Spark Learn How the Power in Your Home Actually Works

How the Power in Your Home Actually Works

By Anthony Hall, Software Engineer Basics 8 minute read

Power arrives as two 120 volt legs, and the panel splits them into the circuits that feed every room.

The short version

  • Two legs at 120 volts combine to make 240 for big appliances.
  • A breaker protects the wire, not the thing plugged into it.
  • The neutral carries current back and the ground carries faults.
  • Everything past the meter is yours to maintain.
Transformer Street 120 V 240 V Theirs Yours Meter Everything past it is yours Main 200 A 15 A 20 A 40 A Your panel Two bus bars, one per leg Lights One leg, 15 amps at 120 volts Outlets One leg, 20 amps at 120 volts Car charger Both legs, 40 amps at 240 volts Ground rod Leg one, 120 volts Leg two, 120 volts Neutral, the middle tap Ground A circuit in the house
The transformer on the pole has a tap halfway along its winding. That tap is the neutral. From either leg to the neutral you get 120 volts, which runs the lights and nearly every outlet in the house. Across both legs you get 240 volts. That is what a range, a dryer and a car charger need, and it is why those breakers are twice as tall and take two slots. The meter is the dividing line. Everything past it is yours.

From the street to the meter

Power leaves a substation at high voltage and drops down through a transformer on a pole or in a green box at ground level. From there three wires come to the house, two hot legs and one neutral.

They land at the meter, which counts what you use, and the meter belongs to the utility. Everything on the house side of it belongs to you, which is why a problem at the meter is a phone call and a problem at the panel is a quote.

What the panel does

Inside the panel the two legs land on two bars. A normal breaker clips onto one bar and gives you 120 volts. A wide double breaker straddles both and gives you 240 volts for a dryer, a range, an air conditioner or a car charger.

Each breaker feeds one circuit, which is a loop of wire out to a set of outlets or lights and back. The number stamped on the breaker is the most current that wire can safely carry.

Three wires and three jobs

The hot wire brings power to the device. The neutral carries it back to the panel to complete the loop. Those two do the work every second the light is on.

The ground does nothing at all until something goes wrong. It gives fault current a fast path back so a breaker trips instead of the metal case of an appliance going live. That is why the third pin matters and why old two prong outlets are a real gap.

Why a breaker trips

An overload means you asked for more than the wire can carry, usually by running a heater and a microwave on the same kitchen circuit. It takes a moment and then it trips.

A short circuit is a hot wire touching a neutral or a ground, and it trips instantly. A ground fault is current leaking somewhere it should not go, and a protected outlet catches that one. The breaker is doing its job every time. A breaker that trips over and over is telling you something real.

What the numbers mean

Volts are the pressure. Amps are the flow. Watts are the two multiplied, which is the actual work done. A 1500 watt heater on 120 volts pulls about 12.5 amps, which is why two of them on one 15 amp circuit will not work.

A hair dryer, a space heater, a microwave and a toaster are each close to a full circuit on their own. Almost every nuisance trip in a house comes from running two of those together.

Questions people ask

How do I know what size service I have
Look at the number on the main breaker at the top of the panel. It reads 100, 125, 150 or 200 on most homes.
Is it bad to reset a breaker
Resetting once is fine. Resetting the same breaker again and again without finding the cause is not.
Why do my lights dim when the air conditioner starts
A motor pulls a large gulp of current at startup. A small dip is normal and a big one every time is worth a look.

Keep reading

How to Read Your Own Electrical Panel

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