Many owners, developers, and architects are puzzled when an electrical engineer's load calculations, and the distribution infrastructure designedaround them, differ considerably from the capacity the utility company will provide. For example, it's common for us at Peter Basso Associates to calculate a building load of 1,000 kVA, only for the utility to provide 500 kVA of service capacity. That can seem odd, even to electrical engineers. So why the difference?
As the building's engineers, we are required to design its electrical infrastructure in accordance with the National Electrical Code (NEC), published by the National Fire Protection Association as NFPA 70. The NEC prescribes specific methods for calculating service and feeder loads, and we must follow them to comply with the code. Those methods are intentionally conservative and leave little room for engineering judgment or for how the building will actually operate.
The utility company, on the other hand, does not have to follow the NEC when designing its infrastructure. It operates under a different set of rules—typically the National Electrical Safety Code (NESC) and its own design standards. They are responsible to provide infrastructure to support their customers but have a large database of operating condition for similar facilities. They can also load transformers and cables beyond their nameplate ratings for limited periods, something the NEC does not allow inside a building.
The easiest way to understand and reconcile these differences is this: the utility companies' equipment and cables are outside our building. The point where the utility connects to the building's electrical system is called the service point, and everything from that point into the building must comply with the NEC. If the utility overloads its equipment or cables, it happens outside the structure. If equipment or cables inside the building are overloaded, they present a fire risk to the building and the people in it. That is exactly what the NEC is designed to prevent: its purpose is the practicalsafeguarding of people and property from electrical hazards. The minimumstandards and requirements set forth in the NEC are about protection ofproperty and people.
A gap between the engineer's calculated load and the utility's service size isn't a mistake on either side. It's the result of two different sets of rules applied on opposite sides of the service point. Inside the building, the NEC requires conservative sizing to protect people and property. Outside the building, the utility sizes its own equipment based on how similar facilities actually perform. To avoid surprises, involve the utility early in design, share load information openly, and remember that the buildingss service equipment must still be sized to the NEC, regardless of the capacity the utility provides.