Power Grid Basics

Understanding electrical grids: outlets, AC vs DC, voltage, and three-phase power.

Published January 1, 2022 ET
electrical-engineeringscience

How Many Things Can I Plug Into an Outlet?

As many things as you want, as long as the combined current drawn by all devices doesn't surpass about 80% of the current available through the outlet (about 20 amps). Therefore, the combined current shouldn't surpass 16 amps (0.8 × 20).

Power strips run in parallel (not in series):

  • In a series circuit, current delivered to each destination is the same, and voltage drops with each new destination
  • In a parallel circuit, current delivered to each destination is proportional to the resistance provided by each destination, and voltage at each point is the same

Why Does the Grid Use AC Instead of DC?

AC has at least three advantages over DC in a power distribution grid:

  1. Natural generation: Large electrical generators happen to generate AC naturally, so conversion to DC would involve an extra step

  2. Transformers require AC: Transformers must have alternating current to operate, and the power distribution grid depends on transformers

  3. Conversion economics: It is easy to convert AC to DC but expensive to convert DC to AC, so if you were going to pick one or the other AC would be the better choice

Voltage and Current in Your Home

When electricity leaves the power plant, voltage is in the range of 155,000 to 765,000 volts. Typical maximum transmission distance is about 300 miles.

Commercial electrical generators produce 3-phase AC power. The three phases are offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral/ground common to all three.

The Journey from Plant to Home

  1. Power plant: Generates electricity at thousands of volts
  2. Transmission substation: Transformers step up voltage to extremely high levels for long-distance transmission
  3. Power substation: Steps transmission voltages (tens/hundreds of thousands of volts) down to distribution voltages (typically less than 10,000 volts)
  4. Your home: Further stepped down to usable voltage

A power substation typically:

  • Has transformers that step down voltages
  • Has a "bus" that can split distribution power in multiple directions
  • Often has circuit breakers and switches for disconnection when necessary

Why Three Phases?

The basic answer is that three phases achieve a nice balance of power delivery for the level of complexity required.

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