What a building’s electrical service rating tells you

A property's electrical service rating is a starting point for assessing power, but three different quantities need to remain separate: the rating of the existing service, the additional load the building's installation can accommodate, and the additional load the utility network can supply.

The first describes equipment. The second depends on existing demand, the proposed equipment and the limits of the installation. The third depends on the utility's network and its assessment of the project. Converting a listing's amp rating into a power figure does not answer all three.

What amps, voltage and power describe

Current is the flow of electric charge, measured in amperes, or amps. Conductors and electrical equipment have limits on the current they can carry. Voltage is the electrical potential difference between two points: it describes the energy transferred per unit of charge. An amp rating needs a voltage and supply configuration before it can be expressed as a power rating.

Real power, measured in kW or MW, is the rate at which electrical energy is transferred into uses such as heating, lighting and motor operation, including losses. Energy, measured in kWh or MWh, accumulates over time. A facility's monthly energy consumption does not reveal its highest power demand.

Apparent power, measured in kVA or MVA, expresses the combined voltage and current loading of an AC supply. Equipment must carry that current even when some of it does not contribute to net energy transfer. Power factor is the ratio of real to apparent power; it is a property of the load and operating conditions, not a universal conversion factor. The US Department of Energy's motor-systems guidebook explains this relationship in its chapter on power factor correction.

One conversion, with its limits

Consider a hypothetical service rated at 1,200 A and 480 V, three-phase. For a balanced three-phase supply, using RMS line current and RMS line-to-line voltage:

Apparent power = √3 × voltage × current ÷ 1,000
√3 × 480 × 1,200 ÷ 1,000 ≈ 998 kVA.

This expresses the stated service rating in another unit. It does not establish that the installation can continuously supply 998 kVA to new equipment, or that the utility has that much additional capacity. Permissible loading depends on the actual installation, protection, operating conditions and applicable requirements. Existing load also uses part of the service.

A kW figure requires an appropriate power factor as well. Even then, it remains a conversion of the stated rating, not a measurement of demand or a utility commitment. Published capacity fields require a further check of their definitions; see when an MVA label does not mean you should convert to MW.

What to establish next

• Existing installation: verify the service voltage, phase, equipment ratings and restrictions with the site's electrical professional.
• Existing demand: obtain representative demand records and understand seasonal peaks and operating schedules.
• Proposed load: identify expected coincident demand, significant starting loads and the timing of expansion.
• Utility supply: confirm the serving equipment and the utility's requirements for accommodating that load.

Published network data can make the utility inquiry more specific. Nearby capacity records do not identify spare capacity inside the building, prove which circuit serves the parcel, or replace the utility's project assessment.