USA Residential Power Line Ampacity

[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]

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.

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]

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.

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.

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-.)

[Monkeh]

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.

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.

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-.)

Ah, well, you get 60A and 80A fuses to fit the same carier. Does vary area to area, most newer construction should be 100A supplies. Although we're long past the point we should be running 60A 3-phase..

[bdunham7]

Anyone who can furnish more light on the subject is invited to respond.

A 'rating' is not a definitive statement of the maximum capabilities of something and different rating methods may have vastly different objectives and safety factors.  The ampacity ratings the NEC uses are very conservative and intended to minimize temperature rise at connections and voltage drop, as well as tolerating overloads long enough for circuit protection to kick in.  In a different environment, you might see a a 6AWG wire rated for only 55A in a 60C rated application, but in an automotive starter system the same size wire might carry 400A or more.  The utility company, as you discovered, does not follow NEC rules and they don't like to waste copper (or iron and aluminum).

As far as the distribution network not being able to handle 100% load (everyone drawing 200A) I'm sure that is all figured in, but if you look at the capacity of the transformer I think you'll be even more surprised.  The pole pig in my backyard feeds 4 houses and is rated at 50kW.  Old ones were only rated for 25kW.  If we all have 200A service (very likely) then that would be 192kW!  The key is in how the transformers are rated.  IIRC, at standard temperature they are rated at 175% continuous overload and 400% for one hour.  So even if all 4 of us charge our Teslas at 80A at the same time all should be good.

[wn1fju]

Thanks for the comments.  I guess the power company can look at the usage history from each home and get a rough idea of the capacity that best serves the neighborhood. 

Around here, the houses all use heat pumps for winter heating and they usually have anywhere from a 5 kW to 15 kW auxiliary heating coil in the inside air handler.  These auxiliary heaters kick in when the outside compressor unit can't meet the heating demand. 

Many years ago, we had a multiple-day outage due to a blizzard resulting in everyone's inside house temperature falling into the 50's (F).  I happened to have my 2 meter handheld transceiver tuned to the electric utility communications up around 450 MHz.  I heard one guy on the radio say that he was about to throw the switch, having completed the repairs.  Moments later, the power in the neighborhood came back on.  Usually you can't tell where the workers are by listening to the radio, since they only identify locations by telephone pole number.  Anyway, about a minute later, someone else, presumably at one of the control stations, comes on the radio and asks the lineman if he did the repairs correctly since the power consumption was rising too fast and too high.  I really wanted to key up my transmitter and scream, "that's because everybody's auxiliary heat just kicked on." 

[ajb]

If you go through the NEC, you'll find that it doesn't require the capacity of your electrical service to be equal to the sum of all your breakers' ratings either.  There's simply no realistic need for that level of capacity in a normal residential or commercial context.  You might have a whole 15A breaker or two just for lighting, for example, but would be hard pressed to reasonably consume 1.8kW that way, especially with modern LED sources.   Your appliances also probably don't add up to anywhere near the consumption that the sum of their breaker ratings would imply, even if you managed to get them all running at exactly the same time.  All of that is considered in the NEC. Plus there's a practical lower limit to the breaker and wire sizes you would want to install, so small loads are effectively rounded up to that minimum in terms of breaker capacity. 

Anyone who can furnish more light on the subject is invited to respond.

A 'rating' is not a definitive statement of the maximum capabilities of something and different rating methods may have vastly different objectives and safety factors.  The ampacity ratings the NEC uses are very conservative and intended to minimize temperature rise at connections and voltage drop, as well as tolerating overloads long enough for circuit protection to kick in.

Furthermore, the wires the NEC are concerned with are generally in cables inside walls where dissipating heat is more difficult versus utility wiring that is often single conductors in open air, so even if you apply the same engineering rules you'd have a higher ampacity for the latter than the former.  Underground wiring would be different for the same reasons.  There are a bunch of rules in the NEC for how you derate wiring based on environmental and installation conditions like these, and it acknowledges in places that specifically engineered solutions may be required for situations it doesn't adequately cover -- which certainly applies to utility wiring, which is why there's a whole separate code for it! 

[IanB]

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.

[Gyro]

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.

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.

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]

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.

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.