When calculating branch circuit loads, what's the maximum load you’re allowed? The limit is this: The total calculated load on a given branch circuits can't exceed the rating of that circuit [220.18]. When calculating, don't forget to multiply continuous loads by 125%.

But suppose you're designing a fabrication shop. The tenant owns a single arc welder and needs to use it all over the shop. So you run a single branch circuit with 10 receptacles on it. Since only one receptacle will be used at any time, what's the calculated load? Answer: It's the load presented by that single arc welder. This assumes there are controls (administrative or otherwise) in place to ensure there won’t be a second arc welder (perhaps a rented one) also in use.

The principle is "load diversity," which the NEC mentions in several places [e.g., 310.15(B)(3)] but doesn't define. Account for load diversity so that you don't grossly oversize the conductors.

A common example for residential applications is the fact that you aren't going to run air-conditioning and electric heat at the same time. Therefore, you don't need to size the service to accommodate both loads; you need to accommodate only the larger of the two. Only when two loads are non-coincidental (or, ideally, mutually exclusive) should you consider them diverse.