How does a Theory (evolution) differ from a Law (gravity)? How much proof does a theory require before it becomes a law?

Three-phase, 4-wire panelboards are usually constructed with an even number ofspaces available for each phase; thus, the total number of spaces would be in incre-ments of 6, such as 12, 18, 24, 30, 36, or 42. how many spaces would be available forthe later addition of circuit breakers in panelboard p-1 after the panelboard has been equipped as scheduled?