# Rule of thumb for sizing fuses?

#### danton133

Joined Mar 12, 2010
7
Assuming a load without significant inductance or capacitance am I correct in thinking the fuse should be about 1.3 times the expected operating current then use the next available sized fuse above this?

Additionally, at what point is the fuse size too large? What the general rule here? Is it at twice the operating current or 3, 4, 5 times?

I assume this depends on whether it’s being used to protect against short circuit or overload. How would you decided what to use based on protecting from:

• Short circuit, and
Example, I need to protect a electronic device (a 12v counter) from overload. Operating current is 10mA and I’m using automotive style blade fuses. The smallest available is 1A, which is 10 times the operating current, is this too much to offer adequate protection?

Joined Jul 18, 2013
25,979
As a general rule, 1.5x the operating current of the device is the criteria for fusing.
But the type and fuse value depends on what style of protection you need, very fast or can the device tolerate an over current for a short period of time.
If yours is critical at 10ma, then you require a fuse rating close to the consumption of the device.
Use a different style fuse, or modify the automotive style to the suitable value , if needed

#### MrChips

Joined Oct 2, 2009
27,116
1A is 100 times 10mA.
Are you sure you didn’t mean 100mA?

#### dl324

Joined Mar 30, 2015
15,110
Example, I need to protect a electronic device (a 12v counter) from overload. Operating current is 10mA and I’m using automotive style blade fuses.
What are you trying to protect with the fuse? Fuses are generally used to prevent fires, not to protect downstream devices.

#### bertus

Joined Apr 5, 2008
22,094
Hello,

Have a look at the attached selection guide.

Bertus

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#### GetDeviceInfo

Joined Jun 7, 2009
2,118
Generally, you want to protect the smallest conductor in the circuit. Short circuit currents are a consideration for low impedance sources such as transformers.

#### prairiemystic

Joined Jun 5, 2018
290
Sizing fuses according to electrical safety standards gives 150% to 167% of nominal load.
You want to protect your wiring/connectors/PCB traces, so that they do not melt and burn if there is a short.
You want to protect the load from overheating as well.

Automotive blade fuses look at the fuse curves but a 1A ATO will carry just over 1A forever, and 1.5A for 10 seconds. Fuse ratings are generally not when they will open, but what they will carry.
At low current say 50mA, consider using a polyfuse in your project instead.

Joined Jul 18, 2013
25,979
And of course one other thing to consider is the type of fuse, i.e. Sweep-Through speed?

#### danton133

Joined Mar 12, 2010
7
1A is 100 times 10mA.
Are you sure you didn’t mean 100mA?
Sorry I meant the consumption of the device is 100mA not 10

#### danton133

Joined Mar 12, 2010
7
What are you trying to protect with the fuse? Fuses are generally used to prevent fires, not to protect downstream devices.
Thanks for the reply Dennis. Could you explain why this is the case? (that they are fused for fire prevention rather than for device protection)

#### MrChips

Joined Oct 2, 2009
27,116
Thanks for the reply Dennis. Could you explain why this is the case? (that they are fused for fire prevention rather than for device protection)
Without a fuse to break the circuit, a continuous current could cause overheating which can start a fire.

Chances are that any sensitive component has already been destroyed by the over current.

#### dl324

Joined Mar 30, 2015
15,110
Could you explain why this is the case? (that they are fused for fire prevention rather than for device protection)
Regular fuses don't react fast enough to protect sensitive components. For that, you need to employ other, faster acting solutions.