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Mnemonic for estimating wire capacity (reposted) A friend asked about the current-carrying capacity of cables. We have discussed this before, and since some people found the calculations complicated, here is a mnemonic to help us remember it. I. Current situation: A 1-square-millimeter copper wire allows different amounts of current to flow through it depending on the voltage applied; theoretically, the maximum value is 15A. In normal use, we can assume a current range of around 9A–13A. 1 mm² copper wire: 15A for overhead wiring, 11-12A when installed in walls, around 9A for sheathed wires, and 5-7A when run in conduits. In other words, for a square copper wire, a stable current-carrying capacity of 6A is no problem. Therefore, when it comes to the current-carrying capacity of wires, disputes often arise, and it is difficult to determine a very precise value for this capacity. Due to differences in environment, what we provide are generally estimates for reference, so as to give us some idea in practical project applications. II. Mnemonic for estimating cable current-carrying capacity: For 2.5, multiply by 9; then decrease by 1 and move forward sequentially. Thirty-five times 3.5, pair them up and subtract 0.5. If conditions change, a discount is applied; at high temperatures, copper upgrades at a 10% discount. Two, three, four roots in the tube; 80%, 70%, 60% capacity for current flow. Note: The multiple decreases as the cross-section increases. “\"2.5 down multiplied by 9, then subtract one and move forward in sequence\" refers to aluminum-core insulated wires with cross-sections of 2.5 mm² or less; their current-carrying capacity is approximately 9 times the value of their cross-sectional area. For a 2.5mm’ wire, the current-carrying capacity is 2.5×9=22.5(A). The multiple relationship between the current-carrying capacity and the number of cross-sections for wires of 4 mm² and above is arranged in ascending order of wire size, with the multiple decreasing by 1 at each step: 4×8, 6×7, 10×6, 16×5, 25×4. “\"Thirty-five times 3.5, in pairs subtract 0.5,\" means that the current-carrying capacity of a 35mm” wire is 3.5 times the area of its cross-section; that is, 35×3.5=122.5(A). For wires of 50 mm² and above, the multiplicative relationship between current-carrying capacity and cross-sectional area changes such that wire sizes are grouped in pairs, with the multiplier decreasing by 0.5 for each pair. That is, the current-carrying capacity of a 50mm wire is 3 times its cross-sectional area, which is 50×3=150A ； The current-carrying capacity of 70, 95, and 120 mm” wires is 2.5 times their cross-sectional area, and so on. “If conditions change, a discount is applied; at high temperatures, a 10% discount applies and the copper grade is upgraded. The above formula is established for aluminum-core insulated wires installed exposed, under an ambient temperature of 25°C. If aluminum-core insulated wires are installed exposed in areas where the ambient temperature remains above 25°C for extended periods, the current-carrying capacity of the wires can be calculated using the method outlined above, and then that value should be reduced by 10% ； When insulated copper wires are used instead of aluminum wires, their current-carrying capacity is slightly higher than that of aluminum wires of the same specification; the current-carrying capacity can be calculated by using the method outlined above, by increasing the wire gauge by one level compared to aluminum wires. For example, the current-carrying capacity of a 16 mm² copper wire can be estimated based on that of a 25 mm² aluminum wire. “\"For 2, 3, or 4 wires, the current-carrying capacity is calculated at 80% of the normal value; for 3 wires, it’s 70%, and for 4 wires, it’s 60%.\" In other words, when 2 wires are used, the current-carrying capacity should be calculated at 80% of the standard value, 70% for 3 wires, and 60% for 4 wires. The safe current rating for ordinary copper wires is approximately 28 A, which is the safe current-carrying capacity of 2.5 square millimeter copper power wires. The safe current-carrying capacity of a 4 mm² copper power cable is around 35A. The safe current-carrying capacity of a 6 mm² copper power cable is around 48 A. The safe current-carrying capacity of a 10 mm² copper power cable is around 65A. The safe current-carrying capacity of a 16 mm² copper power cable is around 91 A. The safe current-carrying capacity of a 25 square millimeter copper power cable is around 120A. For aluminum wires, with the same current-carrying capacity, the diameter should be 1.5 times that of copper wires. III. Examples of common application scenarios are as follows: 1. 1.5mm2—spotlights ； 2, 2.5mm2---Sockets, such as air conditioners (1.0p or 1.5P air conditioners), main lighting fixtures, electric water heaters, etc ； 3, 4.0mm2---For particularly large electrical appliances, service entrance wires, or in special situations such as cabinet air conditioning. Air conditioning. The safe power rating for a 2.5mm2 wire is at least 3.2KW. If the air conditioner is a 1.5p unit, the horsepower rating refers to the power consumption of the appliance; 1 horsepower equals 735W, so the total power is 1.5*735=1102W. Some friends mentioned that using 2.5mm2 wire is sufficient, but we still recommend using 4.0mm2 wire. The other one is the kitchen, which needs to accommodate the simultaneous use of lighting, a range hood, a gas water heater, a disinfection cabinet, a refrigerator, and a microwave oven. If an electric water heater is used in the bathroom at home, it is also recommended to provide a separate circuit with a capacity of 4.0 mm2 for that bathroom, so as to accommodate the simultaneous use of devices such as the electric water heater, ceiling fan, heater, and lighting. In summary, it’s this image: http://img.civilcn.com/d/file/zhishi/jzdq/2019-04-29/df9fbf954e45400442afb9e700d7e2da.jpg