Elk M1, 3-wire motion sensor?

[snakevargas]

It occurred to me after wiring my motion sensor that maybe this could maybe be done with 3 wires instead of 4? I'm thinking the "NEG" wire that's used to power the detector is electrically the same as the "NEG" wire that's used for the sensor loop. I could just run one NEG wire to the detector and use it to serve both purposes. Is this correct? Good idea? Bad idea?
 
I don't need to do this now, but it seems like it could be a useful technique to run both a PIR and a contact from the same 4-wire bundle in the future.

 

[Del91574]

It's possible, as the panel shares a common negative, however a PIR should be a home run, as should a contact. You should avoid running items to other unrelated items and splicing them, it's creating a failure point.
 
I wire PIR's as 3 wire just to avoid making a splice with a chicklet and EOLR inside the unit and it's easy enough to figure out why a zone isn't working...
 
Usually going the common negative route is done when there's no other choices, such as in an outbuilding and not enough conductors.