In a perfect world, all electrical wiring would be in conduit. EMT, or thinwall metal conduit, is about the safest option inside a barn, arena, or any outbuilding. In the real world, most existing facilities aren’t equipped with all wiring in conduit. So, the final determination should be made with your local electrical inspector. One of the reasons for having all wiring fully encased in metal tubing is because, as electricity flows through the wiring, a strange phenomenon occurs that actually produces a slight salinity, or salt. Mice can detect the salt and will try to get at it by chewing through the outer plastic casing of unprotected wiring. Of course, this will be their last meal, but by then the potential fire hazard has already been realized. With all the dry hay, wood shavings and exposed lumber that are present in horse structures, this is obviously a concern. It doesn’t happen very often, but it does happen. This is one area where you don’t want to take any chances. 

Utilities...