To enable Imperial to Metric connections for simple joints

Note: In AutoCAD Plant 3D 2014 and later imperial-to-metric connections are enabled (or disabled) for each joint type. You can modify joints, or add a new joints to set the imperial-to-metric value.

1. In the Setup tree view, expand Plant 3D DWG Settings. Click Configure Piping Connection Settings.
2. Under Simple Joints, select the joint that you want to enable imperial-to-metric (for example: MixedButtweld).
3. Click Modify
4. In the Modify Joint dialog box, select This Joint Connects Imperial to Metric Components.
5. In the Display Name box, enter a description that identifies a mixed-metric connection (for example: Buttweld imperial to metric joint)
6. Click OK

This joint now connects only if one component is imperial and the other component is metric. If you also want to support connections between same-unit components use the Add Joint dialog box.