Keep those questions ready to ask the code panel after it has eventually made the spare capacity for an electrical system a requirement.(now it is only a recommendation.See Sec 90-1B)How are we to know how much spare capacity to be prepared for? Would we then need to increase capacity when we use up the spare capacity so there is more spare capacity again? What if we have 600 amp service - with 200 amps of spare capacity, then owner decides to put on addition that requires 800 amps of capacity for the addition alone? What good did that extra 200 on the original installation do for us?