Let’s get this out of the way; wire sizes can be confusing.

Some countries have different ways of measuring electrical wire size, making it tough to standardize one set of measurements. If you’re in the United States or Canada, you’re probably familiar with the American Wire Gauge (AWG) wire sizes. In other countries overseas, like Britain, you might see the now withdrawn Standard Wire Gauge (SWG) or the more widely used IEC 60228, the dominant gauge sizing system found in most of the world.

We’ll stick with AWG for this article since that’s the gauging Kris-Tech uses to produce its copper wire products.

How Does Wire Gauging Work?

Copper wire is sized using the American Wire Gauge, or AWG. But what do all the numbers mean?

A wire is sized by cutting a cross-section of AWG-sized wire and measuring its diameter. Size charts then correspond to the wire’s diameter using a scale ranging from #40 (a super fine wire with a diameter of .00314 inches) up to 0000, or 4/0, which is .46 inches across.

When a wire is larger than 0000, it has to be measured in MCM (thousands of circular mils), or kcmil. MCM and kcmil can be used interchangeably, but most people are familiar with MCM measurements. MCM sizing starts at 250 kcmil – slightly larger than a 4/0 wire.

Understanding how AWG sizing works goes a long way toward helping answer other common electrical questions, including how much current it can safely handle.

Wire gauges also relate to carrying capacity, telling us how much current a copper conductor can safely carry without damaging itself or its insulation. When electricity is carried by a conductor, it generates heat as a by-product. As heat is generated, it needs somewhere to go – if it doesn’t dissipate fast enough or if too much heat gets built up, it could damage the wire or its insulation.

Besides the heat that naturally builds up in a conductor during regular use, other factors can impact the ampacity of a wire, including where you install them. The ambient temperature of the air around the cable can dictate how the conductor and insulation perform, especially in tight spots like conduits where wire size is critical to the installation’s overall safety.

How do you prevent heat issues from impacting conductors? See if the wire you’re using for the job has a higher ampacity than what you expect the wire to carry. This way, there is less risk of the conductor overheating, allowing it to safely carry current and maintain its durability for longer.

What Is a Mil When Measuring Copper Wire?

When someone is measuring the diameter of a piece of copper wire, you might hear it measured in mil or mils.

If you’re trying to convert mils to inches, a mil equals 1/1000th of an inch, or about 0.0254 mm. Mils should not be confused with MCM, which stands for thousands (M) of circular (C) mils (M). It might seem confusing, but you use MCM to find the area of a circle whose diameter is measured in mils. A circular mil, by definition, is the area of a circle with a diameter of one mil.

Because MCM measures area and mils measure diameter, converting a wire from MCM to mils involves several formulas and added steps.

When ordering large gauge copper wire, you’ll typically see it measured in MCM. This sizing ranges from 250 MCM on the small side to 2,000 MCM on the large end of the scale.

How Do You Convert MCM to mm2?

It’s not a perfect conversion, but a simple rule of thumb makes it easier to convert MCM measurements to mm².

If you’re trying to figure out the area of a cross-section of copper wire measured in MCM, remember that 2 MCM equals roughly 1 mm². Drilling down even further, 1,000 circular mils (1 MCM) equal .5067 mm.

Putting this to use with a real-world example, a 500 MCM (or kcmil wire) will measure about 253 mm², though it might be short by a small amount.

Don’t Let AWG Sizes Confuse You

Unlike clothing sizes which can vary wildly from retailer to retailer, AWG sizing is standardized across all wire manufacturers in the U.S. and Canada.

Rest assured that the wire you buy from one company will be the same size as copper wire bought from another manufacturer. The uniformity helps keep workplaces safer by ensuring the wire performs up to the standards needed on the job site long after it has been installed.

Knowing how to convert wire sizes also helps determine how much ampacity a conductor has, making it less likely for problems to occur. Those conversions also make it easier to figure out how much space a conductor will take up within a conduit or other raceway, based on NEC guidelines. (For a refresher on conduit fill percentages, click the link.)

Next time you order wire, don’t forget to consider sizing, ampacity, and fill rates. It might save you from ordering the wrong product or setting yourself up for problems down the road.

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