DIYbanter - View Single Post - What's the largest Main Breaker available?

dpb

On 4/5/2012 12:43 AM, wrote:
....

As much wiring as I've done, I have never understood how the 3 phase
systems get the desired voltages. Just like how does one get 277v from
480v (half is 240). Then too, why use 277v for lighting? Unlike large
motors and electrical heating sources which are more efficient at higher
voltages, lighting is one thing that surely does not need higher
voltages and would require special bulbs, ballasts, and probably
switches too. Seems sort of pointless....

As the other posters note, you need to consider the phaser diagram. The
Wikipedia article isn't _too_ bad; reading it won't hurt.

But, as to the question posed above, look at it the other way 'round...

Suppose we want to take two phase wires (of a three-phase distribution)
and a neutral to make a three-wire household service supplying 120 V
between each hot wire and ground. The neutral will become the grounded
conductor, the two phases the hot and neutral conductors. If we grant
this, the wye voltage is 120, so the delta voltage will be ˆš3 x 120 =
208 V. This is the three-phase line voltage necessary in this case
which is why that is a common distribution value for three phase
systems--it was worked out so that the end result will be that desired.
Note that the two 120 V sources are not opposite in phase, and will
not give 240 V between them.

On the other hand, suppose we do want a 240 V service. Then this must be
the line voltage, and the voltages to neutral will be 139 V, not 120 V.
A 120 V three-phase service will give only 69 V from line to neutral.

http://en.wikipedia.org/wiki/Three-phase_electric_power

Also as noted, the reason for the higher voltages in commercial building
is that the lower amperage requirements for smaller conductors
translates into a less expensive installation overall. The cost-savings
escalate the larger the facility.

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