The last time you went shopping for an electrical cable, your retailer probably asked you to specify the gauge, which, in turn, sent you to a wire ampacity chart like this one
|Wire Gauge Size||60˚C|
THW, THWN, SE, USE, XHHW
THWN-2, THHN, XHHW-2, USE-2
THW, THWN, SE, USE, XHHW
XHHW-2, THHN, THWN-2
But if you’re new to wire ampacity charts, these tables only added to your confusion because you don’t know how to interpret them. Fortunately, wire ampacity charts are not that difficult to understand. This is what they have to say:
The first column in a wire ampacity chart is the wire gauge. If you’ve seen ‘AWG’ on a cable’s jacket, those letters stand for ‘American Wire Gauge,’ a unit of measurement that reveals a conductor’s physical size.
To be more specific, the AWG points to the thickness. The thickness matters because it shows how much current a conductor can transmit without overheating. AWG, which comes from the United States, is popular because of its simplicity.
You can determine a wire’s ampacity instantly by looking at the AWG, especially if you’re familiar with the NEC’s wire ampacity chart. The wire gauge contains more information that laypeople realize, including:
- Diameter – The gauge will tell you the wire’s diameter in inches. For instance, 40AWG equates to 0.0031 inches. Some contractors prefer the diameter to the gauge because of the unit’s precision.
- Cross-Sectional Area – This is the area of a cross-section taken perpendicular to the wire’s length and measured in square millimeters or square inches. If you want to calculate a conductor’s resistance, the cross-sectional area will help you. A larger cross-sectional area translates into a lower resistance per foot. The AWG can show you the cross-sectional area.
- Resistance – The resistance is the opposition a current encounters as it flows through the wire. Resistance matters because it influences the amount of heat a current generates as it passes through a wire. The resistance, wire length, and thickness go hand in hand. Some people use the wire thickness to calculate the resistance. Others use tables like this one from The Engineering Tool Box, which reveals the wire gauges and their corresponding cross-sectional areas, diameters, and resistance.
- Amps – Most laypeople use the gauge to find the amps. You can go either direction. That is to say; you can use the gauge to find the amps or the amps to find the gauge.
Don’t expect to find this information in a conventional wire ampacity chart (except for the amps). But once you know the gauge, other tables can tell you everything you need to know. You can also calculate the other variables.
The ampacity and wire size are intimately intertwined. The first column of a wire ampacity chart shows the wire size. Every other column shows the ampacity of each gauge under different conditions. Most laypeople will only open a wire ampacity chart to examine the wire gauge and ampacity.
This is why the majority of wire ampacity charts online only show the AWG and amps. The other variables matter to engineers and contractors. But a homeowner trying to select a suitable gauge for their extension cord is primarily interested in the gauge and ampacity, which is why tables like this one
Their simplicity makes them easier to interpret. But what is the ampacity, and why does it matter? This is what you should know:
- The ampacity is the current-carrying capacity of a conductor.
- Many people describe the ampacity as a representation of the current a conductor can transmit, but that is not necessarily true. Contractors look for the ampacity in a wire ampacity chart because they want to know the amount of electricity a wire can transmit continuously while staying below its temperature rating.
- The worst outcome is to force a conductor to carry so much power that it catches fire. In other words, the objective is to prevent overheating. This is why safeopedia has emphasized the ability to dissipate heat while defining a conductor’s ampacity.
- You can already see how these factors bleed into one another. For instance, ampacity is concerned with the conductor’s ability to transmit a current without overheating. And as you now know, resistance influences the amount of heat a current generates as it flows through a conductor.
Selecting a wire gauge means taking all these variables into account. Most of the time, the ampacity is out of your hands. In fact, you typically consult the wire ampacity chart because you know the application’s ampacity, and now, you need to find the corresponding gauge.
Have you seen the row at the top of the chart, just below the row that shows the wire materials (copper/aluminum)? This row is hard to miss because it has seemingly random combinations of letters such as ‘THWN’ and ‘UF-B.’ What do those letters mean? Do they matter?
Those letters reveal the cable types. You can’t buy an electrical wire from a retailer by simply asking for an ‘Electrical wire.’ The retailer will ask you to specify the type because it affects the attributes you will get. These three types are the basic options consumers usually encounter:
If you’ve heard the term ‘Romex,’ it refers to nonmetallic cables (NM). People think Romex is a type of cable, but it’s actually a brand. It has two or more wires inside a plastic sheath. You find this type in outlets and light fixtures in sizes ranging from 14AWG to 6AWG.
If you’ve noticed NM-B in your chart, the B stands for ‘Building.’ Contractors will pull these lines through the wall and floor. They don’t like wet locations.
Many laypeople have never heard of this type because contractors usually bury these cables. Therefore, homeowners rarely encounter them. Each wire has a solid plastic cover. UF cables are expensive, but the hefty price tags are worth it because you don’t need a conduit to run these lines underground.
These are single conductors. The insulation has a unique color that tells you the conductor’s attributes. Additionally, each letter means something. For instance, there’s T for ‘Thermoplastic,’ H for ‘Heat-resistant,’ HH for ‘Highly Heat-resistant,’ W for ‘Wet Locations,’ and N for ‘Nylon-coated.’
They rely on conduits for protection. Contractors use them inside (basements, garages, inside the house, etc.). You may notice additional letter combinations in the table, such as ‘THW’ and ‘XHHN.’ These are all single conductor wires, but the attributes will change depending on the letters. For instance, X refers to a flame-retardant synthetic polymer.
I want consumers to keep phone cables and data wires in mind, but they don’t matter to consumers looking for conventional electrical wiring. You should also consider armored cabling (AC, BX, MC). These lines have a metallic sheath that enhances their durability.
Contractors usually know the wire type they need. They base their decision on the setting. They use the wire ampacity chart to find the appropriate gauge for the wire type they’ve chosen.
The temperature rating and wire type share a row. Those numbers you see above the wire type refer to the temperature rating. Your options include 60, 75, and 90 degrees C. A higher temperature rating typically translates into a higher ampacity.
This makes sense once you remember that the ampacity concerns a wire’s ability to dissipate heat. Ambient temperature and the number of current-carrying conductors determine the rate at which heat is dissipated.
The number of current-carrying conductors may come as a surprise because it doesn’t sound like it should matter. However, the more conductors you have in an enclosed space, such as a conduit, the more ambient heat you will encounter.
The experts at iconn systems have noted that contractors take the ambient temperature into account when determining ampacity. The role of the ambient temperature is easier to understand. A higher ambient temperature translates into a slower transfer of heat out of the cable.
The temperature rating is one of the reasons why laypeople consult electricians when selecting a cable. You need an expert to analyze the conditions in your setting before identifying a suitable temperature rating because the environment affects the temperature. For instance, do you want the wire to run in a wall where ventilation is poor or out in the open where the heat can dissipate quickly?
The top row looks at the material. Notice that it only mentions copper and aluminum. These are not the only conductive materials in the world. But where conventional electrical lines are concerned, copper and aluminum are your only options.
You will notice that the ampacity is higher in the ‘Copper’ columns. This is because copper is more conductive than aluminum. Aluminum’s current-carrying capacity is lower by 40 percent. It also offers greater resistance.
Therefore, people flock to copper. But if you can’t afford the hefty price tags associated with this material, a wire ampacity chart will show you an aluminum cable whose gauge and amp rating match the copper lines you want to replace.
Aluminum can work in place of copper if you select a higher gauge. But you must take the wire type and temperature ratings into account.