Heating WireResistance heating wire is used in a variety of applications for generating heat. Household use can be found in toasters, portable space heaters, warming plates and more. Stove burners are an example of an electrical element used to create heat. Industrial furnaces and dryers use wire elements to produce heat. Ceramic materials are often used as an insulator to encase the wire. |
American Wire Gauge (AWG)
It is helpful to understand the AWG system when working with heating wire. As the gauge number of the wire goes down, the diameter size increases.
Gauge (AWG) | Diameter (in.) | Diameter (mm) |
16 | 0.0508 | 1.291 |
18 | 0.0403 | 1.024 |
20 | 0.0320 | 0.812 |
22 | 0.0253 | 0.644 |
28 | 0.0126 | 0.321 |
30 | 0.0100 | 0.255 |
Relationship Between Resistance & Temperature
Heat is produced when electric current meets resistance. The heat is a loss of power over the circuit. Energy does not disappear, it changes from one state, or form, to another. The energy or power lost in a circuit becomes heat. The resistance produces thermal energy, felt as heat.
Resistance increases linearly with temperature. The higher the temperature, the higher the resistance. For example, if you double the length of a piece of wire, the resistance of the wire will double. If you double the diameter, moving to a larger wire, the resistance will be cut in half. If the resistance of an element, increases or the current increases, the temperature will increase.
Calculations
The relationship of the reaction of producing energy and creating heat is known as Joule's first law. Joule's law states that the rate of heat produced by a steady direct current is directly proportional to the square of the current and the resistance of the circuit. This is the same as the formula for power, P = I2 x R, or current squared times resistance. If you have two amps with 100 Ohms of resistance, you would have 400 Watts.
As applied to heating, the heat produced can be expressed in calories. H = I2 x R x t. The "t" symbol stands for the amount of time the current is flowing. Note: One calorie = 4.184 joules.
Resistance = rho L/A. rho is the constant resistivity of a given material. L is the length and A is the cross sectional area.
Example Calculation
Nichrome, specific heat capacity = 450 J/kg C
Using 800 volts on 48 ohms = 16.6 amps
P = 16.6 amps squared times 48 ohms = 13,227 watts
1 W = 1 J/s
A 1 kg resistor made of Nichrome receiving 13.3 kW will have an increase in temperature of 29.6°C for every second of power applied.
Now take this factor of 29.6°C and divide it by the actual mass of the resistor to determine the temperature in Celsius per second. Example: 2 kg of Nichrome will increase 14.8°C per second. The increase will continue until it reaches a maximum or power balance. This does not take into account for heat lost due to convection.
Why is Resistance Wire Used?
Heating elements must be manufactured to withstand the extreme heat they are required to generate. The elements must also hold up to environmental factors, including moisture, which can cause corrosion. Heating wire has a high resistance and resists oxidation. It is able to withstand a high surface load. Other considerations that make wire beneficial are its ability to resist sagging and deforming while being light in weight.
Alloy | Resisitvity at 20°C (68°F) Ω mm²/m (Ω/cmf) |
Max. Continuous Operating Temperature |
Nichrome 60 | 1.11 (668) | 1150°C (2100°F) |
Kanthal A1 | 1.45 (872) | 1400°C (2550°F) |
Kanthal D | 1.35 (812) | 1300°C (2370°F) |
Kanthal A-1A-1 wire is often used for industrial applications. It can be found in heating elements for high temperature furnaces used in glass, steel and ceramics industries. Kanthal heating wire provides a consistent resistivity in all order runs for to facilitate production. Kanthal DKanthal D wire is used in both household and industrial applications. In the home it is often used for heating elements in dishwasher, embedded in ceramic for heating panels and used heating cables. Heating cables are wrapped around or run along plumbing pipe to prevent freezing. Our heating wire charts are available to provide you with the resistivity of each type of wire. By calculating the voltage you will be applying, you will obtain the current for your element. Using the current and the resistance, you will can determine the power or wattage. Adding in resistivity and mass will provide you with temperature. |