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What Is Electric Current?

on January 29, 2016

What is Electric Current?Your breakers are rated by amps, your lightbulbs are listed in watts, but everything that plugs into the wall has to be 120 volts.  What’s the difference?  Why are all of these things listed with different units when they all use electricity?  Today, let’s try to unpack what these are to give you a better understand of what each measurement is telling you about your device.


The standard example for how a circuit works involves plumbing.  Voltage is the difference between two points, current is the rate of flow, and resistance is the how much a material resists flow.  As an example, if you have two tanks of water with a valve in the middle, the difference in water level between the two tanks is the voltage (potential of difference), the valve is your resistance (restricts water flow), the current is how fast water is flowing between the two tanks.

For any device to operate, all three of these need to exist.  Resistance consists of the physical medium (wires and the device itself), while current is the rate at which charge is passing through those wires to the device.


Current, Voltage, and Resistance are all tied together in a balanced circuit.  As one changes, so do the others.  This is demonstrated by a formula known as Ohm’s Law: Voltage = Current x Resistance or V=IR.  This is why your circuit breaker is measured in Amps, the unit of current.

Most of the circuits in your home are set at 120 volts.  The only exceptions to this are a few specialized appliance circuits which operate on a higher voltage.  Since the voltage is kept constant, and you cannot directly control the resistance of the circuit, the remaining variable is current.  Current is adjusted by adding or removing power loads to a circuit in the form of devices.

  1. Adding Devices: Whenever you add a new device such as a phone charger, the total power draw of your home electrical circuit changes.  Each device added brings with it additional power draw.  While a phone won’t add very much load to the system, a high powered computer or a space heater can put a great deal of power draw on the system, increasing the amount of current over the line. This is normal and perfectly fine.
  2. Current Overload and Runaway: There is another way to increase current on a circuit, and that’s by overheating the wires. Copper wires are not perfect; they contain a small amount of resistance.  As electricity flows through them they heat up slightly.  Because metal expands as it heats, the surface area of your conductor (the copper wire) increases.  Electricity travels over the surface (now increased due to heat), which is what limits the flow of electricity.  As wires heat up, current increases.  This is normal and is fine, until it reaches a point of runaway.  Runaway is when heating of the conductor causes the current to spike (heating increases, so current increases, so heating increases, etc).  This is not fine and will certainly cause a breaker trip.

This issue with increase in current is part of why circuit breakers use a measurement of current to control when the breaker needs to be tripped.  It’s a reliable measurement and is directly related to power usage.  Usually current overload is caused by high-power loads being added to the circuit.  You can easily adjust for this by moving heavy-load devices (space heaters, or microwave ovens) to a separate circuit.  If this doesn’t fix the problem, then you may have a wiring problem instead.

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