# electric power transmission: how many steps between power plant and outlet?

Here in the US, residential power outlets operate at around 110 VAC. The power plant where electricity is generated outputs something higher than that.

How many different voltage levels does the power go through before it arrives at my outlet as 110 VAC?

Bonus question: Wikipedia says that large transformers can be up to 98% efficient. What is a typical efficiency for a much smaller transformer, e.g. one that outputs 250 watts?

IIRC, there’s the little transformer on the power pole in your neighborhood. Prior to that is the local substation with a larger transformer. And that is fed from the big transmission lines coming directly from the plant. However, this is just what I know about locally-generated power. There may be other steps in long-distance transmission, like in from Canada to New York City.

Here’s the typical story, in brief:

Power stations generators put out something like 25000 volts AC. This is stepped up to 130kv - 700kv for long-distance transmission.

A substation near you transforms this to 7200v for local distribution. A transformer near your house produces 240v that feeds your power panel. This 240v consists of two wires each at 120v with respect to ground; a circuit that uses one of these and a ground wire yields 120v.

I assume we are talking of just the network, so our start point has to be the switchgear from the generator switchboard to the network.

You will have various steps up to line level, in UK this means getting it up all the way up to 400KV.

Some parts of our network operate at lower voltages, such as 132KV and then on down to lower levels.

It may depend upon lots of things because there may also be reactor correction devices that look similar to transformers, these are in place to set up various safe operating conditions to prevent cascade shutdowns or limit short circuit currents.

Now that I think about it, you are probably better off starting at your end, the consumer and work your way backwards to the highest transmission line voltage.

Now if you are in the US it may well be different, you may have local utility which has generators feeding a switchboard at something like 66KV which also has a supply coming in the other end from somewhere else that was transformed down to 66KV from higher voltage transmission lines.

You should be able to google up some sort of very basic diagram like this

http://www.roymech.co.uk/Related/Electrics/Electrical_transmission.html

This is extremely simplified, at each voltage level change there will be some sort of switchboard which will have various feeders - especially when you start getting to the ‘lower’ voltages such as33KV and 11KV where you might well get lots of small infill generation such as windfarms, smaller CHP stations etc

Electric power turbines output at varying voltages. I’ve seen turbines with output voltages at from 2kV to 18kV. (there may be much higher output voltages but I just haven’t had that much experience that close to the output terminals)

Since a lot of power plants have more than one generator and sometimes different styles and makes of generators sometimes there is a dedicated transformer which converts the generator voltage to the generation plant bus voltage (Ex 18kV to 12kV).

Then a transformer steps the plant bus voltage up to transmission voltage (example 12kv to 161 kV). If transmission voltage is fairly high then downstream there could be another transmission transformer which steps the voltage down to a lower transmission voltage(Ex: 161kv/69 kV).

Somewhere near the customer is a distribution substation which steps the voltage down to distribution voltage (Ex 69kV to 12.47kV)

That voltage is carried to the pole or pad-mount transformer where it is converted to 240V for the customer. Industrial customers sometimes have 480V or higher and require different transformers.