When applying the NEC, is a municipal firehouse considered a dwelling unit. It appears as if it is, according to the definition of dwelling unit given in article 100.

Frank

It looks like the firehouse meets the NEC definition to me, too. You might want to look into the Life Safety Code since this is also a mixed occupancy. The NEC requirements for dwellings might only apply to the areas that involve sleeping, cooking, bathrooms.

A firehouse IMO is multiple occupancy. Typically in a situation is when you have a conflict in the rules, typically the more sticter prevails. The Life Safety codes are only applicable if it is adopted like the electrical code ot a building code. At a minimum though they can shed light on the matter pending what you are trying to address. You may want to contact the AHJ if you have codes overlapping.

Out this way, a firehouse is often a light steel garage, a 'manufactured home,' and a covered passage between the two. In that design, it's pretty simple to separate the structure into different occupancies.

Our building code department indicated it was a class "B" occupancy; but didn't expand on it.
Also, when calculating the service size for a new "mixed occcupancy" building what does the design professional use, the residental standard calculation or commercial calculation?

Frank

Typically they use comercial calculations, for example 180VA per recept, 100%DF on lighting loads, etc.