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.
If you're starting from a physical connector rather than a requirement, run the shared Connector Identification Workflow first, then come back here.
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 (creepage = shortest path along a surface between two conductors; clearance = shortest through-air gap), 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.
How to read a derating curve
A contact-current derating curve plots the allowable current per contact (often as a percent of the single-contact rating) against the number of energized contacts, at a stated ambient temperature. To use it: find your loaded-contact count on the x-axis, read the allowable percentage (or current) for your ambient, apply it to every current-carrying contact, and keep a design margin below that.
Illustrative only — use the manufacturer's actual curve: a contact good for its full rating with a single pin energized might be derated to roughly 70–80% in a fully-loaded insert, and lower again at elevated ambient.
Why fully-loaded is worse: every energized contact dissipates I²R heat, and closely packed contacts warm each other (mutual heating), so the whole insert runs hotter and each contact must carry less to stay within its temperature limit. The falloff steepens as the loaded-contact count and ambient rise — which is exactly why "current rating is not one number."
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?
Two procurement traps in particular: crimp contacts are often sold only in minimum order quantities (bags of ~100), and the connector, its contacts, the backshell, and the crimp tooling each carry independent lead times — order them together and early, not as an afterthought.
For production cable and wire harness workmanship, inspection, and acceptance criteria, use IPC/WHMA-A-620 (Requirements and Acceptance for Cable and Wire Harness Assemblies) or the program/customer-required equivalent — and always follow the applicable contract, drawing notes, customer standard, and the manufacturer's application specification for the exact connector/contact/tooling system.1 Concretely, the crimp part of the design package should name: the exact contact P/N, the wire gauge and insulation range, the approved crimp tool and positioner/die, the strip length, the pull-test or inspection requirement, the insertion/removal tools, the acceptance standard/inspection class the shop will build to, and any technician notes.
The items that most often fall off the BOM or the drawing at this step are collected in What People Forget.