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Section 220.87 provides methods for determining existing loads.
To add additional load to an existing installation, the NEC requires understanding what the existing load actually is.
The entire load could be recalculated using the main rule, but if there is already data for a period of time, then the rules in Section 220.87 for feeders or in Section 220.16 for branch circuits can be used. Both of these are based on the same essential concept with some variations.
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