I have a 12Volt 20Amp 240Watt regulated power supply to use for a table top pinball game - comprising of solenoids (2@ 12V 8A peak), some LEDs and a micro controller. The unit has an internal fuse but I also need a fuse for the AC IEC socket (which has an on-off switch and separate fuse) on the back of the cabinet, and also one for the main wall plug.

If fuses are rated approx 125% of the normal operating current:

(240W/240V) * 1.25 = 1.25A

Would a 3A fuse be fine for both the socket and the wall plug, or should I use a different combination?

  • \$\begingroup\$ Must use a slow blow but yes 3A SB is ok on AC \$\endgroup\$ Dec 29, 2018 at 15:53
  • \$\begingroup\$ Thanks @TonyEErocketscientist - does that apply for both the switched socket and the plug too? \$\endgroup\$ Dec 29, 2018 at 16:04
  • \$\begingroup\$ I'd take Tony's advice, but just for background, you want a fuse to be set at the lowest possible value that will never cause a nuisance trip. Slow blow fuses allow a fuse to have a lower overall rating while allowing surge currents from things like solenoids and motors to pass without tripping. Electrical and manufacturing codes will indicate maximum settings for overcurrent devices, and the formula you are quoting is similar to those found in the Canadian Electrical Code, however be aware that the best value may be lower than the formula indicates. \$\endgroup\$
    – K H
    Dec 29, 2018 at 16:27
  • \$\begingroup\$ The overcurrent device, at minimum and in every case, should protect the downstream conductors, so if the load is unknown, as with a receptacle circuit in a dwelling for instance, you would instead rate based on the smallest downstream conductor, so a 20A receptacle on 15A wire would use OC of no more than 15A, and a 15A receptacle on 50A wire(silly, but just for the example) also requires overcurrent of no more than 15A. \$\endgroup\$
    – K H
    Dec 29, 2018 at 16:31

1 Answer 1


The biggest primary surge is power on. Normally PSU's will store enough energy for 1 cycle dropout. If solenoid pulse is near this duration yet <= 10% of the primary voltage, it will also be <= 10% of the 2ndary current, so this is trivial compared to surge for 1 cycle of storage power on startup that may or may not be much more than 3x the 240W.

The fuse isolates a fault and protects the wire and maybe some thermal failures, but cannot protect the electronics from every failure mode.

A good PS design protects itself so nothing needs to be added. CHeck the specs and suggested protection as required or your regional rules.

I suggest if you want to isolate a failure on a breaker, the PS ought to have a fuse recommended by the supplier. 3A Slo-blo may be suitable. I did not check elsewhere.


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