4. Connector Selection — Think in This Order
Connector selection is multi-variable optimization. An excellent choice on most criteria that fails one critical criterion is still wrong. Work the steps in order — don't start from a connector you like.
If you are still trying to turn a vague need into useful search terms, start with How to Search for Connectors before you begin narrowing families.
Step 1: Define the interface
Before picking a family, write down: What crosses this boundary (power / signal / data / RF / fiber / fluid)? Internal or external? User-, technician-, or factory-accessible? Disconnected often? Safety-critical? Does it cross an environmental boundary? Must it be mated/unmated under power?
Step 2: Electrical requirements
| Criterion | What to evaluate | Common mistake |
|---|---|---|
| Voltage | Use the specified AC and DC working-voltage ratings; account for RMS/peak, transients, creepage/clearance, pollution degree, altitude, and the applicable safety standard | Using DC rating on an AC circuit; ignoring transients/peak |
| Current | Per-contact rating at temperature, bundle-derated per the manufacturer's derating curve. Apply a conservative design margin. | Using max rating with no thermal/bundle derating |
| Signal type | Discrete, analog, thermocouple, encoder, CAN, RS-485, Ethernet, RF — each has different needs | Mixing high-current power and mV analog in one insert |
| Pin count | Required + shields + spares + growth (see spare-capacity note below) | Filling every contact, then re-engineering for one more pin |
| Contact size | Drives both current capacity and wire-gauge range | Cramming wire that's out of the contact's range |
| Wire gauge | Work forward: current → gauge → contact size → connector series | Working backward from an available connector |
| Shielding / EMI | 360° backshell shield termination vs. pigtail; per-group shields | Pigtail terminations; shield grounded one end only without reason |
| Isolation | Segregate noisy motor/power lines from low-level analog/data | Routing PWM motor lines next to encoder feedback in one insert |
Add spare positions where feasible and justified. Unused cavities must still be sealed, documented, and configuration-controlled. Spare capacity is useful in expandable systems, but shell size, sealing plugs, weight, cost, panel space, and customer/program requirements may override it.
It depends on contact size, number of loaded contacts, ambient temperature, wire gauge, enclosure heat, and acceptable temperature rise. Always consult the manufacturer's derating curve for a fully-loaded connector.
Step 3: Mechanical / environmental requirements
| Criterion | Questions to answer |
|---|---|
| IP rating | Sealed mated? Unmated (capped)? Panel-mount sealing? At what depth/duration for IP68? |
| Vibration / shock | Test category? Positive locking? Anti-decoupling? Strain relief? |
| Mating cycles | Production-only? Test? Field service? Daily? (service life with margin below the rated cycles) |
| Keying | Can two similar cables be swapped accidentally? Use alternate keying. |
| Locking | Threaded/self-locking is often preferred for severe vibration; bayonet is fast; push-pull/latch/jackscrews depend heavily on application, qualification, retention, and service needs |
| Mounting | Cable plug, panel receptacle, jam nut (compact, can rotate), flange (rigid, better gasket control), PCB |
| Backshell | Straight / angled; shielded / unshielded; environmental; strain relief — not optional |
| Cable jacket | OD range vs. gland/clamp range; material; temperature; chemical exposure |
| Serviceability | Can a technician reach it with gloves and the right tool, from the needed side? |
| Availability | Can procurement actually buy it in time? QPL / COTS / commercial-equivalent? |
| Documentation | Can you fully define it in a drawing, BOM, ICD, and assembly procedure? |
Step 4: Production reality (the step beginners skip)
This is where the technically perfect connector turns out to be dead on arrival. Ask: Who crimps it? What tool is required? Is the crimp tool calibrated? Are contacts removable? Insertion/extraction tools available? Spare contacts stocked? Mating caps stocked? QPL-qualified, COTS, or equivalent? Second source? Lead time vs. schedule? Strip length / crimp spec defined? Inspection / pull-test plan?