Circuit amps vs device amps

Question

I've always struggled with some really basic questions around amperage. Like, if you're plugging a 2 amp device into a wall socket, but that circuit is a 20 amp circuits why doesn't it blow up?

Then I realized, and want to ask this question: In reference to circuits in a building, like a standard 20a 120v circuit, is that referring to amperage capacity and the amperage on my phone charger is referring to actual amperage drawn? If so, why is it never explained?? I know that is really simple but I've never understood it and wish somebody had said that to me years ago.

Basically, a 20a circuit can provide UP TO 20 amp, and a device draws what it says it requires.

Actually, it's explained over and over -- at least, I'm finding myself explaining it over and over. Note that this applies to wall-warts as well as wall sockets -- a 5V, 2A wall wart is capable of supplying 2A, but what it does supply is 5V at whatever current the load pulls (but it shuts itself down or otherwise enters a fault condition if the load pulls more than 2A). — TimWescott, Mar 07 '21 at 18:29
While this information is in the public domain, it is, evidently, something that can be difficult to grasp and generally requires a certain amount of discussion and explanation. Another gem of wisdom is that the flow of electrons is called current rather than amperage :-) — Frog, Mar 07 '21 at 19:14
Just because noone has specifically mentioned it, I(current)=(equals/is determined by)E/R(Voltage divided by resistance, or the ratio between voltage and resistance). That's Ohm's law, one of the first formulas for electricity. It gets a bit more complicated, like if there is inductance or capacitance in a circuit we start talking about impedance instead of resistance, and for complex electronic loads drawing a certain amount of power, we can discuss "effective resistance". — K H, Mar 08 '21 at 04:53
When electronics are involved, loads can also draw less current as voltage increases. Many lighting ballasts, for example, are designed to work from 110V to 277V. They maintain a constant wattage output, so when source voltage increases, they actually draw less current to maintain the output. — K H, Mar 08 '21 at 04:54

score 1 · Accepted Answer · answered Mar 07 '21 at 19:11

Basically, a 20a circuit can provide UP TO 20 amp, and a device draws what it says it requires.

That's it. You've answered your own question.

To add on an extra layer - that is true for any constant-voltage distribution system; AC, DC, whatever. The way to analyze it is that in such a system, the output voltage of a theoretically perfect voltage source never changes no matter what the load is, and the output impedance of the source is zero ohms, or a very low number. The output impedance is essentially a very small resistor in series with the output of the perfect power source. The complete circuit is the voltage source, its output impedance, and the effective impedance of the load device (light bulb, vacuum cleaner, whatever), all in series to GND.

Thus, the source's output impedance and the load form a voltage divider; the midpoint of the divider is the effective output voltage of the source as seen by the load. With Ohm's Law, you can see that the load impedance must be very low before the voltage it sees begins to sag. This means that the current through the circuit is almost completely dependent on the impedance of the load, from a cell phone charger all the way up to an electric clothes dryer.

There are other types of systems, such as constant current and constant power, but those are not nearly as common, and more complex to analyze.

I'd say constant current is very common, actually--if you include things internal to integrated circuits, at least. Current sources are very handy in all sorts of analog circuitry. — Hearth, Mar 08 '21 at 16:12
Handy, but ((relatively)) rare, compared to the zillions of voltage amplifier circuits in the world. AND, the thread is about the "basic" aspects of a constant-voltage, medium-power distribution network, not opamp internal circuitry. — AnalogKid, Mar 08 '21 at 16:55

score 0 · Answer 2 · answered Mar 07 '21 at 18:58

Breaker Current rates are not to be exceeded by the sum of all the nameplate ratings on each device. This is how circuits are organized and balanced.

However each device might not be drawing rated current at startup or steady-state with no load which determines the actual current, NOT THE SUPPLY> If you short the protected AC supply at home, you can expect up to 10kA until it trips and your screwdriver (shorting the outlet) to explode with sputtered copper and sparks all over your safety glasses (assuming you were not in the Darwinian Awards Club)

Circuit amps vs device amps

2 Answers2