# How damaging are inrush currents?

Typical power supplies will have 30 to 100 amps inrush current at 220v, sometimes less, sometimes over that. How is it that the power strip rated at max 10amps at 220v doesn't get damaged?

Also, if I connect a wattmeter rated similarly at 10amps at 220v, will it get damaged if I plug in my computer to judge the power consumption?

The mechanism of damage for most current limitations is heat. While the inrush current may be very high, the total energy of the event can be quite low, for example if the inrush is a sharp spike. Very small devices such as the actual silicon inside a diode or transistor can still be quite sensitive to these events though, since they can't dissipate heat quickly. The wires in your power strip, or the shunt resistor in your wattmeter, are not very sensitive to short durations of overcurrent.

• In fact you can just look on youtube for demonstrations where people purposefully overload wires to see what happens. It often takes several seconds of severe overload before anything happens at all, and minutes before anything catches fire. Of course small overloads take even longer. Apr 4, 2023 at 13:25
• Whenever inrush current value of a PSU is mentioned it is always done so with the corresponding input voltage. For eg: A PSU might have 40A at 115v and 65A at 220v. And so I thought it must really be 65A at 220v i.e. 14,300W. (I have no knowledge of electronics). Is this correct or does the inrush current take place at a lower voltage so that the power in watts does not exceed the max that the socket can deliver, which is 2200W. Is there some study material which confirms this that you can point me to? Thank you. Apr 5, 2023 at 2:17

Heat is what destroys most components. Heat is generated by power, not by pure current or voltage. Power loss (e.g. in a power strip) happens when there's a resistance in the power strip which causes a voltage drop. Since a power strip consists of quite thick wires, the voltage drop is typically negligible and therefore it doesn't get hot. The 220V 10Amps is the nominal limit for which it is designed, which means you can safely connect appliances that use at most 2200 Watts.

A good power supply has an outrush current in the order of 30 or even 100 Amps, but at a much lower voltage. The product of the voltage and the current must not exceed the 2200 Watts which your power strip safely provides, so a power supply providing up to 100A output cannot do this at more than 22V. A typical power supply for a PC has a rated power output (for different voltages) of between 300 and 500 Watts. So this is well bellow the limit of your power strip as well as the wattmeter.

In fact, the breakers of your house will typically limit the power from a single socket to 10A, so trying to get more power out of a single socket will just blow the fuse and leave you in the dark.

• Hello, thanks for the answer. There are some things I don't understand like the outrush current. Now I guess it is probably the opposite of inrush current. That is, the current which is transferred when the device is switched off. But I don't know how it's relevant to inrush current. Also whenever inrush current value of a PSU is mentioned it is always done so with the corresponding input volatage. For eg: A PSU might have 40A at 115v and 65A at 220v. And so I thought it must be 65A at 220v i.e. 14,300W. (I have no knowledge of electronics). Apr 4, 2023 at 16:08
• From what I understand, you say that the inrush current takes place at lower voltage and so the power in watts does not exceed the max that the socket can deliver, which is 2200W. (Right?). Is there some study material which explains inrush current in detail, that you can point me to? Apr 4, 2023 at 16:12
• The law of electrical power says P = U * I. In an ideal power supply (one without internal loss). the value of P (measured in watts) stays equal on both sides. Hence to output say 20 Watts power at 10V the input current at 220V is only approx 0.05A
– PMF
Apr 4, 2023 at 20:07

Also, if I connect a wattmeter rated similarly at 10amps at 220v, will it get damaged if I plug in my computer to judge the power consumption?

That's an average power rating. Many wattmeters won't explicitly specify the power-vs-time curve for derating. It takes some assumptions like the meter being fit for intended purpose. Inrush current isn't exactly unheard of, so a mains wattmeter that doesn't withstand typical inrush currents would be pretty useless. If anything, the wattmeter should have at least a catastrophic protection fuse built-in, so if it's severely overloaded, that fuse will open and will have to be replaced (it's often soldered in).

In your case, if the wattmeter fails in a typical application you state, it's a pretty lousy wattmeter and either has to be modified to perform better (by someone with relevant training), or tossed out for being useless.

Inrush current from power supplies mostly for led lighting have short duration but high amp.They destroy circuit breaker contacts,switches,timer contacts,contactor contacts and Eazy can trigger C type circuit breakers.This is huge problem in recent years.There is no correlation of energy taken from LEDs but driver construction for multiplayer of nominal current vs inrush current.Suppresion of inrush current and mitigation of the problem can be achieved by design circuits for multiple drivers,inrush current limiters fitted after circuit breakers,double switching with one tungsten contact ect.Yes they are short in duration but huge problem these days.