It's a Canadian thing and it's in the 2009 code as Appendix I, Rule 26-724(a). This seems to contradict the wording of the actual rule, but there it is, 'no' means 'yes'.
A separate circuit outdoors is a good idea in the north, and especially in Canada where we use 15 amp circuits for almost everything. We plug in our cars with block heaters and interior warmers.
