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USA Residential Power Line Ampacity
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wn1fju:
We just had our underground electric service replaced in our neighborhood (Maryland).  From an underground transformer, they ran a single 500 kcmil aluminum wire to a midpoint between four houses.  Then they ran four 4/0 aluminum service lines from that junction to the four individual houses. 

Each house has a 200 amp service.  I thought to myself, what would happen if all four of us drew 200 amps?  There is no way that single 500 kcmil wire is rated to 800 amps (the rating seems to be slightly less than 400 amps).  I realize that situation has a probability near zero, but I asked the contractor pulling the cable that question and basically got the answer, "that's the way we do things around here."

Some perusing on the forums scattered around the internet led me to something I find curious (although maybe known to everybody else but me). There is the National Electrical Code (NEC) and the National Electrical Safety Code (NESC).  The former is what us homeowners have to follow.  The latter is what the power companies follow.  The two are not equal and the result is that the power companies calculation of max amps vs wire gauge can be as much as twice what the NEC tables show.  So maybe that 500 kcmil wire is good enough for even that zero probability 800 amp case!

Anyone who can furnish more light on the subject is invited to respond.
tom66:
There will be an large fuse at the distribution end (substation or pole pig) which will pop in the unlikely even that you all pull 200A. 

The same design is used all across the world, look up "electrical diversity".  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.  It is quite common for two adjacent homes to be connected to the same fused cable (so-called loop bundling) which reduces costs but further limits capacity.  The cabling to supply all homes is typically rated somewhere around 10A per home continuously (per phase) so if the street has, say, 100 homes, the cable might be sized around 300A per phase.  That means if only 3 homes consume the maximum current coincidentally on the same phase, you are at the cable rating already.  Typically every home is on a different phase to its neighbour, if it is not loop-bundled.

There are concerns over these current limits and continuous loads from EVs, home solar/batteries and heat pumps.  It's reasonably likely that 300A cable will need to be dug up and replaced with something like a 1000A cable in the future.  Though the biggest constraints are not on the local distribution but at the 11kV (medium voltage) side.
soldar:
For the same reason that you do not connect all your loads at home at the same time and you contract for less that that theoretical maximum, the power distribution companies count on not all customers drawing max power at the same time.


https://www.tutorialspoint.com/demand-factor-load-factor-and-diversity-factor

Diversity Factor

The diversity factor of the power station is defined as the ratio of sum of individual maximum demands to the maximum demand on the power station,

The diversity factor of a power station is always greater than 1. The diversity factor plays a vital role in the determination of cost of generation of power. The greater is the diversity factor, the lesser is the cost of generation of power.
Monkeh:

--- 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.

--- End quote ---

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.
tom66:

--- 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.

--- End quote ---

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.

--- End quote ---

That wasn't what I was told by the DNO tech at my local hackerspace.  He said they are only 70A fuses and 70A (continuous) supply ratings.  The fuse holder is rated to 100A (which is where the labelled rating comes from, I believe), but almost no one uses 100A fuses in those.

Maybe it varies from area to area.

(edit: I see the error, I should have noted that they are 100A fuse holders with lower rated fuses within, not 100A -fuses-.)
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