Friday, April 27, 2018

Where and why is a 277Vac input used in the USA?

The AC power supplied and used by household and commercial buildings in the USA is primarily 120Vac.  This is suitable for most electrical and electronic equipment in the home or office, but for the higher power requirements of electric driers, air conditioning and electric ovens, 208Vac or 240Vac is available. 

The 208Vac is derived by connecting across two phases of a “Y” or “WYE” three phase supply as shown in Figure 1.

 Figure 1
The 240Vac is supplied from a distribution transformer as shown in Figure 2.
Figure 2
In larger industrial properties, in addition to 208 and 120Vac, 480Vac three-phase is supplied to the building in a "WYE" (or Y) configuration as shown in Figure 3.

Figure 3 480Vac three-phase
The 480Vac three-phase is used to power larger electrical equipment like fork lift battery chargers, and heavy machinery.  Less current is drawn by the load at this higher voltage, enabling the use of lighter gauge wire, and the current is balanced equally in each phase.  This improves both power distribution and generation efficiency.

Measuring across each line to neutral we see 277Vac (480Vac divided by the square root of 3)  This voltage is used in the USA for powering lighting and HVAC (Heating, Ventilation, and Air Conditioning).  Using 277Vac instead of 120Vac reduces the current drawn by lighting fixtures by over 50%, enabling smaller gauge (lighter) and lower cost wiring.  Alternately more lighting fixtures can be connected to a 277Vac feed than with a 120Vac feed.  Power losses in the wiring is also reduced (P=I2 x R).
The awareness of the existence of 277Vac feeds in commercial buildings was very limited until the introduction of LED lighting in the late 2000s.  The AC-DC power supply industry until then had focused on products that operated between 85 – 265Vac.  As LEDs require low voltage DC current, LED “drivers” came on to the market, capable of operating from input voltages of 90 to 305Vac.  This voltage range covers 100V, 120V, 230V and 277Vac nominal inputs (with a regulatory +/-10% tolerance for line variations) to enable use world-wide.

Some of the control infrastructure for smart buildings is now using 277Vac for automatic window sun filtering, intelligent thermal monitoring and security.  Often the location of the controllers and monitors are in places where 120Vac is not available and 277Vac can be easily dropped down from above the ceiling tiles.

TDK-Lambda is one manufacturer that is providing industrial power supplies capable of operating from 90-305Vac.  These are an alternative to using constant voltage LED drivers that potentially can have short product lifecycles.  Engineering resources then have to be diverted to find alternative products.  Power supplies developed for the industrial market tend to be produced for ten years or more.

Below are two such products; the 2 to 4W rated pcb mount KAS series and the 40 to 65W rated chassis / DIN rail mount CSW65 series

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