The arithmetic, in one line
The existing load on the service, established from what it actually drew, taken at 125%. Plus the heat pump, counted the way Article 440 counts it. The total has to fit under the rating of the service.
The existing term is established from the maximum demand over 12 months of utility records, or a 30-day recording where those are not available, under NEC 220.87. For a heating conversion that recording has to include the heating season, because a shoulder-season window captures neither the heating peak nor the cooling peak and the section requires the larger of the two to be in the figure.
How the heat pump is counted
At the rated-load current marked on the equipment nameplate, under 440.6, or the branch-circuit selection current where one is marked and it is larger. Never the horsepower, and never the running amps somebody measured on a mild afternoon.
A heat pump does not heat and cool at the same time. Under 220.60, where two loads are unlikely to run together, only the larger needs counting, so the heating and cooling modes of one unit are counted once, at the larger. Where the heat pump is the largest motor on the service, 430.24 puts it at 125% and the other motors at 100%.
The trap is the backup heat
Many heat pumps carry electric resistance backup that runs during defrost or on a cold morning, with the compressor, not instead of it. When it can run at the same time, the two are no longer noncoincident and both go in the calculation. That combination is frequently larger than the compressor alone by a wide margin, and it is the reason a heat pump conversion that looked easy on the compressor nameplate lands above the rating.
Whether the backup can run with the compressor is a fact about the equipment and its controls. A load study asks it rather than assuming it, and a controls setting that locks out the backup above a set temperature changes the answer.
Replacing gas, and what the meter never saw
A facility that heated with gas has a demand record with no heating load in it. The heat pump is entirely new electrical load on a service that was sized without it, so the study adds the whole unit, backup included where it applies, to a measured baseline that is honest about everything except the thing being added. That is the correct arithmetic, and it is also why the answer for a gas-to-electric conversion is less often yes than for adding cooling to a facility that already had it.
Questions people ask
Does a heat pump count as heating and cooling load?
Once, at the larger of the two, under 220.60, because it does not heat and cool at the same time. Electric backup heat that can run with the compressor is counted with it.
What figure on the nameplate do I use?
The rated-load current, under 440.6, or the branch-circuit selection current where one is marked and it is larger. Not the horsepower, and not a measured running current.
Why does the study have to happen in winter?
Because NEC 220.87 requires the larger of the heating or cooling load to be in the existing-load figure, and a recording that misses the heating season misses it. Where the window cannot be scheduled into winter, the missing load is added by calculation and the report says so.
Can the backup heat be locked out to make it fit?
Where the controls prevent the backup from running with the compressor, the two become noncoincident and only the larger is counted. Whether that is acceptable for the facility's comfort and for the authority is a separate question from the arithmetic.