What is the formula for calculating electrical load?

The base formula is watts equals volts times amps, and its rearrangements give you any missing third value. For three-phase, multiply by the square root of three, 1.732.

Sizing a service takes the extended version, the NEC Article 220 method: general lighting VA equals square footage times 3, plus 1,500 VA per small-appliance circuit, plus fixed appliance nameplates, with demand factors applied because loads never all run together.

Amps for the service then come from total VA divided by 240.

The formula everyone forgets is the 125 percent continuous-load factor: anything running three hours or more counts at 1.25 times its value before the totals are struck.

The electric load calculator runs the full chain from square footage to service amps.