220.87 Determining Existing Loads. The calculation of a
feeder or service load for existing installations shall be
permitted to use actual maximum demand to determine the
existing load under all of the following conditions:
...
Note that the language of the Code is "feeder or service load". If it was only intended to apply to the main service, then it would not say "feeder." If it was intended to apply to branch circuits, then it would say "branch circuits."
Let's say your meter is in a switchboard named "MS" -- this is your service, and you can use maximum demand information from the POCO to determine the existing load on "MS" by following NEC 220.87. Now, let's say that Switchboard "MS" feeds a panelboard named "A", and that Panelboard "A" feeds another panelboard named "B".
If you want to add load to Panelboard "B", you will need to justify your distribution system's ability to handle the additional load. You start by looking for the original permitted drawings, but you quickly discover that they were lost long ago (and even if you could find them there have been several circuits added without a permit ever being pulled, so they wouldn't do you much good anyway). Your next step is to use 220.87(1)
Exception.
Going by the letter of 220.87, you would need records from the POCO to establish the maximum demand on Switchboard "MS", PLUS you would need a 30-day load test on Panelboard "A" AND a 30-day load test on Panelboard "B". Why? Because when you add load to Panelboard "B", you are also adding load to Panelboard "A" and Switchboard "MS". If your POCO doesn't keep records of maximum demand, then you will need to do a 30-day load test on Switchboard "MS" as well.
You might be able to reduce the number of load tests necessary (or eliminate the necessity altogether) depending on the particulars of your installation. For example, let's say your service ("MS") is 600 amps, Panelboard "A" is 400 amps and Panelboard "B" is 225 amps. If the POCO says the maximum demand on your service in the past year was only 150 amps, then you can probably convince your Plan Checker or Inspector that you don't need a 30-day load test, as long as the load you're adding is less than 37.5 amps. You do this by saying, "Let's assume ALL 150 amps of the load is connected to Panelboard 'B'. 125% of 150 amps is 187.5 amps, which leaves us 37.5 amps to work with before we max out our panel." While lucking out like this isn't exactly commonplace, I've had it happen a couple of times.
If that doesn't work, you can look at what's connected to your panelboards and make some educated guesses. If Panelboard "A" only feeds Panelboard "B" and a couple of other small loads, you may be able to make an educated guess that a load test on Panelboard "A" will be able to justify the additional load on Panelboard "B" as well. Of course, if you guess wrong, you have to go the the expense of doing another 30-day load test, plus your project is delayed by a month (at least).
As iwire said, 220.87 is often ignored when adding small loads, but there's always a bit of risk involved because the Inspector
could choose to red tag you for it. If you're familiar with and on good terms with the Inspector, the risk is probably reduced.
