Beatrice Lang
Automotive cables are sized by their total conductor diameter, unlike household wiring, which is measured by cross-sectional area. The right cable size and type for each circuit in a vehicle are essential, as it enhances the system's efficiency and safety. Various terminologies are used with cables, so it is important to understand the correct meaning. For instance, the term 1mm or 2mm does not refer to the cable's diameter but to the conductor's cross-sectional area, which is usually measured in mm². The conductor number and size refer to the number of conductors and their respective diameters, while the nominal current rating indicates the maximum continuous current capacity in Amperes (A). The overall diameter includes the cable's insulation, and resistance is measured in Ohms per meter (Ω/m). When measuring multi-stranded wires, it is crucial to add the cross-sectional measurements of the individual copper strands. Additionally, voltage drop and cable length influence each other, with longer cables causing a more significant voltage drop in low-voltage circuits. Cable materials also vary, with thin-wall cables offering higher temperature resistance and flexibility trade-offs. Tinned copper cables resist oxidation better than plain copper, making them suitable for moist conditions. Calculating the current rating required is an important step, and while it can be done using wattage/voltage, it is best to use a higher current-rated cable to ensure safety.
| Characteristics | Values |
|---|---|
| Conductor's Cross-Sectional Area | Usually described in mm², it shows the total area of the copper conductor. |
| Conductor Number and Size | Defined as the number of conductors and their respective diameters, e.g., 28/0.30 refers to 28 strands of 0.30mm each. |
| Nominal Current Rating | The cable's safe maximum continuous current capacity, represented in Amperes (A). |
| Overall Diameter | Indicates the cable's total diameter, including its insulation. |
| Resistance | Represented in Ohms per meter (Ω/m), it plays a role in determining voltage drop. |
| Cable Material | Cables can be made of various materials, each with its pros and cons. For instance, thin-wall cables are lighter and withstand higher temperatures than standard PVC ones but might be less flexible. |
| Voltage Drop | The voltage drop should not be more than 3-4%. |
| Core Number | Most equipment cable is single or 2-core, but applications including automotive, and marine may require up to 13 cores. |
| Core Type | The conductor can be available as a stranded or solid core. Solid core has a higher current capability but is more brittle, while stranded core has a larger surface area, helps dissipate heat, and is more flexible. |
| Core Material | The most common conductor is copper, which provides excellent conductivity and is more cost-effective than silver or gold. Tinned copper cables resist oxidation better than plain copper ones, ensuring longevity in moist conditions. |
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What You'll Learn
Calculating the current rating
When it comes to automotive electrical cable sizing, it is essential to calculate the current rating required. This is because the cable's current rating determines its capacity to safely carry a certain level of electrical current.
The current rating, also known as the nominal current rating, is represented in Amperes (A) and indicates the maximum continuous current the cable can safely handle. This rating is crucial for ensuring the cable can handle the circuit's current with a safety margin.
To calculate the current rating, you can use the formula wattage divided by voltage, which equals the current. For instance, 100W/12V = 8.33A. It is important to note that it is not advisable to run electronic components at their maximum capacity. Therefore, if you require a 5A cable, it is recommended to choose a cable with a higher current rating to ensure safety.
Additionally, consider the voltage drop that occurs in electrical circuits, including the cable. While a small voltage drop is acceptable, a long cable length in low-voltage circuits can result in a significant voltage drop, affecting device performance. Thus, it is crucial to ensure that the chosen cable does not induce a voltage drop of more than 3-4%.
Various online resources, such as cable size calculators and charts, can assist in determining the appropriate cable size based on the calculated current rating. These tools help convert between different measurement units, such as AWG (American Wire Gauge) and metric sizes, to ensure the correct cable is selected for automotive applications.
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Measuring multi-stranded wires
When measuring multi-stranded wires, it's important to note that they consist of multiple thin wire filaments twisted together to form a composite wire. The first step is to strip the insulation from the wire using a wire stripping tool. Place around 2 inches of wire through the tool, clamp it down, and pull off the insulation to expose the individual wires. Next, untangle and straighten the wires.
To calculate the gauge of the stranded wire, you need to find the diameter of a single wire strand, typically in millimetres or inches. A vernier caliper is a suitable tool for this step, and you should record the average measured diameter in millimetres. You can also use a ruler or a wire and conduit measuring tool for this step.
After measuring the diameter of a single wire strand, double that value to calculate the gauge of the stranded wire. Then, multiply this number by the total number of individual wire strands to find the Circular Mils (CMA) value. CMA is a commonly used wire-gauge scale that calculates the circular area of wires.
Finally, refer to an online table or a physical comparison table to determine the corresponding American Wire Gauge (AWG) value. The AWG scale measures wire gauge sizes from 0000 (or 4/0) to nearly 60, with larger wire diameters corresponding to smaller gauge numbers.
It's important to note that stranded wire diameters can vary depending on factors such as the number of strands, the amount of twist, and manufacturing processes. Therefore, if you know the wire manufacturer, it is recommended to refer to their website for more accurate information. Additionally, other methods and tools are available for measuring stranded wires, depending on the level of accuracy required.
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Cable materials
Cables are constructed from a conductor and an insulator. The conductor transfers electricity along the cable, and the insulator is a non-conductive material that surrounds and protects the conductor. The most common conductor is copper, which provides excellent conductivity and is more cost-effective than other conductive elements such as silver or gold. Copper is also used in the form of a stranded cable in vehicles to prevent 'work hardening' of the cable, which would make it brittle over time due to continuous vibration and shocks.
The conductor is often available as a stranded or solid core. Solid cores have a higher current capability but are more brittle, while stranded cores have a larger surface area that helps dissipate heat and makes the cable more flexible. This makes stranded cores more suitable for automotive applications.
To prevent oxidation in environments with moisture, tinned copper conductors are used. Each strand is coated in a small amount of tin, which helps prevent oxidation and ensures longevity. Tinned copper conductors are advisable for harsh environments like marine and some outdoor applications.
The insulation material of a cable is crucial, and different environments within a vehicle require specific insulation materials. For example, cables running through the engine bay should have heat-resistant insulation, while those exposed to moisture should be water-resistant. PVC is a commonly used insulation material that provides adequate protection against heat and abrasion. It is also cost-saving compared to other insulation materials.
Automotive cables must meet stringent technical requirements to resist the extreme environmental conditions road vehicles encounter, including wide temperature ranges, flame propagation, mechanical stress, chemical exposure, and electrical aspects.
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Voltage drop
Every element in an electrical circuit, including the cable, contributes to some energy loss. In low-voltage systems, even a few meters of cable can cause a significant voltage drop, affecting device performance. Longer wires will have a larger voltage drop than shorter wires of the same size. The amount of current being carried also affects voltage drop levels; an increase in current through a wire results in an increased voltage drop.
The wire size is another critical factor in determining voltage drop. Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length. Copper is a better conductor than aluminium and will have less voltage drop for a given length and wire size. It is recommended that the voltage drop should be less than 5% under a fully loaded condition.
To measure voltage drop, a digital multimeter is the fastest way to get an accurate measurement. If the multimeter does not have auto-ranging capability, use a low-voltage (0 to 1V) scale for voltage drop testing.
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AWG vs. Metric cable size
When it comes to auto electrical cable sizing, it's important to select the right cable size and type for each circuit to ensure both efficiency and safety. The two main systems for measuring cable size are AWG (American Wire Gauge) and the metric system.
AWG is the US standard for defining the cross-sectional areas and diameters of solid, round electrical wires. It is based on the number of dies or wire drawing operations required to reduce the raw copper wire to the appropriate size. The higher the AWG number, the thinner the wire. For instance, a 26-gauge wire requires more passes through the system of reducing dies than a 0000 or 4/0 wire, which is the largest AWG size. AWG measurements are determined by first calculating the radius of a wire squared, multiplied by pi. The term ""circular mil"" is often used, referring to the area of a circle with a 1/1000 (0.001) diameter.
On the other hand, the metric system is used in Europe and is based on the wire's cross-sectional area in square millimetres (mm²), as defined in IEC 60228. This measurement is calculated by multiplying the averaged measured diameter of the conductor strands in millimetres. While AWG and metric cable sizes may not always directly equate, close approximations can be made. For example, 1 MCM (1000 circular mils) equates to approximately 0.5067 square mm, so a ratio of 2MCM to 1mm² can be used with a very small error of 1.3%.
When measuring multi-stranded wires, it is important to add together the cross-sectional measurements of the individual conductor strands. This can be done by counting the number of strands, measuring the diameter of each strand, and then multiplying by the total cross-section. It is important to note that the term "circular mil" refers only to the wire itself and does not include the wire's jacketing or insulation.
In summary, while both AWG and metric measurements are used to determine electrical cable sizes, they differ in their bases, with AWG focusing on wire diameter and the metric system on cross-sectional area. The choice between the two depends on regional standards, with North America typically using AWG and Europe favouring metric sizes.
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Frequently asked questions
Automotive electrical cable size is commonly measured by the total conductor diameter, not the cross-sectional area. To measure multi-stranded wire, add together the cross-sectional measurements of the individual copper strands.
In Europe, stranded cables typically use metric sizes, while North America uses the AWG standard.
It is important to select the right cable size and type for each circuit in your vehicle to enhance efficiency and safety. Consider the cable's current rating, voltage drop, length, material, and flexibility.
The current rating, measured in Amperes (A), indicates the maximum continuous current the cable can safely handle. You can calculate it using the formula: wattage/voltage = current. For example, 100W/12V = 8.33A. Always choose a cable with a higher current rating than required to avoid running components at maximum capacity.
