The Short Answer
Electrical service size is determined by a load calculation performed per the National Electrical Code. Engineers total the building's connected loads — lighting, receptacles, HVAC, kitchen and process equipment, and special loads — then apply NEC demand factors that account for the fact that not all loads run at full capacity simultaneously. The resulting calculated demand, coordinated with the serving utility, sets the service ampacity, transformer, and main equipment sizing documented on stamped electrical drawings.
NEC Load Calculations
The foundation of service sizing is the load calculation required by the NEC. Engineers inventory every load the building will serve and categorize it — general lighting and receptacles, HVAC and motor loads, kitchen equipment, water heating, and any special or continuous loads. The code specifies how each category is calculated, including minimum values for certain occupancy types and rules for continuous loads that must be sized at higher percentages.
Demand Factors
If service were sized to the sum of every connected load, it would be oversized and expensive, because loads rarely all peak at once. The NEC allows demand factors that reduce certain load categories to a realistic simultaneous demand — for example, applying reductions to large numbers of receptacles or diversifying kitchen equipment. Applying demand factors correctly produces a service that's adequate under real operating conditions without being wastefully large.
Special and Future Loads
Some loads require careful attention. Motors and HVAC compressors have starting characteristics that influence sizing. Continuous loads are calculated at increased percentages. Owners often want spare capacity for future expansion, EV charging, or tenant fit-outs. Engineers balance current code-required capacity against reasonable future flexibility, documenting the assumptions so the service supports the building's intended life.
Utility Coordination
Service sizing isn't complete without the serving utility. The utility sets transformer size, service voltage and configuration, metering requirements, and the point of connection. Engineers submit load information to the utility and coordinate the service entrance, transformer pad, and metering to match utility standards. For service upgrades, the utility's available capacity and lead times can affect the project schedule, so early coordination matters.
When Do You Need This?
A service-size calculation is needed for new construction, service upgrades, major tenant improvements that add significant load, and projects adding large equipment such as commercial kitchens, EV charging, or new HVAC. If you're unsure whether your existing service can handle new load, an engineer can perform the calculation and tell you whether an upgrade is required.
Frequently Asked Questions
Can I add new equipment to my existing service?
It depends on the spare capacity in your existing service and panels. An engineer performs a load calculation on the existing plus new loads to confirm whether the current service is adequate or an upgrade is needed.
What documents show the service size?
The one-line diagram, panel schedules, and the load calculation together document the service size and distribution. These are part of the stamped electrical set submitted for permit.
How long does a utility service upgrade take?
That varies widely by utility and location, and it's outside our control, but early coordination helps. We provide the utility the load information they need and coordinate the service equipment to their standards.