Conduit Fittings

It depends on several things. Number one, do you want to run the wiring in conduit. Number two, do you HAVE to run the wiring in conduit. From personal experience, as long as you aren't going to be constantly taking it off and putting it back on, any metal conduit should work fine for you. Plastic conduit would work as well. However, I don't know what any of your electrical codes are, so it's hard to give a good answer.

It really shouldn't matter unless you are intending to mix the conduit types and adaptor types. If you are intending on running it in pvc, then use pvc throughout. If you are intending on using EMT, then don't try to put a pvc male adaptor on the end of a stick of EMT, because it isn't intended to be used that way.

I would bet that most methods don't employ a conduit system at all, rather they use a plenum rated cable.

Optimusprime; is the detector 24 volts or 120 volt? maybe a bushing & thermostat wire

VERY GOOD POINT! You wouldn't want to run individual conductors (for 120volt installations) without a conduit system, unless you used MC, or a Romex (type) of cable. If using a MC or Romex cable (for 120vac system) then a connector is a MUST.

To answer the original question, is does not matter what material the connector is made of, as long as the associated conduit system meets NEC. Based on my experience and understanding of the NEC, EMT is the minimum.

After Reading some of the other posts I would also add I think PVC is not allowed in air handling spaces, so the use this type of conduit system would be excluded. Please forgive me if I'm incorrect as I don't have a copy of the NEC in front of me.

I cannot comment on the use of a cable assembly such as MC or BX cable: I don't see anything wrong with it but I have not seen it used in that application.

As far as any non-metallic clad cable assemblies, it sound suspect to me. I would make sure it is an FM approved methodology before using it.

None the less, some kind of connector would still be required, one compatible with its conduit /cabling system.

MC cable is approved for the use and UL listed and can be installed anywhere metal raceway is specified, including open to the weather with the correct weather resistant jacket.

Plastic jacketed cable such as NM is not approved. EMT is not the minimum and open wire is allowed if all the requirements of the NFPA codes are followed....and you are correct about PVC conduit in a plenum space as defined by UL (United Laboratories) and NFPA (National Fire Protection Association)

just my two cents on a quarter.

The housing of a duct detector is not predicated by the NEC ro grounding. So the housing can be plastic. If the conductors are shielded, then they should only have one grounding point. As far as two connection points. If the system is a class "A" 4 wire then you have an in/out in case of a failure. In the case of a class "B" then the in/out can be in the same conduit; usually the case in addressable systems. hope this helps.