Selecting the correct fuse

Fuses come in various ratings, but the common sizes are 3 A, 5 A and 13 A.

The current flowing through an appliance can be calculated using the equation:

Electric power P = voltage V x current I

P = VI

Once the current is known the next highest fuse rating is chosen.

Example

A toaster has a power rating of 750 W, 230 V.

P = VI

P = 750 W

V = 230 V

750 W = 230 V x I

I = \frac{\text{750 W}}{\text{230 V}}

I = 3.26 A

The normal current for the toaster is 3.26 A. Hence a 5 A fuse would be selected.

A 3 A fuse would “blow” when the normal operating current flowed.

A 13 A fuse would allow dangerously high current to flow and still not blow.

Question

A bed side lamp is rated 60 W, 240 V.

Calculate the size of fuse that should be fitted to the lamp for it to operate safely.

The fuse available are 3 A, 5 A and 13 A.

P = VI

P = 60 W

V = 240 V

60 W = 240 V x I

I = \frac{\text{60 W}}{\text{240 V}}

I = 0.25 A

The normal current for the lamp is 0.25 A. Hence a 3 A fuse would be selected.