Three-Phase Power for Commercial Buildings

What Three-Phase Power Is

Single-phase power delivers electricity through two conductors — one hot and one neutral. It oscillates in a single wave. Three-phase power delivers electricity through three hot conductors, each carrying an alternating current offset by 120 degrees. The three waves overlap, which means power delivery is continuous rather than pulsating. For the same amount of power, three-phase uses less conductor material, produces less heat, and runs motors more efficiently.

Most residential buildings run on single-phase, 120/240-volt service. Most commercial buildings run on three-phase service. The two common commercial voltage configurations are 208Y/120 and 480Y/277.

• 208Y/120: Three hot conductors at 208 volts between any two, and 120 volts between any hot and the neutral. Used in most commercial buildings for lighting, receptacles, and small equipment.
• 480Y/277: Three hot conductors at 480 volts between any two, and 277 volts between any hot and the neutral. Used in larger commercial and industrial buildings. The 277-volt circuits power lighting directly. The 480-volt circuits feed HVAC, motors, and step-down transformers that supply 120-volt receptacle circuits.

When a Building Needs Three-Phase

A building needs three-phase service when it runs equipment that requires it. The most common drivers:

• Commercial HVAC: Rooftop units above 5 tons typically require three-phase power. A 10-ton unit on a single-phase circuit would draw so much current that the conductors and breaker would be impractically large.
• Motors: Three-phase motors are simpler, more reliable, and more efficient than single-phase motors of equivalent size. Any motor above 5 horsepower is typically three-phase.
• Commercial kitchens: Ovens, dishwashers, and walk-in refrigeration in restaurants and food service facilities run on three-phase.
• Industrial equipment: Compressors, pumps, conveyor systems, and manufacturing equipment.

NEC Article 430 governs motor circuits. Motor full-load current (FLC) determines conductor size, breaker size, and overload protection. The FLC is found in NEC Table 430.250 for three-phase motors. The conductor must be sized at 125% of FLC for continuous-duty motors. The breaker can be sized higher — typically 250% of FLC for inverse-time breakers — because motors have high inrush current at startup.

Service Upgrades to Three-Phase

Some commercial buildings were built with single-phase service and need an upgrade to three-phase. This is a significant project. It requires:

1. A load calculation under NEC Article 220 showing that three-phase is justified by the connected load.
2. Coordination with the utility company. The utility must have three-phase service available at the building location. In older areas of Broward County, three-phase may not be available on the pole without an extension — which the property owner typically pays for.
3. Replacement of the service entrance conductors, meter enclosure, and panelboard with three-phase-rated equipment.
4. A new grounding and bonding system sized for the new service.
5. Reconnection of all existing single-phase loads to the new panel. Single-phase loads connect to one hot and the neutral. On a 208Y/120 system, this provides 120 volts — the same as before the upgrade.
6. Permitting and inspection by the local authority.

The utility company's role is critical. Florida Power & Light (FPL) provides electrical service to most of South Florida. FPL determines whether three-phase service is available, what transformer is needed, and whether the customer pays for line extensions. This decision can take weeks and affects whether the project is feasible at a given location.

Load Balancing

On a three-phase system, loads must be balanced across all three phases. If phase A carries 200 amps, phase B carries 80 amps, and phase C carries 90 amps, the system is unbalanced. Unbalanced loads cause voltage drop, overheating in the neutral conductor, and inefficiency in transformers.

Balance is achieved by distributing single-phase loads evenly across the three hot legs. A 120-volt receptacle circuit on phase A, another on phase B, another on phase C. In practice, this means organizing the panel so that circuits are not clustered on one phase.

Balance is estimated during panel design and confirmed after installation, measured with a clamp meter on each phase conductor. If the imbalance exceeds roughly 10 percent, circuits should be redistributed. The NEC does not set a numerical balance requirement — this is a design practice, not a code threshold, but an unbalanced panel wastes capacity you have already paid the utility to bring in.

Transformers

Buildings with 480Y/277 service need step-down transformers to supply 120-volt receptacle circuits. The transformer steps 480 volts down to 208Y/120. This requires a separate panelboard — a distribution panel — fed by a breaker in the 480-volt panel.

Transformer sizing follows NEC Article 450. The transformer kVA rating must exceed the calculated load. A 75 kVA transformer can supply approximately 208 amps at 208Y/120 — enough for a typical office floor's lighting and receptacle load.

Transformers generate heat and need ventilation. In South Florida, where ambient temperatures are already high, a transformer in a closed electrical room can overheat. Ventilation requirements for transformers are covered in NEC Article 450, and the clearances a dry-type unit needs depend on its size and enclosure. Many commercial buildings use dry-type transformers, which do not use oil and are suitable for indoor installation.

Power Factor Correction

Three-phase commercial buildings with many motors — HVAC, pumps, compressors — often have a low power factor. Power factor is the ratio of real power (watts) to apparent power (volt-amps). A power factor below 0.85 means the building is drawing more current than necessary to do the same work.

Low power factor causes higher utility bills, overloaded transformers, and voltage drop. FPL and other utilities charge commercial customers for low power factor. Correction is done by installing capacitor banks that offset the inductive load of motors. The capacitors are sized based on a power factor study and installed at the main panel or at individual motor circuits.

NEC Article 460 governs capacitor installations. Capacitors must be disconnected when motors are off, either through the motor controller or a separate disconnect. This prevents the capacitors from overcorrecting the power factor when the inductive load is removed.

South Florida Considerations

South Florida's commercial building stock ranges from 1960s and 1970s retail — often with undersized single-phase service — to modern multi-tenant buildings with 480Y/277 and individual tenant meters. The older buildings in Fort Lauderdale, Hollywood, and Hialeah frequently need service upgrades to support modern loads. Newer buildings in Sunrise, Plantation, and Miramar typically have adequate three-phase service but may need additional capacity for tenant build-outs.

Hialeah has a concentration of older industrial buildings with three-phase service that predates modern code. These buildings often have cloth-insulated conductors, screw-in fuses, and inadequate grounding. A service assessment is the first step before any tenant improvement.

Davie has light industrial and flex space where tenants run equipment that draws significant three-phase load — compressors, CNC machines, welding equipment. The existing service may be adequate for the square footage but not for the specific equipment the tenant brings. A load calculation specific to the tenant's equipment is essential before signing a lease.

24/7 Electrician is a licensed and insured electrical contractor based in Hollywood, Florida, serving South Florida. Flat-rate pricing is quoted up front. A certificate of insurance is provided for commercial properties. Call (954) 602-0050.