# AC Vs. DC

What’s this thing about AC anyway? A little history lesson might be in order. It all started when Edison built the first incandescent bulb and the power station to light it. A big disadvantage of DC electricity immediately revealed itself: you couldn’t build the power houses any further than a few blocks from where the electricity was produced.

Why? The resistance of the wire consumed a portionof the power. Houses close to thepower station had brilliant lights andthose at the furthermost reach had dimones. The last guy on the line had it bad. When someone close to the powerhouse turned on a whole bunch of lights, his would dim even further. And pity the poor farmer! He couldn’t get any power out there in the country until a few companies got smart and started manufacturing wind-electric machines.

Then along came Nikola Tesla, thefather of AC motors and generators. Ifyou make AC at any voltage, it can be transformed through use of the highly efficient transformer to any other voltage. Stepping up the voltage has the effect of stepping down the current for the same power transfer.

## Why does this matter?

Power delivered to the “load” (anything which uses power) is defined as the product of Amps and Volts, or P=IV. On the other hand, line losses (the energy lost in the transmission wires) are determined by the product of Amps squared times ohms (the resistance of the wire), or P=RI2.

Note that voltage plays no part in line losses.

With AC, then, the transformer stepped up the voltage (or down, depending on the ratio of the number of windings of input and output) to hundreds of thousands of Volts. Naturally, since “power out” must equal “power in” (minus losses), the AC current decreased in the same proportion.

Thus, super-high voltage and super-low current meant very low line losses irrespective of how far you needed to send it. Of course, very high voltage is dangerous stuff for appliances, lights, and motors.

With AC, however, once you get the power to the home, farm, or shop, a second transformer (on the utility pole) would step the voltage back down for use.

## The point of this historical review?

There’s nothing really “sacrosanct”about 120-volt, 60-cycle AC. It is convenient for the utilities to use because it’s the only way they have to transfer power over long distances. If your power is homegrown, you don’t have their problems, so why necessarily accept their solution?

True, after you’ve considered all the factors, you may decide that high voltage will work best for you in your situation.

SOURCE: Backwoods Home Magazine – January/February 2000

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#### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

1. ankushbahale
Aug 08, 2013

Please give me details regarding working principle and construction of Synchroscope.

2. ervinothkumarv
Apr 08, 2013

If we connect the loads in parallel connection means the voltage is became constant right??? then what is the point to consider the voltage drop???? voltage stepup and stepdown only the advantage in AC

• Edvard
Apr 08, 2013

Voltage drop depends on conductor length, cable factor and cable cross section.

3. jdgoy
May 06, 2011

Can you touch on HVDC transmission? It was my understanding that DC power transmission has no reactive power and therefore is highly more efficient than AC which will have a reactive component. Is this true? One drawback I read about was that DC at high voltages is much more dangerous than AC at high voltages, could you explain that as well? Thanks.