Something to consider when using the calculating method is that if it passes the test it does so under current usage patterns. If usage patterns change, say for example that now with L2 and L1 charging capabilities in the garage, a homeowner decides to charge 2 cars simultaneously. We now have a load that was not part of the calculation that resulted in a passing score. Maybe grandma moves in and likes to keep a 1500W heater going in her room most of the time. It passed the test, however, there’s no proof that things won’t change. This test has flaws, particularly if there is guesswork involved in the calculations.
With the metering method, there is no proof that a maximum load condition was ever measured. Again, it passed the test, however, measuring the load for a few seconds out of every hour, doesn’t show us what was happening the other 59 minutes and 30 seconds of each hour. This test is probably the most flawed of the three.
The sum of individual breakers method is the most likely to guarantee a service panel will not become overloaded, provided that the sum is less than the main. However, that situation rarely exists.
To be clear, I am not suggesting that the tests are useless or should be disregarded. A homeowner who is attempting to decide whether or not a circuit can be added to a panel should be aware that passing a NEC test, does not offer proof that the panel will not become overloaded at some point.
