General > General Technical Chat
USA Residential Power Line Ampacity
Gyro:
--- Quote from: Monkeh on February 07, 2024, 02:39:42 pm ---
--- Quote from: tom66 on February 07, 2024, 02:10:08 pm ---In the UK homes are connected to 100A fuses at 240V. Nominally, the capacity is 24kVA, but the cable is typically only rated at around 70A continuous. So the fuse is sized so that 100A can be maintained momentarily (several minutes) but operation at 70A or more continuously will eventually lead the fuse to open.
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Que? A 100A fuse holds 100A indefinitely. The buried cable for a 100A supply will be 16mm², which is rated for 100A (give or take an amp) in open air in a 30C ambient, and more than that buried.
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The house feed cable is typically (assuming that TNC-S installations are mostly typical these days) 25mm2 Al/CU BS7870-3-11. This is rated at around 120A surface, and probably significantly higher buried. You're unlikely to come across a service cable that is less than 13mm dia - it would be hard to joint and bury. This makes the 100A max fuse rating is fairly conservative, even taking into account the dwell time on sustained fractional overload.
Gyro:
--- Quote from: IanB on February 07, 2024, 04:18:14 pm ---I was just going to post something like what ajb just posted. Namely, the response and consequences of an overload on cables in different situations may vary.
Overloading a cable inside the walls of a house is different from overloading a buried cable under the street. The buried cable may have better heat removal to the surroundings, and the consequences of a higher operating temperature in a buried cable may be less significant.
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Yes, once the cable comes out of the ground, heating becomes a significant factor if it passes through a building cavity, or even worse, thermal insulation.
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