How service voltage is chosen for a new load
There is no national voltage class system in the United States. Each utility publishes its own electric service requirements, and the voltage a site is offered comes out of a conversation about load size, available distribution in the street and what the tariff allows. The thresholds below are common industry practice, not a code rule.
| Peak demand | Typical service | Category |
|---|---|---|
| Up to 10 kW | 120/240 V single-phase | Secondary |
| 10 to 75 kW | 208Y/120 V three-phase | Secondary |
| 75 to 1,000 kW | 480Y/277 V three-phase | Secondary |
| 1,000 to 10,000 kW | Primary 4.16 to 34.5 kV | Medium voltage |
| Above 10,000 kW | 69 kV and above | Transmission |
Estimated current follows from apparent power: amps = kVA ร 1,000 รท (โ3 ร volts) for three-phase service. A 500 kW load at 0.9 power factor is 555.6 kVA, roughly 668 A at 480 V, which is why loads much beyond a megawatt usually move to primary service rather than pushing secondary conductors any further.
The crossover point matters commercially as well as technically. Taking primary service shifts transformer ownership, maintenance and relay testing to the customer while lowering the rate, so the break-even depends on the local tariff. Figures are current as of 2026; confirm the actual offering with the serving utility early, and have a licensed electrical engineer review the service study before committing to a design.
Frequently asked questions
No. Unlike countries with a single national tariff, US service voltage is set by each utility in its electric service requirements handbook and in the service agreement. The thresholds here reflect common practice, not a code requirement, so the serving utility always has the final word.
The customer owns and maintains the transformer, primary switchgear and protective relaying, and usually provides a pad or vault plus metering at primary voltage. Capital cost and maintenance go up, but demand and energy rates on a primary tariff are normally lower, so large loads come out ahead.