Let’s face it; there are a lot of numbers to keep track of when we’re discussing wire and cable. 

Understanding the numbers associated with wire is sometimes like learning a new language. But knowing them and how they impact an electrical installation can be the difference between a safe project and an unmitigated disaster. 

So, what do we need to know when talking about electrical wire, and how do the different pieces of information relate to one another? 

Every wire installation requires a few critical pieces of information: gauge size, amperage, and voltage. Without them, it’s nearly impossible to safely complete a project. 

But how do the three measurements relate to each other, and what do they mean? 

What is Gauge Size? 

Gauge size is a measurement used to determine an electrical conductor’s size. Based on your location, wire can be measured using either the American Wire Gauge (AWG) or the International Electrotechnical Commission’s (IEC) standards. 

In North America, gauge size is measured using the AWG system, with each step corresponding to a specific inch/millimeter diameter. Sizes range from 40 to 0000, commonly seen as 4/0, before moving into MCM (thousands of circular mils) measurements.  

Remember that gauge size and wire diameter/area have an inverse relationship when measuring conductors using AWG sizing. As the gauge number gets larger, the diameter and area of the wire are getting smaller. For example, a #10 AWG wire has a larger diameter than a #20 AWG one. 

Although the system seems confusing at first, it’s easy to use AWG to find the diameter of any given wire – plug the gauge size where “n ” is in the formula: 

D(sub n) = 0.005 in. x 92(to the 36-n/39) 

If you’re measuring in millimeters, replace 0.005 in. with 0.127 mm. 

The gauge can also find a wire’s relative diameter or cross-sectional area. Using the American Wire Gauge system, each six-gauge decrease doubles the diameter of the wire – for example, this means a #14 AWG wire is double the diameter of a #20 AWG wire. 

The same can be said for the cross-sectional area – in this case, a three-gauge decrease will double the area of a wire.  

Wire Gauge and Amperage 

Now that we know what a wire gauge is and how to perform conversions, we can jump into how they affect ampacity. 

Ampacity is a measurement dictating how much current-carrying capacity an electrical wire has. Like how AWG size is inversely related to diameter and area, amperage decreases as AWG numbers increase. 

Think of it this way: you wouldn’t attach a garden hose to a fire hydrant because the pressure and amount of water would be too much for the hose to handle. You probably also wouldn’t wire a house with speaker wire because the gauge and ampacity ratings are far too small to deliver the power needed. 

Understanding how amperage works reduces the risk of a potentially dangerous situation. Conductors can quickly overheat when they can’t handle the current flowing through. Eventually, the heat melts the conductor’s insulation, exposing the bare metal and creating a fire risk. 

How Does Gauge Size Impact Resistivity? 

The easiest way to explain resistivity is driving along the Interstate. 

A six-lane highway can support thousands of cars, quickly getting them where they need to go. Now take the same number of cars and reduce the lanes to two. The vehicles have fewer places to move, forcing everyone to slow down. 

The same concept applies to electrical wire and cable. As the gauge size increases, the amount of resistance, measured in ohms, also grows. This is because the wire is smaller, so the amount of current it can move also decreases. 

Resistance doubles or halves for every 3 AWG you move up or down the scale. In this instance, a #7 AWG would have half the resistance of a #10 AWG, which, in turn, has half the resistance of a #13 AWG.  

Going one step further, whenever AWG is increased or decreased by 10, resistance is multiplied or divided by a factor of 10. Let’s say you have a #2 AWG wire and a #12 AWG wire. Their resistances will be 0.1563 Ω/kft and 1.588 Ω/kft, respectively. 

Let’s Talk More About Amperage 

As previously mentioned, amperage is tied to current flow, determining how much current a conductor can accommodate without damage. 

Amperage is measured in amperes and is impacted by several variables, including metal type and wire size. Copper, for example, has a higher ampacity than aluminum and carries a lower resistivity than aluminum in the same size conductor. Because of this, aluminum conductors need to be larger than copper ones to move the same amount of current. 

Ambient temperatures also affect ampacity. When temperatures rise, resistivity increases while ampacity decreases. Large gauge wires (smaller numbers) have higher ampacity to support current flow. 

The same rules apply to multiple conductor cables, like tray cables. In this case, electrical installers should reference NEC (National Electrical Code) Table 310.15(B)(2)(a) to figure out temperature ratings and ampacity. Installers need to reference the NEC ambient temperature table because conductors give off heat when current passes through them, so the more conductors there are, the more heat gets generated. 

NEC Article 310 guidelines sometimes call for all conductors in the raceway to be derated to limit heat generation and maximize heat dissipation. Derating means operating the wire at less than its maximum capacity when ambient temperatures are more than 30 degrees Celsius and when more than three cables are included in the circuit, like tray cable. 

Where Does Voltage Fit In? 

Voltage measures a current flow’s strength within an electrical system.  

Sometimes called a difference of potential, voltage is calculated using Ohm’s Law. Ohm’s Law is a formula that calculates voltage by multiplying the system’s current (I) by its resistance (R). The formula ends up looking like this: 

V = IR 

Voltage is all about pressure, so if there’s a lot of pressure in a system, there’s also a lot of voltage. Think of it this way – if you open a flat bottle of soda, your drink stays in the bottle because there’s no pressure built up. But what happens when you vigorously shake a full soda bottle before unscrewing the cap? The soda sprays out because you’ve built up pressure in the closed system. 

Addressing Voltage Drops 

Unlike amperage, which is dependent on temperatures and metal types, voltage is affected by resistance and distance. The longer a conductor is and the higher its resistance, the worse the voltage drops. 

Voltage drops are important to keep in mind because every manufacturer has a minimum operating voltage for their product, as seen in NEC code 110.3(B), which requires all equipment to be installed based on the manufacturer’s instructions. The NEC’s rule corresponds with ANSI (American Nationalsaysards Institute) rule C84.1, which says that the minimum voltage should not be lower than 90% of the nominal system voltage.  

For a standard 120V system, the minimum voltage must be above 108V to stay in compliance. This means the wire feeding electricity to the appliance or machine needs to be large enough to maintain the minimum 108V.  

To reduce the voltage drop across the system, you either need a larger wire or something to increase the voltage. You can also change the type of material used in the conductor to decrease resistivity – like moving from an aluminum conductor to a copper one. 

As a real-world example, electrical generation plants step up voltages using transformers to make up for voltage drops as electricity travels through transmission lines from the plant to the substation. Once the power reaches the substation, the voltage will be stepped down several times before making it to homes and businesses. 

In other cases, voltages can be stabilized and supported using other methods, such as voltage regulators, capacitors, load rebalancers, and multiphase conductors. 

One Number, Many Uses 

We often have a lot to think about when dealing with electrical wire, but understanding how gauge sizing works can go a long way during your next installation. 

One piece of information can help you choose the best wire or cable for your installation to prevent fires and protect systems from overheating. Proper sizing also reduces voltage drops, ensures you don’t over- or under-engineer projects using the wrong gauge, and helps you better anticipate how the wire will react in different situations. 

So, while it’s only one number, correct gauge sizes result in better installations, safer projects, and more reliability during the wire’s lifespan.

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