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Length vs. AWG vs. Ampacity: How Do These Parameters Impact Cable Choice?
A relationship between the length of the cable, its gauge, and ampacity is a common concern for our customers. Let's assess the difference between the three in simple terms.
What are Length, AWG, and Ampacity?
The length of the cable you buy is determined by how much cable you need to get from point A to point B.
AWG, or American Wire Gauge, is a standard measure of cable conductor size used in the United States, North America, and some countries worldwide. A small gauge indicates the large diameter of a cable. For instance, 2 THHN THWN-2 Stranded Copper Building Wire is way more significant than 14 AWG THHN THWN-2 Solid Copper Building Wire. The gauges between 2 and 14 are the most common on the market. When you buy a cable, you always pick a particular gauge, one accurate measure of cable size.
Amperage is the common term that indicates the strength of the electrical current. The term amperage relates to the cable circuit. However, when it comes to wire and cable, a more relevant term is ampacity, which is the highest amount of current that a wire and cable conductor can carry in the circumstances of use while sticking to its proclaimed temperature rating. The cable's ampacity is measured in amperes, or, shortly, amps. The ampacity sizes are available for the electrical circuits. For instance, a typical ampacity of the electrical cable in a circuit in a private home that is used for home wiring is 20 amps, while a lighting circuit requires a lower current capacity of 15 amps.
How Are Length, AWG, and Ampacity Related?
The greater the amperage of the electrical circuit, the higher the cable ampacity and the smaller the gauge. Smaller-diameter wires (higher AWG numbers) have lower ampacity because their smaller cross-sectional area can dissipate less heat and is more susceptible to overheating at lower currents. The cable should be thick enough to deal with the particular amperage so that it does not warm up to a critical temperature. Higher amperage calls for thicker wire (lower AWG number).
Does wire length affect amperage?
- The length of the wire does not directly impact amperage, but it does affect voltage through resistance. The longer the wire, the higher the cable resistance and voltage drop, affecting how effectively they carry current without significant losses.
- Resistance is the term that indicates how the wire or cable resists the electrical current flow. The measurement of resistance impact is voltage drop, which indicates the decrease of the cable's electrical potential in a current. A voltage drop that is less than 3 percent is acceptable. In case the resistance from a particular cable does not impact the voltage drop for more than 3 percent, the impact of the wire length on resistance is not really significant, and the cable will work great without heating up. In some cases, you might have to use a smaller gauge than you otherwise would if the cable is too long. This relates mostly only to cables of very long lengths.
- The voltage drop is a critical factor in longer cable runs due to increased resistance. Voltage drop is directly proportional to the length of the wire and the resistance per unit length. It's also inversely proportional to the cross-sectional area (related to AWG). A longer wire or a higher AWG (thinner wire) will have a more significant voltage drop.
- AWG and resistance have an inverse relationship. Higher AWG numbers mean smaller wire diameters, translating to higher resistance per unit length. This is critical in determining how much current a wire can carry safely without overheating.
- In practical applications, as the length of the wire increases, you may need to choose a wire with a lower AWG (thicker wire) to compensate for increased resistance and voltage drop, to ensure that the ampacity is not compromised.
- You can click on this page to calculate the voltage drop of a particular cable in a specific circuit to see if it is significant.
Case study: Residential home wiring for a detached garage
To illustrate the connection between length, AWG, and ampacity, let's consider a practical example of choosing a cable for wiring a detached garage in a residential setting.
Scenario: A homeowner wishes to extend electrical service from their main house to a newly constructed detached garage, which is 50 meters away. The garage will feature several light fixtures and power outlets, which together require a total of 20 amps.
Requirements:
- Total amperage needed: 20 amps
- Length of cable required: 50 meters
Decision process:
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Choosing the right AWG:
- The primary consideration is selecting a wire gauge that can handle 20 amps over a distance of 50 meters without significant voltage drop or overheating. A general rule is to opt for a lower AWG number (thicker wire) to manage the voltage drop effectively over longer distances.
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Calculating voltage drop:
- Assuming the house is supplied with a standard 120V system, the acceptable voltage drop should be less than 3%, which equates to 3.6 volts (0.03 x 120V).
- To calculate the voltage drop, we use the formula or a voltage drop calculator online. This calculation will help determine whether 12 AWG or 10 AWG is more suitable.
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Selecting cable type:
- For residential applications, THHN wire in a conduit is typically chosen considering the insulation type and environmental protection needs.
Example calculations:
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Using a Voltage Drop Calculator:
- Input parameters: 20 amps, 50 meters, 120 volts
- Results for 12 AWG: Voltage drop = 3.85 volts (3.21%)
- Results for 10 AWG: Voltage drop = 2.42 volts (2.02%)
Conclusion:
- The 12 AWG wire results in a voltage drop slightly above the acceptable limit (3.21% vs. 3%), which might compromise the performance of sensitive equipment.
- The 10 AWG wire maintains the voltage drop well within the acceptable range (2.02%), ensuring efficient operation without risking the safety or efficiency of electrical devices.
Final recommendation:
- 10 AWG THHN wire should be used to connect the main house to the detached garage. This gauge ensures that the voltage drop remains under the acceptable limit, providing a reliable and safe power supply. Using a conduit will protect the wiring from environmental factors and physical damage, which is crucial for an external run.
