Calculating the existing service is critical to adding new loads. Taking amp meter readings is only a snap shot in time. Now, following 220.87 is an option and other than an actual load calculation is the only option.

So, you need demand data for 1 year. Now, their is an exception for 30 day, 15 minute peak data to be used.

So, you take the max demand load provided and multiply it by 125% plus the new loads and it is not to exceed the existing service or feeder rating. In Lexi's case it did so it means that 220.87 was ONLY to be used to determine if the existing service can be used....

Since it can't a NEW load calculation on the entire building is required and you DO NOT use the values given by the utility.