What do the installation instructions say?

They don't specify wire size, but the wires on the fan and the wires coming off the control look to be about 18 awg.

Yes, it's not legal (unless these fan controllers have their own overcurrent protection in them). Either drop the breaker down to 15A or use 12g wire.

The control is rated at 5 amp maximum.

Give us the brand and model of the control.

Canarm #FRMC5

Are you in Canada or the US?

I'm in the US - SC.

My original comment stands.

Would your opinion be different in Canada?

I don't have a CEC nor am I overly familiar with the vagaries of Canadian practices (other than what I've picked up here and from watching Mike Holmes :-)).

Any suggestions on making all that crap fit in a single gang box. 12/4 plus feeding a second control is going to be impossibly tight - and the control faceplate is only geared for a single box.

I guess I'm missing something in the description about what you want to do.
12ga from source to fan control..
14ga from control to fan?? doesn't the fan draw the most current? so why run 14ga from the control to the fan?
wouldn't 12ga be better? for sure I can think of many cases where this wouldn't be the case.
1. 12ga from source to fan control..control to a remote switch( this control a relay in the fan control box)..sure 14ga would be over kill, but should have a 15a breaker for its power
2. a remote 24Vac switch to control a relay in the fan control box. 14ga is over kill and over current most likely isn't needed

-dkenny

Since it is probably exposed (not dealing with existing walls) Get a large 4 and 11/16" square box and a single gang mudring for it (which can also add more volume) You will then have the equivalent of a 2 gang deep box with just a single opening. Even a deep 4X4" box would like be within volume but the 4 and 11/16" box would be even more room. By the way, not 100% sure that MC cable is appropriate for a horse stable. In the eyes of the code it is about the same as NM cable, even though it is a bit more robust. I don't know much about the capabilities of a horse but they might be able to chomp it up. May be better off with a conduit installation for sections within reach.

Wiring after the control unit that is not directly connected to branch circuit conductors does not have to follow all the branch circuit rules.

The manufacturer should have provided guidance.

Nothing's in reach of the horses. The MC cable is for the mice.

Mudring is a good idea. I'm using weatherproof boxes for everything, so I've only been focused on the standard weatherproof cover options.

If you are required to use weatherproof boxes, then MC cable is not acceptable. That is unless it is inside of a finished wall and only the box is subject to weather. For a weather proof installation, use EMT conduit with compression fittings, liquid tight flexible conduit, or PVC conduit. Set a 2 gang weatherproof box for the input and output power, power out to the motor, and a conduit with only the wires needed for the controller itself, heading to a single gang box for the controller.

Horse barns are a good example of a time to ask your AHJ what would be approved. Art. 547 covers agricultural builings and is specific as to what wirinig methods are allowed. Due to dust, water, and corrosive nature of horse urine, EMT and non-jacketed MC are NOT approved for exposed locations in a horse barn. That being said, damn near every barn I've seen uses both. I have also asked one of my third party inspectors if he would approve EMT or MC and he said yes, he would. Our personal barn and any barn I wire will use PVC conduit and liquid tight were exposed even though I could get away with using something else. If I lose the job so be it. Between horses and hay, a barn is not the time to look for the answer you want. I would ask your AHJ for the answer you need.

All wiring below 12 feet is contained within walls and the MC cable comes out right into the back connection of the weatherproof boxes. The MC cable that's exposed is at or above 12 feet to lights and fans over the breezeway or to the horse stalls.

Local inspector said that EMT is not recommended in any barn or horse stable, but MC cable is fine (as little sense as that makes). However, inspection is a non-issue since they sign off on construction, electrical,plumbing, etc. as soon as the frame of the building is erected for agricultural buildings in our county.

So any other opinions whether or not 14 awg is sufficient to run the 4 conductors (plus ground) between the 5 amp control and the .84 amp fans (up to 4)?

I'd say run #14 if you want. Fit a 15 amp breaker so the overcurrent protection is right for #14. Tag the #12 in the panel with a label indicating that the circuit contains #14 wire, so nobody later decides to use a 20 amp breaker.

Call the manufacturer and ask them for a wire size recommendation.

Controller wiring (even in the NEC) has its own separate rules.