Solid vs. Stranded Resistive WireYour specific application will determine whether you should use solid or stranded resistive wire. Here are several considerations. - Stranded wire is preferred when it will be put under mechanical stress or movement. |
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Wire Manufacturing Basics
Wire is fabricated by drawing the raw material – rods of copper, aluminum or other metal alloys – through a series of dies. A typical rod, perhaps 8mm in diameter, is drawn (pulled) through a series of dies. Each die gradually narrows until, at the final die in the series, the exit is the desired diameter of the finished wire.
As the rod passes through each die, it stretches. The degree of stretching a given metal can withstand without breaking is referred to as its ductility. Ductility is measured as a percentage of elongation. For example, if a wire sample two inches long is stretched to three inches before breaking, its ductility is 50 percent.
In the United States, wire is manufactured in various gauges (which indicate the diameter of the wire) using the American Wire Gauge (AWG) standard. The AWG notations run from 00000000 also notated as 8/0 (0.7314 inches in diameter) to 56 (.00049 inches in diameter). Higher AWG numbers refer to thinner wire diameters.
A bundle of wire can be twisted together to make a stranded wire or cable. The manufacturing process uses automated machinery to twist a specific number of single wires together in a spiraling formation.
The roundness of each strand of wire requires that the twisting be done with specific numbers of individual wires. For example, beginning with a single wire (shown in blue), an additional six wires (yellow) can be twisted together to create a stranded wire. Adding an additional 12 wires for a total of 19 strands surrounding the center wire results in the image shown.
Solid wire is less costly to manufacture than stranded wire. It is used when the wire will remain in a fixed position and flexibility is not needed. Stranded wire delivers far more flexibility than solid wire of the same gauge. It is used when the wire will be subjected to movement as in household extension cords, the wires that connects your mouse, webcam and keyboard to your PC, wiring to moving parts of industrial machinery, outdoor power lines and similar applications.
As the wire gauge increases, the capacity of the wire to carry more current increases proportionally. For applications where high current and voltages are needed, several stranded wires can be twisted together to create cables with very large cross sections.
Resistive Wire
When an application calls for excellent conductivity of electrical current, wire made of copper, platinum, gold or silver are used. They all pose very little resistance to the flow of electrical current due to the mobility of electrons in their outer shells (the conductance band). Other metals with fewer available electrons in their conductance band resist the flow of electrons. Applying current to wire made of such material causes the wire to heat up. Just as the tungsten filament in a conventional light bulb becomes hot, special wire alloys have been developed to produce heat that's needed for various industrial and consumer applications.
Your hair dryer, your toaster, electric oven and stove all use resistive wire components. The most common and least costly resistive wire is Nichrome, an alloy of nickel, chromium and iron first patented in the early 20th century. Nichrome 60 contains 60 percent nickel, 16 percent chromium with the balance being iron.
Nichrome 60 melts at 1350°C (2462°F), giving it a practical maximum operating temperature of 1150°C (2100°F). The actual temperature reached is dependent upon several factors.
Voltage
Greater voltages force increased current flow through the wire, causing its temperature to increase. Using a variable transformer known as a Variac, you can adjust the voltage and current flow continuously to reach the precise temperature needed for a given application.
Length
How much wire do you need? All wire poses some degree of resistance to the flow of current. The longer the wire, the greater resistance. When high temperatures are needed but the physical space you have to work with is limited, you might want to wind resistive wire into a coil to increase its overall length.
Most electric hair dryers consume 1,500 Watts of power. With a 120 VAC supply to the dryer, the current flow is 12.5 Amperes – well within the rating of a typical household 20 Amp service. However to deliver enough heat to dry your hair, a quick look inside your dryer will reveal coiled resistive wire: a long length is needed to produce enough heat. On the other hand, a typical toaster consumes up to about 750 Watts.
Single strands of resistive wire stretched between insulating supports and un-coiled are all that's needed to turn bread into toast. Industrial applications follow the same rationale. A bending table for forming acrylic plastic needs only a single strand of uncoiled Nichrome, while an electric kiln used to melt metal or fire pottery uses many lengthy coils of wire.
Temperature
As resistive wire heats up with the passage of electrical current, its resistance increases. This increase is known as the wire's resistivity factor.
Resistivity Factors for Nichrome 60
| Temperature | Resistivity Factor | |
| °F | °C | |
| 68 | 20 | 1.000 |
| 212 | 100 | 1.019 |
| 392 | 200 | 1.043 |
| 572 | 300 | 1.065 |
| 752 | 400 | 1.085 |
| 932 | 500 | 1.093 |
| 1112 | 600 | 1.110 |
| 1292 | 700 | 1.114 |
| 1472 | 800 | 1.123 |
| 1650 | 900 | 1.132 |
| 1832 | 1000 | 1.143 |
Approximate Amperes to Heat Nichrome 60
| Gauge (AWG) | Diameter | Temperature | |||||||||
| Inches | Millimeters | 400°F (205°C) |
600°F (316°C) |
800°F (427°C) |
1000°F (538°C) |
1200°F (649°C) |
1400°F (760°C) |
1600°F (871°C) |
1800°F (982°C) |
2000°F (1093°C) |
|
| 18 | 0.04 | 1.016 | 4.8 | 6.5 | 8.2 | 10.1 | 12.2 | 14.8 | 17.7 | 20.7 | 23.7 |
| 20 | 0.032 | 0.813 | 3.8 | 5.1 | 6.3 | 7.6 | 9.1 | 11 | 13 | 15.2 | 7.5 |
| 22 | 0.0253 | 0.6426 | 2.9 | 3.7 | 4.5 | 5.6 | 6.8 | 8.2 | 9.6 | 11 | 12.5 |
| 24 | 0.0201 | 0.5105 | 2.21 | 2.9 | 3.4 | 4.2 | 5.1 | 6 | 7.1 | 8.2 | 9.4 |
| 26 | 0.0159 | 0.4039 | 1.67 | 2.14 | 2.6 | 3.2 | 3.8 | 4.5 | 5.3 | 6.1 | 6.9 |
| 32 | 0.008 | 0.2032 | 0.68 | 0.9 | 1.13 | 1.36 | 1.62 | 1.89 | 2.18 | 2.46 | 2.76 |
Gauge
The greater cross section of a wire, the more free electrons it has to flow. 12 AWG wire is a better conductor than 36 AWG wire, for example. Consequently, as you choose resistive wire for your application, be sure to check the specifications for the gauge your project requires. Below you'll find resistance per foot at room temperature for Nichrome 60.
| Gauge (AWG) | Resistance (Ohms/ft. @ Room Temp) |
| 18 | 0.4219 |
| 20 | 0.6592 |
| 22 | 1.055 |
| 24 | 1.671 |
| 26 | 2.67 |
| 32 | 10.55 |