Choose by required circuits
Select every circuit the trailer actually needs. Running, stop/turn, and ground are included as the four base circuits.
Connector shape shows available positions, not proof that each circuit is present. Verify both connector faces, circuit functions, tow-vehicle support, and the installed product instructions before connecting wires.
Use the selector above after listing every circuit the trailer must operate. It recommends connector capacity, not a finished wiring design. Vehicle support, connector assignments, protection, and product limits still require verification.
List the trailer functions that must operate
The base four are ground/return, tail/running, left stop/turn, and right stop/turn. Add reverse or surge-brake lockout, electric-brake controller output, and +12 V auxiliary only when the trailer actually needs them. Treat each as an independent circuit position.
Do not choose by trailer size alone. Two visually similar trailers can need different connectors because their brake, reverse, charging, or accessory systems differ.
Use actual equipment as evidence. Inspect the trailer for electric-brake wiring and axle documentation, a breakaway kit, reverse lamps, a surge actuator with electrical lockout, a house or accessory battery, powered accessories, and manufacturer-assigned auxiliary circuits. Do not infer those systems from connector shape alone.
If a required function is unknown, leave it unresolved instead of selecting the smallest plug. The cost of one unused position is usually less important than silently omitting a brake or safety-related circuit.
Map functions to independent circuit positions
| Required set | Minimum common capacity | Unresolved check |
|---|---|---|
| Four base circuits only | 4-way flat | Verify all four functions |
| Base plus one assigned circuit | 5-way flat | Verify the fifth assignment |
| Base plus one or two assigned circuits | 6-way round or 7-way RV blade | Verify 6-way assignments and vehicle compatibility |
| Base plus brake, auxiliary, and reverse | 7-way RV blade | Verify every installed position |
Connector count is not circuit proof. The CURT wiring reference documents common connector functions and convention differences, while the exact installed products control their own assignments.
Work through the selector inputs
| Question | If yes | Evidence to collect |
|---|---|---|
| Does the trailer need only base lighting? | Start with 4-way capacity | Lighting inventory and verified return path |
| Does it need exactly one additional assigned function? | Compare matched 5-way options | Product instructions for that fifth function |
| Does it have electric service brakes? | Include a brake-output position | Axle and controller documentation |
| Does it need +12 V auxiliary power? | Include a separate auxiliary position | Load, protection, vehicle, and charging documentation |
| Does it need reverse or another auxiliary function in addition to brakes and power? | A matched 7-way commonly has the required capacity | Exact connector assignment at both ends |
The tool recommends the minimum common interface that can carry the selected independent functions. It does not choose wire size, fuse rating, controller, battery charger, or vehicle harness.
Compare compatible connector families
A 5-way is compact when one extra function is known and matched at both ends. A 6-way can carry two extra assigned functions, but layouts vary and must be verified. A 7-way RV blade provides capacity for the usual brake, auxiliary, and reverse additions and is often easier to match with modern tow-vehicle equipment.
When moving from an existing 4-way vehicle output to a 7-way trailer plug, use the adapter capability guide. An adapter housing does not create the extra signals selected here.
Interpret the result as capacity, not permission to tow
- A 4-way result means the selected task needs the four base positions. It does not prove the installed lamps or ground pass inspection.
- A 5-way result means one additional position may be sufficient. Confirm which function occupies it on both halves.
- A 6-way or 7-way result means more positions are required. Verify the connector family because round and blade interfaces are not interchangeable pinouts.
- A 7-way result for brakes also implies a compatible controller path and brake-system verification. The receptacle alone is not a controller.
If the vehicle and trailer use different bodies, first ask whether an adapter passes all required existing circuits. If circuits are absent at the vehicle, the task is an electrical conversion, not only a mechanical adapter choice.
Verify vehicle support and installed assignments
Open the connector types guide, identify both halves and viewing direction, then test the disconnected vehicle connector one function at a time. Confirm the trailer-side paths and product manuals. Label verified functions rather than copying colors from an unrelated harness.
Keep a short result record: selected functions, recommended capacity, actual connector family, vehicle circuits proven active, trailer paths proven connected, documents used, and anything still unresolved. This prevents a later replacement plug from being wired from memory or insulation color.
Stop if the selected connector lacks positions, the vehicle does not supply a required circuit, assignments conflict, the ground is weak, or protection and product limits are unknown.
Sources
Last verified: August 25, 2026.