STD-8739.4A, Workmanship Standard for Crimping, Interconnecting Cables, Harnesses, and Wiring. NASA lists it as active; the base document is dated June 30, 2016, and Change 4 is dated April 13, 2022. Its scope concerns cable and harness assemblies used to connect electrical, electronic, or electromechanical components in critical work.
The standard is not a household wiring code, and its requirements do not automatically apply to every consumer, automotive, or construction repair. Actual NASA or aerospace work may also be controlled by engineering drawings, project work instructions, material specifications, NASA-STD-8739.6 implementation requirements, soldering requirements such as IPC J-STD-001, and procurement or subcontract terms. NASA-STD-8739.4A states that NASA-STD-8739.6 takes precedence if the documents conflict; its training section has been superseded by that document.
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Section 19 recognizes multiple configurations, including lap, lash, solder-sleeve, Western Union/Lineman, solder-ferrule, crimped, modified-crimp-contact, and crimp-ferrule splices. These methods are not interchangeable recipes. Select only a configuration permitted by the approved design and applicable process.
First decide whether the splice is authorized
There is an important distinction between a splice designed into a harness and one added later to fix a problem. Under NASA-STD-8739.4A §19.2.1, unless a splice is identified in manufacturing or engineering documentation, it is treated as a repair and must follow applicable NASA-STD-8739.6 requirements. Nonstandard configurations likewise require the applicable approval process.
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Before work begins, confirm the drawing or repair authorization, wire and insulation type, conductor size and plating, splice parts, environmental needs, inspection criteria, and test requirements. A visual match to an illustration cannot establish flight qualification. For mission or other controlled hardware, use qualified personnel and the project’s approved documentation and quality system.
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Choosing a recognized splice configuration
| Method | Key requirements and considerations |
|---|---|
| Lap splice | Two conductors lie parallel, with an overlap of 3–6 wire diameters. They are not twisted together. A solder fillet is required on both sides along the overlap, with no protruding strands; conductor contours remain discernible after soldering. |
| Lash splice | A lap splice additionally secured with solid lashing wire. Use at least 6 turns; turns must not overlap, and an open spiral can have no more than 2 lashing-wire diameters between turns. Trim ends flush and solder over the overlap and all turns. |
| Solder sleeve | A specified component—not just heat-shrink with solder. Center its solder ring over stripped conductors and place sealing rings over the wire insulation. Apply uniform heat in the manufacturer’s specified range; solder must fully wet the conductors, the ring outline must disappear, and seals must contact the insulation circumference. |
| Western Union/Lineman | When approved, conductors are pre-tinned and each makes at least 3 tight turns around the other, with no gaps or overlapping wraps. Trim ends flush; solder must wet every element and form a fillet around the full periphery. Solder quality must meet IPC J-STD-001FS requirements. |
| Solder ferrule | An end splice only. The ferrule fits over inserted, tinned wire but not insulation; protrusion cannot exceed one wire diameter of the largest wire. Solder must fill the ferrule and be visible at both ends. Secure wires against movement and apply heat away from insulation. |
| Crimped splice | Use a correctly sized contact or ferrule and the specified tooling and settings. For multiple wires, calculate combined circular-mil area and convert it to Equivalent Wire Size (EWS); select a part matching that EWS or the next larger EWS. |
NASA notes that solder-style splices can be smaller and lighter than crimp-style ones, but that does not make solder universally preferable. Solder brings heat exposure, possible solder wicking, and process-control demands; crimping avoids solder heat but depends on a correctly matched part, wire, and tool. A generic crimp connector or solder sleeve is not automatically compliant just because it fits.
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NASA-style lap splice: controlled workflow
The following summarizes the lap-splice requirements; the approved work instruction governs the actual strip length, materials, process settings, and acceptance criteria.
- Confirm authorization and materials. Verify the splice configuration, wire types, conductor sizes, plating, insulation rating, approved solder and flux, cleaning solvent, sleeve, and tooling against the drawing or repair procedure. De-energize the circuit and apply required ESD controls.
- Install the sleeve before joining. Slide the approved insulation sleeve onto one wire and move it well clear of the soldering heat. Forgetting it can force rework; do not improvise a lengthwise-cut wrap unless the approved process explicitly permits it.
- Strip only as specified. NASA requires precision mechanical or variable-temperature thermal stripping tools. The strip length depends on the splice design; there is no one universal length. Do not nick, gouge, ring, stretch, or remove conductor plating so base metal is exposed. Reject damaged wire rather than hiding the defect under solder or heat-shrink.
- Pre-tin the conductors. For the lap splice, pre-tin as required by §19.4.1. Apply solder so strands are bonded without an excessive bulge or a long rigid section. Pre-tinning is not permission to flood the wire with solder. For shield work, NASA distinguishes the drain wire, which is pre-tinned, from the shield itself, which is not.
- Position the conductors. Lay them parallel and in contact for an overlap of at least 3 and no more than 6 wire diameters. Do not twist them together. Neither conductor may overlap the other wire’s insulation; no loose or protruding strands are acceptable. Keep the joint stable during soldering.
- Solder the overlap. Heat the conductors sufficiently for solder to wet the connection, following the controlled process rather than relying on a large blob for mechanical strength. The finished splice needs a fillet on both sides along the full overlap, with no solder bridge, damaged insulation, or excessive wicking. The conductor contours must remain discernible.
- Inspect before sleeving when the design allows. Check overlap, wetting, both fillets, strand condition, insulation, and cleanliness. NASA requires soldered splice connections to be inspected before and after shrink-tube application when the piece-part design allows it.
- Clean the area to be covered. Before applying insulation sleeving, clean areas that will be covered with an approved solvent. Heat-shrinkable soldering splices are exempt from this particular cleaning requirement. Avoid spreading solvent contamination elsewhere in the harness.
- Recover the insulation sleeve. The insulation must completely encapsulate the splice body and extend over wire insulation by at least 2 times the diameter of the largest wire in the splice. If multiple layers are used, each added layer must overlap the underlying layer by at least 2 largest-wire diameters at each end. Follow material and heating instructions; do not scorch, cut, or lift the sleeve.
- Inspect after sleeving. Check full encapsulation, required sealing or bonding, smooth transitions, and absence of exposed metal, cuts, bubbles, scorching, lifting, or trapped contamination. A sleeve can conceal an underlying bad joint, so it is not proof of electrical or mechanical quality.
- Test and record the assembly. Apply the tests required by the approved acceptance procedure and preserve traceable results. Do not choose a test voltage or method from a generic guide.
Inspection and acceptance
NASA-STD-8739.4A calls for visual inspection with 4×–10× magnification and suitable lighting of at least 100 foot-candles (about 1,077 lux) at the assembly surface. An inspection should assess more than appearance:
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- Geometry: approved splice type, correct materials and sizes, proper overlap or wrap count, no insulation overlap, and no protruding strands.
- Solder quality: wetting of required elements, specified fillets, no cracks, voids, bridges, or obvious contamination, and no excessive wicking or insulation damage.
- Insulation: full coverage, specified extension and layer overlap, proper recovery and seal, no exposed metal, and support against flexing or vibration.
- Records: inspection findings, deviations and approvals, assembly identification, test parameters, and traceable test results as required by the project.
NASA-STD-8739.4A Chapter 18 identifies continuity, dielectric-withstanding voltage (DWV), and insulation resistance (IR) among cable-assembly acceptance tests, subject to the standard’s exceptions and engineering documentation. Acceptance procedures must be available for review and approval before use, and records must trace to the cable or harness. The relevant requirements depend on the assembly and its design.
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Common failures and what to do
- Forgot the sleeve: Stop and use the approved rework procedure. Do not assume a slit sleeve wrapped around the joint is acceptable.
- Nicked or gouged conductor: Reject the damaged section and follow the approved replacement or repair procedure.
- Excessive solder wicking: A stiff section can concentrate flexing at its transition. Additional heat-shrink does not fix the mechanical problem; rework or replace as authorized.
- Solder sleeve did not fully melt: Follow the part manufacturer’s approved rework instructions. Replace a sleeve that has been overheated, damaged, or contaminated rather than repeatedly reheating it.
- Continuity failure: Possible causes include incomplete wetting, a broken strand, a wire not captured, movement during soldering, contamination, or incorrect crimp tooling. Treat it as a failed splice and diagnose it; do not inject solder into a concealed joint without identifying the cause.
- IR failure: Look for exposed strands, bridges, residue, sleeve damage, inadequate spacing, or trapped moisture or contamination. Stop acceptance and inspect or remake under the approved procedure.
- Pull test passes but visual inspection fails: A pull test does not replace workmanship inspection. The termination must satisfy all applicable requirements, not only withstand a force.
Training and controlled work
NASA’s workmanship standards are process requirements, not a knot or solder shape that can be copied from a picture. Actual controlled work requires the applicable approvals, qualified personnel, specified materials and tools, inspection, and traceable tests. The JPL Manufacturing Technology Transfer Center crimp, cable, and harness course is a professional training option; its course page states that J-STD-001 Space Addendum certification is a prerequisite.
For ordinary construction or household wiring, use the applicable electrical code, listed components, and qualified electrician rather than treating NASA-STD-8739.4 as the governing code. For aerospace hardware, use the current project-controlled documents; standards and revisions can change.
Quick Recap
Primary references
- NASA-STD-8739.4A standard record — status, revision, and scope.
- NASA-STD-8739.4A with Change 4 PDF — splice, workmanship, inspection, and test requirements, including §§6, 18, and 19.
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