Why Did the U.S. Transition from 110V to 120V Supply?

Why Did the U.S. Transition from 110V to 120V Supply?

In the early days of electricity in the U.S., the standard household voltage was 110 volts, largely influenced by Thomas Edison’s DC systems and early incandescent lighting technology, which was optimized for around 100–110V. However, as technology advanced, electrical loads increased, and better transmission efficiency was needed, engineers and utility companies started looking at ways to improve the power supply system without completely overhauling infrastructure.

One major reason for the shift to 120V was the introduction of AC (alternating current) and modern appliances. AC power systems, promoted by Nikola Tesla and Westinghouse, allowed for long-distance power transmission at higher voltages with lower losses. As demand for more power-hungry appliances like refrigerators, washing machines, and heaters increased, so did the need for a higher and more consistent voltage level. Increasing the voltage from 110V to 120V allowed for more efficient energy delivery to homes and reduced line losses, especially over longer distances.

Another factor was the standardization of equipment and safety regulations. As electrical codes and standards evolved in the U.S., it became necessary to define a nominal system voltage that allowed a range for fluctuation. The National Electrical Code (NEC) and utilities gradually defined the standard voltage as 120V ±5%, allowing for variations while still ensuring safe and consistent operation of equipment.

Importantly, the shift was not abrupt. Utility companies incrementally increased the voltage at transformers to compensate for line drops and improve efficiency. The infrastructure (such as transformers and appliances) was gradually designed or retrofitted to handle 120V without the need to drastically replace household wiring or plugs, which still largely resemble those used in the 110V era.

🔍 Summary:

The U.S. moved from 110V to 120V to improve efficiency, support modern appliances, reduce power losses, and meet updated safety and performance standards. This transition allowed for better compatibility with growing residential loads without major rewiring, as 120V systems could still support legacy 110V devices.