Voltage Classes
480V (480Y/277V) | 240V (120/240V or 208V) | 120V | Low voltage (<50V) | |
System type | Three-phase, 4-wire | Split-phase in homes; 208V is three-phase in commercial buildings | Single-phase branch circuits | Class 2 / Class 3 power-limited circuits |
Typical uses | Commercial and industrial distribution, chillers, rooftop units, large motors, elevators, 277V lighting | Ranges, dryers, water heaters, heat pumps, EV chargers, small commercial equipment | Receptacles, lighting, small appliances, electronics | Data and telecom, PoE devices, security, access control, thermostats, AV, doorbells, nurse call, fire alarm signaling |
Where it lives | Main switchboards, risers, mechanical rooms | Home load centers; panels in light commercial buildings | Every building's branch panels | Separate low-voltage cabling, often run by a different trade |
Why it's used | Low current for big loads, so smaller wire and long runs with little loss | Twice the power of 120V on the same size wire | Safe, universal, and every plug-in device is built for it | Shock-safe and cheap to install, but limited power |
Notes:
- 110V vs. 120V. 110V is an older label. Today's US standard is 120 V nominal, with outlets seeing about 114–126 V.
- 208V vs 240V. Commercial buildings get 208V from two legs of a three-phase system, while homes get 240V from split-phase service. Equipment rated for 240V runs about 13% weak on 208V, which matters for heaters and some HVAC.
- Low voltage can mean two different things. To electricians and in technical codes, it can mean anything up to 1,000 V. In building design it usually means the low-power systems under 50 V.
- Canada often uses 600Y/347V in place of 480Y/277V.
Tradeoffs
The underlying trade-off is simple. Power equals voltage times current, so doubling the voltage halves the current for the same load. Lower current means thinner conductors, lower losses, and longer possible runs. The cost is safety and complexity:
- Higher voltages carry greater shock and arc-flash risk.
- They need qualified workers and more expensive, higher-rated equipment.
- They have to be stepped down before reaching anything people touch.
Buildings therefore carry power at the highest practical voltage and step down close to the load. In a typical office, 480V feeds the HVAC and 277V runs the lighting. A transformer near the users then steps 480V down to 208/120V for receptacles. Low-voltage systems are separate, powered by their own transformers, power supplies, or PoE switches.
Two related variants:
Why voltages and frequencies differ outside North America
The split comes from history rather than engineering superiority.
- Most of the world uses 220–240V at 50 Hz.
- North America uses 120/240V at 60 Hz.
- Parts of Latin America and a few other countries mix the two, such as 127/220V or 60 Hz.
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Voltage
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Frequency
What this means for projects
- Most modern electronics have universal power supplies (100–240V, 50/60 Hz).
- Motors, clocks, heating elements, and transformers are voltage- and frequency-specific. A 60 Hz motor on 50 Hz runs about 17% slower.
- Equipment must be specified for the local system.