Wires, Connectors and Cables
Tinning, splices, solder cups, heat-shrink, strain relief, magnet wire and the resistance of a joint.
Wires: solid, stranded and tinning
Wire is described by its American Wire Gauge (AWG: a higher number is thinner) or its cross-section in mm². Solid wire is rigid and easy to solder but breaks if it is flexed; stranded wire is flexible but wicks solder up its strands. The diameter is d = 0.127 × 92(36 − n)/39 mm.
| AWG | Diameter | Area | Resistance (copper, 20 °C) | Chassis current (typical) |
|---|---|---|---|---|
| 30 | 0.255 mm | 0.05 mm² | 338.5 mΩ/m | 0.5 A |
| 28 | 0.321 mm | 0.08 mm² | 212.9 mΩ/m | 0.8 A |
| 26 | 0.405 mm | 0.13 mm² | 133.9 mΩ/m | 1.3 A |
| 24 | 0.511 mm | 0.20 mm² | 84.2 mΩ/m | 2.1 A |
| 22 | 0.644 mm | 0.33 mm² | 53.0 mΩ/m | 3.0 A |
| 20 | 0.812 mm | 0.52 mm² | 33.3 mΩ/m | 5.0 A |
| 18 | 1.024 mm | 0.82 mm² | 20.9 mΩ/m | 7.0 A |
| 16 | 1.291 mm | 1.31 mm² | 13.2 mΩ/m | 10.0 A |
| 14 | 1.628 mm | 2.08 mm² | 8.3 mΩ/m | 15.0 A |
| 12 | 2.053 mm | 3.31 mm² | 5.2 mΩ/m | 20.0 A |
The current figures are typical for single wires in free air (power supply and chassis wiring); the full derating and the voltage-drop method are on the Electronics Fundamentals pages and in the wire size widget on My Dashboard.
- Strip with a proper stripper, not the teeth: nicked strands break later. Strip only the length you need (about 5 to 10 mm).
- Twist the strands of stranded wire in the lay direction before tinning.
- Tin the end with a small amount of solder and flux, from the insulation outward (the solder wicks) until it is bright and the strands are joined; stop short of the insulation by 1 mm, or it will be stiff.
- Use the right size wire and insulation: PVC melts at about 105 °C and shrinks back; silicone and PTFE take the heat.
Worked example: voltage drop of a 2 m cable
- 22 AWG: d = 0.127 × 92(36 − 22)/39 = 0.644 mm, area 0.33 mm², resistance 53.0 mΩ/m.
- A 2 m run carrying 3.0 A has a loop length of 4 m, so the drop is 3.0 × 211.8 mΩ = 0.64 V. On a 5 V supply that is 13 %: too much, so use 18 AWG (0.25 V).
Joining wire to wire: splices
A soldered splice is only as strong as the wire is held mechanically. Never rely on solder alone to carry tension.
| Splice | How | Good for |
|---|---|---|
| Lap or “Western Union” | Twist the bare wires together along their length, then solder | Strong, for solid wire and field repair |
| Inline butt (side by side) | Lay the two ends parallel, twist or wrap, solder | Small cables, stranded wire |
| Hook (to a terminal) | Hook the wire through the terminal hole and crimp it back, then solder | Terminal lugs and posts |
| Solder sleeve | Pre-loaded heat-shrink with a solder ring, heated with hot air | Fast, neat, sealed |
| Crimp | A butt connector squeezed with a crimp tool | Vehicles, vibration: do not solder a crimp |
Worked example: a lap splice and its sleeve
- Strip 10 mm from each wire; overlap 8 mm, giving a splice about 16 mm long.
- The heat-shrink tube should extend 6 mm past each end: 16 + 2 × 6 = 28 mm. Cut it before soldering and slide it onto one wire first!
- The wires each have an outside diameter of 1.5 mm, so the bundle is about 3.0 mm. A 4.0 mm (2:1) tube shrinks to 2.0 mm: the supplied size is larger than the bundle (slides on) and the recovered size is smaller than the bundle (grips). It fits.
Heat-shrink and solder sleeves
Heat-shrink tubing insulates and supports a joint. Its sizes are given by the supplied inside diameter; it shrinks by its ratio (2:1, 3:1 or 4:1). Pick a tube that slides over the largest part of the joint, shrinks to less than the smallest part and has a wall that thickens enough to grip. Adhesive-lined (“dual wall”) tube seals against water.
| Supplied ID | Ratio | Recovered ID | Suitable bundle diameters (approx.) |
|---|---|---|---|
| 2.4 mm | 2:1 | 1.2 mm | wire bundles from 1.4 to 2.0 mm |
| 3.2 mm | 2:1 | 1.6 mm | wire bundles from 1.8 to 2.8 mm |
| 4.8 mm | 2:1 | 2.4 mm | wire bundles from 2.6 to 4.4 mm |
| 6.4 mm | 2:1 | 3.2 mm | wire bundles from 3.4 to 6.0 mm |
| 4.8 mm | 3:1 | 1.6 mm | wire bundles from 1.8 to 4.4 mm |
| 9.5 mm | 3:1 | 3.2 mm | wire bundles from 3.4 to 9.1 mm |
- Shrink with hot air at 120 to 140 °C (typical), working from the middle outward so that air is not trapped; do not use the iron tip.
- Slide the tubing on before making the joint and push it back out of the heat.
- For a solder sleeve, strip and insert both wires, heat until the solder ring melts and the end seals flow, and let it cool before moving.
Connectors: solder cups, pads and lugs
Many connectors (D-sub, XLR, power connectors, circular military types) have solder cups: small hollow pins into which the wire is soldered. Others have pads, eyelets or lugs. The method is the same: tin the wire, fill the cup, insert and re-melt.
Worked example: filling a solder cup
- A cup of 2.0 mm inside diameter and 4.5 mm deep has a volume of 14.1 mm³.
- A 1.0 mm conductor inserted full depth takes up 3.5 mm³, leaving 10.6 mm³ to fill with solder: about 21.1 mm of 0.8 mm wire.
- Fill the cup with solder first (tin the cup), heat it again and push the tinned wire into the molten solder until it is bottomed; remove the heat and hold still. A cup overfilled with solder overflows onto the next pin.
- Slide the boot and strain relief onto the cable before soldering, check the pinout twice (the mirror image on the other side of the connector is a classic mistake) and bundle the wires with ties so that each can be inspected.
- Insulate adjacent pins with a short piece of heat-shrink on each wire (about 5 mm) to prevent shorts.
- Work in a connector vise, and use a small tip: plastic inserts soften above 150 °C.
- Solder to the board: wires soldered to a pad should be anchored to the board (a hole, a tie or glue) so that the joint takes no pull.
- Shielded cable: terminate the shield with a short pigtail, or better a 360° clamp, and keep the drain wire short.
Worked example: time to wire a 9-way connector
- A 9-pin D-sub cable: strip and tin 9 wires, solder 9 cups, check and fit the shell. At about 90 s per conductor (including stripping, tinning and sleeving) the soldering takes 14 minutes, plus assembly.
Strain relief: why wires break at the joint
Most wire failures are not in the joint but just beside it. Solder wicks up stranded wire under the insulation, making a rigid length (about 4 mm) that ends at an abrupt change in stiffness. Every flex of the cable bends the wire at that line, concentrating strain in the first unsoldered strands, and they fatigue and break one by one: the resistance rises, then the wire opens. This is why solder joints on flexing cables fail.
- Support the cable with a boot, clamp or cable tie, so that the bend is spread over several centimeters.
- Do not wick solder far: keep the tinned end short and use a small amount of solder and a quick contact.
- Use a crimp for flexing cables: a good crimp does not stiffen the wire, which is why crimp contacts are used for vehicles and robots.
- Add a flexible length: a service loop, and adhesive-lined heat-shrink over the wire end to extend the supported length.
- Use silicone-jacketed, high-flex stranded wire (finer strands) in moving applications.
Worked example: bending strain at the joint
- The strain at the surface of a wire bent to radius R is ε = d ÷ (2R). For the 22 AWG conductor (0.644 mm) bent to R = 2 mm: ε = 0.644 ÷ 4 = 16.1 %, which is far beyond the elastic range of copper (about 0.2 %) so each flex deforms the wire plastically.
- If the cable is supported so that the bend radius is 20 mm, ε = 0.644 ÷ 40 = 1.61 %, which is ten times smaller; fatigue life rises by orders of magnitude.
Magnet wire and special wires
Magnet wire (enameled wire for coils and motors) is coated with a thin varnish. Some enamels, such as polyurethane, burn off at soldering temperature and are “self-fluxing”; others must be stripped.
| Insulation | Soldering | Method |
|---|---|---|
| Polyurethane (155 °C) | Self-fluxing at 350 to 400 °C | Solder directly |
| Polyester / polyesterimide | 430 to 470 °C | Strip or heat harder |
| Polyimide (Kapton, 240 °C) | Not self-fluxing | Strip mechanically or chemically |
| Enameled wire with solvent | N/A | Scrape or use a paint stripper |
- Test by touching the tinned end with the iron: if the solder flows and the surface is bright, the enamel has gone.
- Strip by scraping with a blade, fine abrasive paper or a drop of solder on a hot tip held on the wire for a few seconds.
- Litz wire (many insulated strands) needs every strand cleaned; coaxial cable needs a short, hot, quick joint so the dielectric does not melt; aluminum wire cannot be soldered with ordinary solder (use a crimp or clamp).
The resistance of a soldered joint
Soldered joints have low resistance, but they are not zero, and a poor joint (cold, oxidized) can have a resistance hundreds of times higher. The bulk resistivity of Sn63Pb37 is about 14.5 µΩ·cm, 8.4 times that of copper.
Worked example: resistance and heating of a joint
- A solder layer 1 mm long and 0.30 mm² in cross-section (a small splice) has R = ρL ÷ A = 1.45×10−7 × 0.001 ÷ 0.30×10−6 = 0.48 mΩ. One millimeter of 22 AWG copper wire has 0.05 mΩ, so the joint adds the resistance of 9.1 mm of wire.
- At 5 A the joint drops 2.4 mV and dissipates 0.01 W: negligible. A cold joint with a resistance of 50 mΩ would dissipate 1.25 W and heat up, which is how a bad connection in a power cable can burn.
- Use a four-wire (Kelvin) measurement or a milliohm meter to find a bad joint: if the joint is above a few milliohms for a small wire, resolder it.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Wire breaks at the joint | Wicked solder stiffens it; no strain relief | Reduce wicking; add a boot or clamp; use crimps |
| Solder runs up the wire | Too much solder or heat; stranded wire | Less solder, shorter contact; wick it out |
| Insulation melted or shrunk back | Too long or too hot | Quicker joint; heat sink on the wire; right insulation |
| Intermittent connection | Cold joint or broken strands | Reheat with flux; measure resistance; replace the wire |
| Cup overflows onto neighbors | Overfilled | Wick; use less solder; sleeve the wires |
| Enameled wire will not tin | Enamel not removed | Strip further or use a hotter tip; test with a meter |
| Heat-shrink does not seal | Not shrunk enough; wrong size | More heat; check ratio and bundle size |
Related
- Soldering: the overview of this guide.
- Irons, Stations, Tips and Hot Air: tips and temperatures.
- Joint Quality, Inspection and Failure: defects and fatigue.
- System Integration: wiring, cables and voltage drop in machines.
FAQ
Should I solder or crimp?
Crimp for flexing cables and vibration (cars, robots, machines); solder for fixed wiring, boards and where the crimp tool is not available. Never solder a crimped joint: the solder stiffens it and it fails.
How do I tin stranded wire?
Twist the strands, add flux, touch the iron to the underside and feed the solder to the wire until it is bright; stop 1 mm short of the insulation.
Why did my wire break next to the joint?
Wicked solder made the wire stiff at that point and every bend fatigued it. Support the cable with a boot or clamp and keep the soldered length short.
Can I solder aluminum wire?
Not with ordinary tin-lead or lead-free solder: the oxide layer prevents wetting. Use crimps, clamps or special aluminum solder and flux.
How do I solder magnet wire?
Polyurethane enamel burns off at about 350 to 400 °C: tin the end in a bead of solder on a hot tip. Others need stripping first.
References
- IPC/WHMA-A-620, Requirements and Acceptance for Cable and Wire Harness Assemblies; IPC J-STD-001; NASA-STD-8739.4 (crimping, cables and harnessing).
- ASTM B258, Standard Specification for Nominal Diameters and Cross-Sectional Areas of AWG Sized Solid Round Wires.
- Manko, H. H., Solders and Soldering, McGraw-Hill (solder resistivity).
Last reviewed: October 2026. Chassis currents, resistivity and strain figures are typical: derate for bundling, temperature and the standard you work to.