Bunch of questions here, but probably basic for you all.

My wall outlet is 120v, 20amps.

1) What makes it 20amps? Where is the resistance applied? What if it was 30amps instead? Where along the circuit is this resistance applied and with what physical component? Because the copper wire adds more resistance does my outlet always give slightly less than whatever it's rated from the breaker box?

2) If a power supply plugs into this outlet and says it's "20volts, 2.4amps" what is inside this power supply doing that conversion? Key words to Google here would be great!

3) If that power supply was lost, and I couldn't remember it's volt/amps. So I "found" another power supply, that was 20 volts, 5 amps, and plugged it into the device; would the extra amperage damage the device?

  • 5
    \$\begingroup\$ -1 for completely useless title. Think about it a little next time. \$\endgroup\$ Jul 31, 2014 at 20:39
  • 1
    \$\begingroup\$ You should read the basics in electronics as this is always covered. \$\endgroup\$ Jul 31, 2014 at 20:42

3 Answers 3



You might be thinking about amperage in the wrong way. You can do current (amp) limiting techniques if needed, but for the most part power sources are designed as either fixed voltage or fixed current. Fixed current is less common and should be obvious, it tries to maintain the same current output by varying the voltage. However, voltage power supplies (a lot more common), try to maintain the same voltage by varying the current. This means that if you have a 120 volt device that will try to draw a varying amount of power (watts), this will be done by the current draw increasing or decreasing. The 20 amp rating in a voltage supply, is the maximum amount of current draw permissible before damage or fire. In the case you try to draw too much, you will usually run into some form of overcurrent protection (like breakers or fuses). However, if there is no overcurrent protection, fire may result.


This is complex and involves many topics. However, since your house is AC (alternating current), you usually will have some device stepping down the voltage to something it can handle. This is usually done with a transformer, which operates on induction. A transformer can increase or decrease voltage (step up or down) by decreasing or increasing the current, respectively... usually at acceptable efficiencies, though that can vary by a lot of factors. After the step down stage, you will get a rectification stage where the AC voltage is turned into DC (direct current), which is what most devices use internally.


As explained in #1, since the amperage rating of a voltage supply is the maximum rating of current that can be drawn, then you would be fine as the voltage output is the same and the new supply meets or exceeds the previous. Though for obvious reasons, it is much preferred for you to at least have a vague idea of the requirements of the device. Sometime under-voltage is just as bad as over-voltage.

  1. The 20A rating is the maximum allowable current. If you connect something that requires up to 20A you are safe, more than that and you can fry the outlet. Please note that 20A might be the outlet rating, while the wires have a lower maximum rating, or maybe a switch, or whatever.

  2. What's inside a power supply is quite complex, try to search for "transformer" or "switching power supply"

  3. Just read this

  • \$\begingroup\$ So if I plug a device that draws 10 amps into a power supply that can deliver 5 amps what happens? Possible fire in power supply assuming it doesn't have some breaker or fuse part of it? \$\endgroup\$
    – Nathan
    Jul 31, 2014 at 20:47
  • \$\begingroup\$ Usually some form of overcurrent protection would be triggered. Edit: Or fire... if not properly designed. \$\endgroup\$ Jul 31, 2014 at 20:50
  1. 20 amps is not the amount it always is. It is the max amperage that can be pulled safely. (before a chance of a fire) Current is pulled from the device/s connected to the plug.

  2. That is what the power supply supplies. It converts it down to 20 volts. (most likely a transformer is used. 2.4amps is the max current draw before the device can fail.

  3. As I said above current is pulled by the device. so having a 5amp power supply means it can handle up to 5 amps of current being drawn.


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