Key takeaways
- Three-phase power runs large motors, HVAC and commercial equipment more efficiently than single-phase.
- Small commercial buildings often use 120/208 volt three-phase, and larger ones often use 277/480 volt.
- A load calculation comes first. It shows whether the existing service can carry the new equipment.
- A new or larger service needs a permit, an inspection and coordination with the utility.
Three-phase service is what most commercial buildings run on, but plenty of small shops, offices and converted houses in San Antonio still have single-phase power. That works until the business adds a large HVAC unit, a compressor, kitchen equipment or a machine whose nameplate says three-phase. Then the question becomes whether the building's service can carry it.
Single-phase, three-phase and why it matters
Single-phase power is what homes use. Three-phase power delivers power more evenly and lets large motors start and run more efficiently, which is why commercial HVAC, compressors and heavy equipment are often built for it. Small commercial buildings commonly use a 120/208 volt three-phase system. Larger buildings often use 277/480 volts for lighting and equipment, with transformers to step down for receptacles.
Start with a load calculation
Before anyone talks about a new service, the load has to be calculated: what the building draws now, what the new equipment adds, and what the code requires the service to carry. Sometimes the answer is that the existing service is fine and only a panel or feeder needs work. Sometimes it is a larger service. Sometimes it is a change to three-phase.
How a service change runs
- Load calculation and the new service design.
- A permit with the city, and an application to the utility for the new service.
- The new service equipment, panels and feeders installed.
- The city inspection, then the utility connects the new service.
- Circuits moved over and labeled, with the outage planned around business hours.
The utility side has its own timeline, especially when a transformer has to change, so it is worth starting that conversation early. In most of the San Antonio area that utility is CPS Energy, and the electrical contractor coordinates the work with them.
On the home side, the same trade is a 100 to 200 amp upgrade. Our panel upgrade service covers that. For businesses, the work is quoted per site as part of our commercial electrical services.
Signs a business has outgrown its service
- Breakers trip when equipment starts together.
- Lights dim when a large motor or compressor kicks on.
- New equipment arrives labeled for a voltage the building does not have.
- The panel has no spaces left and circuits are doubled up.