I believe this would depend on your quiding rules/regulations. If it is the NEC, then you usually size a breaker to 125% of the largest load on it, then add all the other loads to it (for multiple loads). If the result is not a standard overload amount, you are allowed to go to the next higher rating, in most cases.

If you rules/regulations pertain to the NESC, or perhaps the IEEE, then you will use those standards for figuring the rating of the overload protection. This most likely would be engineered, so you won't have to figure it anyway, just follow specs.

I hope this has helped.

Rick Miell