I have a circuit with 30 computer fans and a DC power supply and they seem to turn on and run well.

The fans take 12V and their rated current is 0.18A. My power supply is 12V 5A.

By my math, this would give each fan actually only about 0.16A, not the 0.18A they are rated for. But when I measure it, it's only about 0.13A per fan in practice

So theoretically the fans should be trying to use more current than the power supply can provide (5.4A total vs 5A) but in practice it seems like they are only using 3.9A. Is this safe?

Wondering why my fans are turning on and seeming to run normally with less current than they're marked for?

Sorry if this is missing key info and or dumb, I'm new to hardware!

  • 1
    \$\begingroup\$ Starting current can be different to running current. \$\endgroup\$
    – Solar Mike
    Aug 30, 2022 at 9:13
  • \$\begingroup\$ depends what's inside the fan. usually there is a sort of a driver, that may either ramp up gradually (you will never see full current) or push more than the rated current on startup. Either way, take some margins, never be on the edge. \$\endgroup\$
    – TQQQ
    Aug 30, 2022 at 9:14
  • \$\begingroup\$ As you discovered, there also isn't a dwarf inside the power supply who shuts off the power if it goes above 5A (there could be a fuse but fuses are not precise and often don't blow if the overload is quite small) and/or there isn't a dwarf inside the fan who keeps the current at 0.18A all the time. These are "nominal" parameters - basically, they are what the designers had in mind when they designed the thing \$\endgroup\$
    – user253751
    Aug 30, 2022 at 9:42
  • \$\begingroup\$ Most nameplates gives worst case continuous current, not nominal. \$\endgroup\$
    – winny
    Aug 30, 2022 at 10:11
  • \$\begingroup\$ Power used by a fan may vary depending on backpressure, too \$\endgroup\$
    – bobflux
    Aug 31, 2022 at 9:25

1 Answer 1


In general, when you see ratings for a load that is supposed to run with a nominally constant voltage, such a computer fan, the rated current is the MAXIMUM current that the load can draw in the worst operating condition, taking into account every variation of, for example:

  • manufacturing tolerances (sample variations)

  • temperature

  • humidity

  • mechanical load

Just to say a few factors that could influence the actual absorbed current.

This means that that 0.18A is the max current you are expected to provide to the fan in the worst operating condition (not when, for example, it is stalled because you blocked the rotor with a pencil).

Your measured 0.16A is well within specification.

Moreover keep in mind that that 0.18A current rating has been established by the manufacturer (usually by design and then confirmed by sampling their production batches) in order to give you a sort of guarantee that any specimen of that fan will never draw more than 0.18A at exactly 12V (the rated voltage), even if the typical fan you use could draw a little less.

Anyway, note that you measured just a 20mA difference from the rated current, which is about a 10% difference. That's more than usual for electric/electronic components not meant for precision applications, where tolerances of around 10% (or even more) are very common.


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