How to Calculate Fuse Rating for Electrical Appliances?

Different Rating's Fuses

Fuse Rating Calculating Guide

The function of a fuse is to break a circuit when a current threshold is exceeded. It’s a single function device, and historically single-use. Calculating the rating of very basic protection equipment fuse, we use the thump rule just select a fuse rated 150% - 200% of normal operating current of the specific circuit. But actually there are numerous calculations are involved for determining an appropriate fuse rating. Frequently necessary to consider other factors like- including ambient temperature, available energy during fault, inrush current, etc.

In order to select the proper rated fuse protective device, the following parameters and criteria need to be considered:

1.    What is the normal operating current of the circuit?

2.    What is the operating voltage?

3.    Is the circuit AC or DC?

4.    What is the operating ambient temperature?

5.    What is the available short-circuit current?

6.    What is the maximum allowable I²t?

7.    Are there in-rush currents available?

8.    Is the protective device being used for short-circuit protection, over-load protection, or both?

9.    What are the physical size limitations?

10.Is the PCB surface mount or thru-hole?

11.Does the fuse need to be "field-replaceable"?

12.Is reset ability an issue?

13.What safety agency approvals are needed?

14.How will I mount the device?

15.What are the cost considerations?

Formula to Calculate the Fuse Rating

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There is a simple and basic formula for working out the fuse rating, voltage or wattage for each appliance:

P=VxI

Where,

P for power in Watts;

V for voltage in volt);

I for current in Amps.

The fuse rating can be calculated by dividing the power used by the appliance by the voltage going into the appliance.

I (Amps) = P (Watts) ÷ V (Voltage).

Fuse Rating Calculation for Motor

A fuse for a machine is rated on the load that the machine carries when running. For example, a 1-HP (746W) motor operating at 115V would draw 746/115 = 6.5A at full load, so theoretically a 10A fuse would be sufficient.

What is Fusing Factor?

Fusing Factor is the ratio of minimum fusing current and current rating of fuse.

Therefore, fusing factor = Minimum fusing current or current rating of the fuse.

The value of fusing factor is always more than 1.

Fuse Size Calculation Formula

Fuse Wire Rating: The melting point and specific resistance of different metals used for fuse wire is as below:

Metal	Melting point	Specific Resistance
Aluminium	240oF	2.86 μ Ω – cm
Copper	2000oF	1.72 μ Ω – cm
Lead	624oF	21.0 μ Ω – cm
Silver	1830oF	1.64 μ Ω – cm
Tin	463oF	11.3 μ Ω – cm
Zinc	787oF	6.1 μ Ω – cm

Many times we have to face some physical places limitation to choose the fuse or circuit breaker mounting sizes.

It is this reason that fuse and circuit breaker manufacturers have created a wide selection of components with varying physical sizes. Typically however, there are a trade-offs that the engineer must consider.

Generally speaking, the smaller the fuse, the less current and/or capabilities that the fuse or circuit breaker may have. For example, a sub-miniature fuse maybe limited to 15A whereas the larger 1/4" x 1 1/4" glass tube fuse can accommodate up to 40A.

Additionally, although the fuse can be smaller, the corresponding fuse holder maybe substantially bigger adding to the consideration.

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