How to Find the Suitable Size of Cable & Wire for Electrical Wiring Installation?
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In the realm of electrical installations, one crucial element often dictates the success of an entire project: the selection of the right cable and wire size. Whether you are working on residential wiring or overseeing a complex industrial setup, the size of cables and wires used can profoundly impact the efficiency, safety, and longevity of the electrical system.
The task of determining the suitable cable and wire size, however, can be a confusing and difficult technical journey. Undersized wires can lead to overheating and potential hazards, while oversized cables might result in wastage and inefficiency. Striking the delicate balance requires a blend of mathematical precision, knowledge of electrical standards, and an understanding of the unique demands posed by each application.
In this article, we shall learn all about how to calculate cable size and wire size for your electrical wiring installations.
Cables and how they can impact voltage drop
It is common knowledge that all conductors (other than some superconductors), cables, and wires are characterised by some resistance. It is important to note that resistance is inversely proportional to the conductor’s diameter and directly proportional to its length.
Therefore, R ∝ L/a … [Laws of resistance R = ρ (L/a)]
The conductor experiences a voltage drop whenever an electrical current passes through it. This drop may be negligible for conductors that are of smaller length. But in the case of longer conductors and conductors with a narrow diameter, considering voltage drops is important if correct installation and safety are to be ascertained.
The IEEE rule B-23 mandates that voltage drop should never be larger than 2.5% of the supply voltage at any place between installation and power supply.
Let us take a look at an example:
If supply voltage is 160V AC, then voltage drop can be allowed up to:
Permissible Voltage Drop = 160 x (2.5/100) = 4V
Voltage drops in cables can be measured using two systems of measurement.
The FPS (foot pound system) utilizes a length-based (100 ft) system to calculate voltage drop.
The SI (system international and metric system) calculates voltage drop in amperes per meter (A/m).
Nowadays, you can search for cable current carrying capacity calculator on the web to access several such free tools.
Why Cable Size Matters in Electrical Wiring?
Selection of the correct cable size is essential for electrical safety and efficiency. This is because inappropriate size can result in overheating and cause potential hazards like electrical fires. Hence, you must know how to select cable size and cable size calculation formula. A proper-sized cable ensures optimal power distribution and energy efficiency. It also prevents damage to electrical systems. Proper selection of cable size also improves system performance and reduces energy losses.
Prevent Overheating and Fire Hazards
An undersized cable may not be able to carry the required load. Hence, it may result in overheating and possibly lead to electrical fires. Correctly sizing cables prevents these dangers and ensures electrical safety.
Improve Energy Efficiency
An appropriate cable size helps minimize energy losses. A correctly sized cable ensures that the current flows efficiently. This results in reducing energy wastage and maintaining overall system efficiency.
Reduce Voltage Drop
Voltage drop occurs when electrical energy is lost due to the resistance of cables. The voltage drop can be minimized by selecting the right cable size. It ensures that appliances and systems function optimally.
Compliance with Electrical Standards
Proper cable sizing ensures that your installation complies with electrical codes and safety standards. Compliance is essential for both residential and commercial electrical systems.
Key Factors that Influence Cable Size Selection
Selection of the right cable size depends on various factors. It includes the load requirement, distance, ambient temperature, and type of installation. Properly understanding these factors ensures the safe and efficient operation of electrical systems. Read below to understand how to choose cable size.
Load Requirement
The amount of current that the cable will carry determines its size. Higher loads require cables with a larger cross-sectional area to avoid overheating and ensure the system operates safely.
Length of the Cable Run
Longer cable runs result in higher resistance. This leads to voltage drops. Larger cables are required to maintain efficiency and prevent performance loss for longer distances.
Ambient Temperature
The surrounding temperature can affect the cable’s ability to carry current. Higher temperatures require larger cables to avoid overheating. This factor should always be considered when choosing cable sizes.
Type of Insulation Material
Different insulation materials have varying temperature and current-carrying capacities. The type of insulation used in the cable influences its size and suitability for specific applications.
All these factors are crucial for the cable selection formula. So, you must understand them for the right selection.
How do I calculate the voltage drop in a cable?
The steps for calculating voltage drop in the cable are as follows:
First, measure the highest permissible voltage drop.
Second, calculate the load current.
Third, you must know that the most appropriate cable will be one that has a current rating closest to the load current calculated. Several charts with cable sizes and corresponding current ratings are easily available online. Use such a chart to select a suitable cable.
Fourth, use the chart to also locate voltage drop in meters or 100ft, according to your system of preference.
Fourth, use the chart to also locate voltage drop in meters or 100ft, according to your system of preference.
Fifth, use the following cable size and current rating formula for calculating voltage drop along the actual length of the wiring circuit.
- 1. If you want to calculate the drop in voltage per meter, (Actual length of circuit x volt drop for 1m) /100
- 2. If you want to find the drop in voltage in 100ft (Actual length of circuit x volt drop for 100ft) /100
After calculating the above, you have to multiply the value you get with the load factor -
Load factor = Load Current to be taken by Cable/ Rated Current of Cable (which should be given in the chart you are referring to).
Now, you have arrived at the voltage drop value of the cable when the load current passes through it.
If this value is less than the maximum permissible voltage drop, then the size of the cable selected should be okay.
However, if the value is higher than the value of the maximum permissible voltage drop, then the voltage drop for the next greater-sized cable according to the referred chart has to be calculated. This process has to be continued until the value of the voltage drop becomes less than the maximum permissible voltage drop value.
Common Cable Sizes for Residential and Commercial Use
Cable sizes vary depending on the application and electrical load. You must understand the standard sizes to choose the correct one for your specific needs.
Residential Wiring
Cables such as 2.5mm² for lighting circuits and 4mm² for power circuits are commonly used for home wiring. These sizes ensure safe and efficient power distribution in household applications.
Commercial Wiring
Commercial buildings typically require larger cables, such as 6mm² or 10mm². It depends on the electrical load and distance. These sizes help in efficiently managing the increased power demand in commercial setups.
Industrial Use
Cable sizes may range from 10mm² to 50mm² or more for industrial applications. Their size depends on the machinery and equipment that need to be powered.
Specialty Applications
Larger cables like 16mm² and 25mm² are chosen for certain specialized installations, such as high-power equipment or outdoor applications. They handle higher electrical demands and external factors.
How can I determine precise wire and cable size for a certain load?
First, you need to keep in mind that cable sizes corresponding to given loads can be ascertained with the help of standardized tables and charts available online but certain rules regarding voltage drop must be heeded.
There must be a 20% additional scope of current for emergency or future requirements for a given load other than the known current value.
Voltage drop must be limited to 1.25% from the energy meter to the distribution board while it should not be more than 2.5% of the supply voltage for the final sub-circuit.
Temperature variations have to be considered as per needs. Temperature factor charts are also easily available online and can be referred to.
You also need to keep in mind the load factor when trying to estimate cable size.
Two other factors that need to be considered in the determination of cable size are the type of wiring system and the diversity factor in installation.
Let us now look at a solved example using a wire size calculation formula to better understand this.
Example of wiring size calculation:
The total load for the installation of electrical wiring in a building is 5.5kW and the total length of cable from the meter to the distribution board is 40 feet. Supply voltage is 220V and temperature is recorded at 35°C. Calculate the most appropriate cable size from meter to sub-circuit for wiring to be installed in conduits.
Solution:
Total Load = 5.5kW =5.5 x 1000W = 5500W
20% additional load = 5500 x (20/100) = 1100W
Total Load = 5500W + 1100W = 6600W
Total Current = I = P/V = 6600W /220V =30A
Now select the size of cable for a load current of 30A from your referred chart or table. This should be around (7/0.044) (34 Amperes). Accordingly, we can use (7/0.044) cable.
Thereafter, you need to match the (7/0.044) cable with the charted temperature factor. This value should be around 0.97 at 35°C and the current carrying capacity of (7/0.044) is 34A. Therefore, the current carrying capacity of this cable at 35°C could be calculated as:
Current rating for 35°C = 34 x 0.97 = 32.98 A
As the calculated value (32.98 A) at 35°C is less than that of the current carrying capacity of (7/0.044) cable which is 34A, this cable size (7/0.044) is also appropriate with the given temperature.
Next, you need to calculate the voltage drop for 100 feet for this (7/0.044) cable from cable size charts with corresponding current ratings and voltage drops available online. This value should be around 5.7V. Since in this case, the length of the cable is 40 feet, the voltage drop for 40 feet cable would be calculated as:
Actual Voltage drop for 40 feet = (5.7 x 40/100) x (30/34) = 2.01V
And Allowable voltage drop = (2.5 x 220)/100 = 5.5V
Therefore, we can see that the actual voltage drop (2.01V) is less than that of the maximum permissible voltage drop of 5.5V. Hence, the most suitable cable size is (7/0.044) for the given load.
Also Read: Type of Electric Cables & Conductors with Application
Get your cables at the Schneider Electric eShop!
Schneider Electric has been a trusted name in the market for electric cables and wires for over two decades. For those who want to avoid the hassles of complicated calculations, visit the Schneider Electric eShop for expert consultation on cable sizes best suited to your exact requirements.
FAQs
Q1. What happens if I choose the wrong wire size?
Ans: The wrong wire size can lead to overheating, electrical fires, or inefficient operation. A wire that’s too small for the load will overheat. On the other hand, the one that's too large may be unnecessarily expensive and difficult to install.
Q2. Can I use aluminum wires instead of copper wires?
Ans: Aluminum wires are sometimes used as an alternative to copper. But, they require larger sizes due to higher resistance. Proper precautions are needed for this to avoid connection issues and potential hazards.
Q3. What is the role of voltage drop in choosing wire size?
Ans: Voltage drop occurs when the electrical current loses energy as it travels through the wire. A higher voltage drop can affect performance.Therefore, an appropriate wire size is important as it minimizes this issue and ensures efficient power delivery.
Q4. What is the maximum allowable voltage drop for residential wiring?
Ans: The maximum allowable voltage drop is typically 3% for lighting and 5% for power circuits in residential wiring. This ensures that electrical systems operate efficiently and prevent excessive energy loss.
Q5. Can I use a smaller wire size for shorter distances in electrical wiring?
Ans: Shorter distances can allow for a smaller wire size. This is due to less resistance and voltage drop. However, it’s important to still adhere to safety standards and ensure the wire can handle the intended load.
Q6. What is the difference between copper and aluminum cables in terms of wire sizing?
Ans: Copper cables generally have better conductivity than aluminum. This means that copper wires can be smaller for the same current load. Aluminum wires require a larger size to handle the same amount of current.
Q7. Do electrical codes affect the wire size selection for residential wiring?
Ans: Electrical codes provide guidelines for selecting wire sizes based on safety standards and load requirements. These codes ensure that electrical installations are safe, reliable, and efficient.
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