selection of incoming voltage

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You guessed at transmission losses as a factor in the service delivery voltage decision. That is usually a concern of the power authority, not the customer, unless the customer has a great distance to cover beyond his service entry point.

I can tell you that the major factors in this choice relate to cost.

Usually power suppliers charge less at higher voltage delivery values since the customer then supplies and carries the equipment cost, maintenance and losses of transforming down to his service voltage.

Amperage will dictate cable and equipment size, directly affecting cost of those items.

Also you should include the square root of 3 in your formula to accurately determine the amperage of the 3 phase system.

Regards, CJM

The really simple answer is that the higher the voltage, the lower the current is needed to produce the same amount of power. Low current means smaller cross sectional diameter cables to carry the current, which means less copper or aluminium in the conductors, which means less cost (as cable cost is proportional to the amount of copper or aluminium included in the cable).

There comes a point in power level where it is cheaper to step-up to a higher voltage transformer, switchgear and cabling to save money due to reduced cabling costs. Cable costs can account for a very large portion of the overall installation cost (especially when long runs of cables are necessary), so moderate savings per meter of cable can add up to big savings really quickly.

Hi all,

thanks for every one's contribution. As per Jack of all trades, (would have been more convinient if ihad known your real name) more voltage means less current ther by saving in conductor size & equipment. I just have a small hitch here. The secondary side i.e end user side voltage is any how constant i.e 415V. How does that contribute in saving the conductor size. And on the primary side that is on the HT side equipments, there is no flow of current (as for as my little knowledge). again here what contributes the saving? could you please clarrify.

PS: Hi Cjm, sure i will get logged in as a permanent user. thanks for your advise.

It depends on who you are considering. Put simply, for the lines company transmission at higher voltage means cheaper cables, for large energy usage customers (like large factories, smelters, etc) it is common for them to have their own transformer on site and run high energy usage parts of their plant of high voltage instead of 415V, again to save costs by running cheaper cable. Medium usage customers can also have the lines company transformer moved close to their premises to reduce the length of the 415V cable (hence reducing costs again). As for small energy usage customers such as houses, there is no need to run high voltage (it costs more so there is no benefit).

And on the primary side that is on the HT side equipments, there is no flow of current

There is still a flow of current on the primary side of the transformer even thou the primary and secondary windings are electrically isolated from each other. Have a look on Wikipedia (or some similar site) for a basic explanation on how a transformer works.

As per Jack of all trades, (would have been more convenient if ihad known your real name)

I prefer to stay fairly anonymous, as it is a small country down here and I don't want people to start spamming me on my business email address. The avatar name is far more revealing due to the very broad nature of the engineering work I am, and have been involved in over the years.

11 kv is considered a distribution voltage where you have residential and small commercial loads. 4 MVA are usually industrial loads and would load an 11 kv feeder down. You might have a VD problem if the 11 KV feeder was very long. 33 kv is an upper level distribution line or a lower level transmission line and is able to handle larger loads. 33 kv usually will be fed from a larger substation.