How is the US 277Vac input voltage derived and why is it used?

Posted by Dulcie on April 4, 2018
Posted in: Power Supply Basics

The most commonly used voltage in the US is 115/120Vac to supply households and commercial facilities. For residential properties two-phase 208Vac (phase to phase) or 240Vac (via a distribution transformer) is available for those users that require higher power for electric dryers, air conditioning and electric ovens. 208Vac three-phase is commonly used in commercial and light industrial facilities.

In larger commercial properties in addition to 208 and 120Vac, 480Vac three-phase power is supplied to the building in a “WYE” (or Y) configuration as shown in Figure 1.

Figure 1: 480Vac three-phase

The 480Vac three-phase is used to power large electrical equipment like fork lift battery chargers, ovens and heavy machinery. Using this higher voltage reduces the amount of current drawn by the load, and is balanced equally between each phase for more efficient power distribution and generation.

With this configuration, a 277Vac supply is derived, (480Vac ÷√3=277Vac) measuring from one of the phases to the neutral connection. This lower voltage supply requires less insulation and can be used with smaller and less expensive switchgear and wiring. In the US this has been used for fluorescent lighting and HVAC (Heating, Ventilation, and Air Conditioning) for many years.

A lighting fixture powered from 277Vac draws 25% less current than one on a 208Vac circuit. This reduces wiring losses I²R (Power Loss = Current² x Wire Resistance) and also enables more light fixtures to be connected on the same circuit. This provides significant energy savings to schools, offices, hotels, restaurants and warehouses.

Until LED solid state lighting became commercialised in the late 2000s, in general the electronic power supply industry did not produce 277V rated AC-DC supplies. General awareness of the existence of the 277Vac mains input was very limited. That soon changed as LEDs require DC power to operate and LED ‘drivers’ quickly came onto the market, rated for use with 100V (Japan), 120V, 208V, 230V/240V (Europe) or 277Vac nominal inputs. For these power supplies to be used globally, an operating input voltage of 90 to 305Vac is required, satisfying the regulatory requirement of a +/-10% input variation on the nominal supply voltage.

The desire to save the environment (and cost) has led to the development of smart buildings and with that a host of new products to reduce and manage lighting, heating and cooling energy usage. More intelligent security features are also being installed. As these devices need a control system and a power source, an easy way to provide AC power to them in the US is to use the available 277Vac circuits located above the ceiling tiles. This saves time and money that would be needed to run additional 120Vac cabling and install extra outlets.

Some power supply manufacturers are now providing non LED driver products that can operate from 90 to 305Vac. TDK-Lambda has recently introduced two industrial grade power supply families; the 2 to 4W rated pcb mount KAS series and the 40 to 65W rated chassis / DIN rail mount CSW65 series.

KAS2-4 Series

CSW65 series

The availability of industrial grade products is important to many designers. Typically these have a long (10+ year) lifecycle, whereas LED drivers are often optimised to drive specific light fixtures and are more subject to early obsolescence. The cost and time to evaluate and recertify power supplies is expensive and diverts resources away from new product development.

For more information about the full range of TDK-Lambda AC-DC power supplies, please visit: www.emea.lambda.tdk.com/uk/products/acdc-power-supplies