# Power Rating of Resistors

Power rating of a resistor indicates the maximum heat energy that a resistor can dissipate in the surroundings without fail. The heat dissipation depends on the power rating and the ambient temperature. The higher ambient temperature cause less heat dissipation, leading burning of resistor.

In this article, we will discuss about the resistor power rating, its formula, and significance. As we know, resistor is an electric circuit element which generates heat when an electric current passes through it. In a resistor, the amount of heat generated depends on the current flowing through it. In other words, we can say that a resistor converts electrical energy into heat.

The heat dissipation in the resistor is a wastage of electric energy, however you can not avoid this. When current flows through the resistor, the entire length and cross section area of a resistor creates hinderance to electrons and heat dissipation takes place in the resistor. Yes, for some of the applications like water heater, space heater, the resistor’s property of the converting electrical energy into heat is useful.

## What is Resistor Power Rating?

The measure of capability of a resistor to convert electrical energy into heat is termed as power rating of the resistor or wattage rating of resistor.

In practice, the resistors are rated in terms of watts (W), and the value of the resistor specified in terms of watts is the maximum amount of heat that the resistor can dissipated without getting damaged.

For every resistor, there is a maximum value of electric power is specified that the resistor can handle safely. More specifically, the wattage rating of a resistor depends on the physical size of the resistor.

As it is a fact that if the resistor has a larger physical area, then it can dissipate higher amount of electric power, and hence higher will be its power rating.

For the safe operation of a resistor, the amount of heat generated by the resistor must not exceed its power rating. The power rating of a resistor is also termed as a wattage rating of the resistor.

The wattage rating of a resistor can also be defined as the amount of heat energy that the given resistor can dissipate infinitely by radiation, convection and conduction without getting damaged or decreasing in its efficiency.

You would have observed that the temperature across the resistor’s body is not uniform. You will find the resistor’s body hotter than its terminal, and the maximum temperature at the center of the body. The temperature rise of resistor must not exceed the specified permissible value. The temperature rise above rated value cause increase in the resistance value and affect the circuit performance. Therefore, heat dissipation to environment is a important factor for a reliable operation of resistor.

In actual practice, most real resistors have their wattage rating specified for a surrounding temperature of +70 °C or lower.

## Units of Resistor Power Rating

As we already mentioned that the resistor power rating is usually expressed in watts (W). However, there are several other smaller or larger units that are also being used for specifying resistor’s wattage rating.

The following table gives some most commonly used units for expressing power rating of resistors in practical applications:

## How to Calculate Resistor Power Rating?

Now, let us discuss how we can compute the wattage rating of a resistor.

There are several different ways of calculating the power rating of a resistor. The simplest one is Ohm’s law. The formula for calculating the wattage rating of a resistor can be derived using the Ohm’s law as follows:

Let us consider a resistor subjected to a voltage of V volts and an electric current of I amperes flows through it. Then, the resistor wattage can be simply given by the product of voltage across and the current through the resistor, i.e.

Also, as per ohm’s law,

Where, R is the resistance of the given resistor in Ohms.

Then, the resistor power rating will be,

And,

Using any of these three formulas, you can easily calculate the wattage rating of the resistor. The wattage rating is calculated in the below circuit diagram.

In practical applications, we always select a resistor which has a power rating two or three times of the calculated power rating for its safe operation in the circuit.

## Types of Resistors and Their Power Ratings

The following are some common resistor types used in electrical and electronics circuits, here we have specified their power ratings:

• Metal Film Resistors – These resistors have very low power rating, typically less than 3 Watts with relatively high stability. Hence, they are widely used in small electronic circuits.
• Carbon Resistors – The carbon resistors also have low wattage rating, usually less than 5 watts, but they have lower stability than metal film resistors. These are also suitable for small electronic circuits.
• Wirewound Resistors – The wire wound resistors are considered high power resistors and also provide high resistance stability. Therefore, they are used in high power electric circuits.
• Power Resistors – Power resistors are special type of wire wound resistors designed to handle high amount of power up to 1000 watts or more. These resistors are made to have larger physical area to dissipate high amount of heat generated. They are widely used motor control circuits, electromagnets, motor braking circuits, and more.

## Resistor Power Derating

As discussed, the nominal wattage rating of resistor is specified at a certain ambient temperature. For example, power rating of a resistor is 10 watts at 75°C, and if you operate it at the ambient temperature of 90°C, does the resistor capable to dissipate the amount of heat generated in its body? No, the heat transfer rate will get slow and resistor will eventually fail. You will observe that the resistor’s body get more hotter at 90°C. This effect is known as derating.

Now, if you want to use the same resistor at increased temperature at 90°C ambient temperature, then you need to reduce the amount of heat generation by reducing the current flowing through it. Thus, the heat dissipation rating of the resistor decreases with rise in ambient temperature as shown in the below graph.

The other factors that impacts the resistors wattage deratings are as follows.

• The electrical circuit is enclosed in a enclosure. The heat transfer takes from the resistor’s body to enclosure wall to ambient temperature. Thus, an enclosure creates hinderance to flow of heat. The design parameters of enclosure like its size, shape ,material, wall thickness and orientation affects the heat transfer rate.
• The types of cooling arrangement affect the wattage rating. The natural air circulation has slower heat dissipation than the forced cooling.
• When the resistors are installed close to each other, the resistor receive the heat of other resistors and affect the heat transformer. The resistors can be installed sparsely in a circuit to improve the heat transfer rate.
• At altitude, the heat transfer happens only through radiation and above 30 KM the heat transfer rate gets adversely affected.

## Solved Problems of Wattage Rating

Now, let us consider some numerical examples to understand how to calculate wattage rating of a resistor.

### Numerical Example(1)- A resistors is subjected to a voltage of 24 V and a current of 2. A flowing though it. What is the maximum power rating of this resistor.

Solution – Given,

V = 24 V
I = 2.5 A

The maximum wattage rating of the resistor is,

P = V × I
= 24 × 2.5
P= 60 W

### Numerical Example (2) – Calculate the maximum power rating of a resistor of 100 kΩ when 220 V supply source is connected across it.

Solution – Given,

R = 100 kΩ
V = 220 V

The maximum wattage rating of the resistor will be,

Hence, this is all about resistors power rating, its formula and significance. Every resistor has a specified wattage rating that must not be exceeded to avoid failure of the resistor.