Fitting an isolator on the battery, do I use a cable on the pos or neg side.

 

If you are talking about an isolator connected to a dual battery setup, then the isolator should be connected to the positive side of the battery. Mine is a Keyline "voltage sensitive" type which also has a small ground wire which needs to be connected to the vehicle ground so it knows when to "turn on." If yours is manual you shouldn't have to worry about a ground wire.

If you are talking about a battery disconnect switch, then it should be connected to the negative side.