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Could people with houses use legacy PSTN wiring to distribute 12V throughout the house?

by tardo99 10y ago

walrus01 10y ago
incredibly inefficiently, and only for loads of 5-10W max... phone wiring is not thick enough in gauge and 12V has very high voltage drop over distance for any appreciable amperage. there is a good reason why we use 110-240VAC to distribute power in houses.
- tardo99 10y ago
  http://www.britishtelephones.com/howtele.htm http://www.britishtelephones.com/howtele.htm Is 16 ohms per thousand feet really that high?
  - imgabe 10y ago
    Yes. Typically devices can tolerate a voltage drop of about 3%. With that resistance, a 1A load can only go 22.5 ft from the source. Considering you might have to run up a wall, across the ceiling and down another wall, you might not even get to the opposite side of a room.
- Johnythree 10y ago
  The phone line is essentially "constant current". There's 48V before you draw current, but that quickly drops to zero when you lift the handset, because there's an exchange relay in series which senses when the phone is "off-hook"
yardie 10y ago
No. In fact most newer solar installations are using microinverters to convert to AC at the source. 20-30amps of DC requires quite a bit of copper to go even a few meters.
- dreamcompiler 10y ago
  20-30amps of DC requires very little copper if it's high-voltage. It's low voltage that requires fat copper, not DC vs. AC.
  - bearbin 10y ago
    The same number of amps will require the same cross-sectional area of Copper, for both DC and AC. You can use less copper for the same power with higher voltage as a reduced current is required for the same power. This is also independent of AC/DC.
    - 2PetitsVerres 10y ago
      (nitpicking) That's correct if you take into account the skin effect when computing the cross-section area (or if you design your transmission line to avoid it. Better solution). But at 50/60 Hz, it's negligible (the "skin depth" of copper at these frequencies is around 9mm)
  - yardie 10y ago
    Except for electric cars there is very little consumer facing HV DC. Solar panels are 12/24/36/48V. With a 20-30 meter cable run it makes sense to convert it to higher voltage at the source. Since it's going to be used in traditional AC grid system, skip a step and convert it to AC as well. Kills 2 birds.
    - dreamcompiler 10y ago
      True. And HV DC (more than 50 volts or so) is more likely to kill you than HV AC if you grab a live conductor. Just the same, if I were building a new PV-powered house today I would try very hard to wire it as a mostly-DC house with 12-gauge or bigger romex and switchable voltages at the breaker box in increments of 12, up to 48. I'd put in point-of-use inverters at the few places that needed them instead of having 100 point-of-use rectifiers like we have today. The overall efficiency of a DC house would be quite a bit higher than a typical AC home.
snowwindwaves 10y ago
POTS wires are 24 gauge [0] which can handle 3.5 amps [1] so you'd be able to transmit 12V * 3.5A = 42 Watts. POTS often had 36V on it to power the ringer on phones so you could potentially up the power transmission by running your system at 14V. Some devices which consume a fixed amount of power would use less current, other devices would just use more power at higher voltage. [0] https://en.wikipedia.org/wiki/Plain_old_telephone_service https://en.wikipedia.org/wiki/Plain_old_telephone_service [1] http://www.powerstream.com/Wire_Size.htm http://www.powerstream.com/Wire_Size.htm
- tardo99 10y ago
  42 watts seems ok for a very small installation, right? I'm thinking enough to charge a phone and maybe a laptop. And, that would be driveable with a small 12V battery and a small solar panel.
  - snowwindwaves 10y ago
    As others have pointed out voltage drop is a significant problem with low voltage systems. #24 is 84 ohms per km, or 0.084 ohms per metre. Say the system is running at 13.4 V and the loads will work on as little as 11.5V before they brown out, that means we can have 13.4-11.5V voltage drop = 1.9V voltage drop. ohms law V=IR so for 3.5A and 1.9V R=0.54 ohms total resistance, and at .084 ohms per metre we can transmit 3.5A 6.46 metres before the voltage drop will be unacceptable. You could charge a phone, but even my small laptop chargers are over 60W. The big ones are 90W. Also don't try this in your house! pushing 3.5A through that little pinner wire it could get hot and start a fire.
    - tardo99 10y ago
      Makes sense. Could you instead apply a DC bias to the AC power jack?
    - Scoundreller 10y ago
      Wouldn't there be 4 conductors, so resistance would be half when you pair them together?
sitharus 10y ago
Yes, but you'd be better off using a higher voltage and switching DC-DC converters at the end. The POTS lines would have ~50V on them in normal operation so they'd be insulated for 48V. However, V=I/R, and the lines have a constant resistance so if you can raise V and lower I you can get the same total wattage for a lower line loss. This is also why distribution networks are at 11kV and up. Modern switching converters are pretty efficient, and you can also tailor them for the device so you don't have to double convert.
- Scoundreller 10y ago
  Since you have 4 conductors to work with, you could run 12V for low-amperage applications and 48V for high-amperage stuff that justifies the switchgear.
dreamcompiler 10y ago
Yes, but only for very small amounts of power, as others have pointed out. A better idea would be to run DC on a select few runs of the existing 14-gauge romex from your breaker box to the rooms they serve. You'd be taking those 110v lines out of service and using them exclusively for DC, but you could pump a lot more power across them than you could across telephone wire. (You still wouldn't be able to run electric heating appliances or big motors or incandescent lights, but pretty much everything else in a modern house would work, modulo the need for switched DC-DC voltage converters.)

Could people with houses use legacy PSTN wiring to distribute 12V throughout the house?

by tardo99 10y ago

Could people with houses use legacy PSTN wiring to distribute 12V throughout the house?

incredibly inefficiently, and only for loads of 5-10W max... phone wiring is not thick enough in gauge and 12V has very high voltage drop over distance for any appreciable amperage. there is a good reason why we use 110-240VAC to distribute power in houses.

tardo99 10y ago
http://www.britishtelephones.com/howtele.htm http://www.britishtelephones.com/howtele.htm Is 16 ohms per thousand feet really that high?
- imgabe 10y ago
  Yes. Typically devices can tolerate a voltage drop of about 3%. With that resistance, a 1A load can only go 22.5 ft from the source. Considering you might have to run up a wall, across the ceiling and down another wall, you might not even get to the opposite side of a room.
Johnythree 10y ago
The phone line is essentially "constant current". There's 48V before you draw current, but that quickly drops to zero when you lift the handset, because there's an exchange relay in series which senses when the phone is "off-hook"

yardie 10y ago

No. In fact most newer solar installations are using microinverters to convert to AC at the source. 20-30amps of DC requires quite a bit of copper to go even a few meters.

dreamcompiler 10y ago
20-30amps of DC requires very little copper if it's high-voltage. It's low voltage that requires fat copper, not DC vs. AC.
- bearbin 10y ago
  The same number of amps will require the same cross-sectional area of Copper, for both DC and AC. You can use less copper for the same power with higher voltage as a reduced current is required for the same power. This is also independent of AC/DC.
  - 2PetitsVerres 10y ago
    (nitpicking) That's correct if you take into account the skin effect when computing the cross-section area (or if you design your transmission line to avoid it. Better solution). But at 50/60 Hz, it's negligible (the "skin depth" of copper at these frequencies is around 9mm)
- yardie 10y ago
  Except for electric cars there is very little consumer facing HV DC. Solar panels are 12/24/36/48V. With a 20-30 meter cable run it makes sense to convert it to higher voltage at the source. Since it's going to be used in traditional AC grid system, skip a step and convert it to AC as well. Kills 2 birds.
  - dreamcompiler 10y ago
    True. And HV DC (more than 50 volts or so) is more likely to kill you than HV AC if you grab a live conductor. Just the same, if I were building a new PV-powered house today I would try very hard to wire it as a mostly-DC house with 12-gauge or bigger romex and switchable voltages at the breaker box in increments of 12, up to 48. I'd put in point-of-use inverters at the few places that needed them instead of having 100 point-of-use rectifiers like we have today. The overall efficiency of a DC house would be quite a bit higher than a typical AC home.

snowwindwaves 10y ago

POTS wires are 24 gauge [0] which can handle 3.5 amps [1] so you'd be able to transmit 12V * 3.5A = 42 Watts. POTS often had 36V on it to power the ringer on phones so you could potentially up the power transmission by running your system at 14V. Some devices which consume a fixed amount of power would use less current, other devices would just use more power at higher voltage. [0] https://en.wikipedia.org/wiki/Plain_old_telephone_service https://en.wikipedia.org/wiki/Plain_old_telephone_service [1] http://www.powerstream.com/Wire_Size.htm http://www.powerstream.com/Wire_Size.htm

tardo99 10y ago
42 watts seems ok for a very small installation, right? I'm thinking enough to charge a phone and maybe a laptop. And, that would be driveable with a small 12V battery and a small solar panel.
- snowwindwaves 10y ago
  As others have pointed out voltage drop is a significant problem with low voltage systems. #24 is 84 ohms per km, or 0.084 ohms per metre. Say the system is running at 13.4 V and the loads will work on as little as 11.5V before they brown out, that means we can have 13.4-11.5V voltage drop = 1.9V voltage drop. ohms law V=IR so for 3.5A and 1.9V R=0.54 ohms total resistance, and at .084 ohms per metre we can transmit 3.5A 6.46 metres before the voltage drop will be unacceptable. You could charge a phone, but even my small laptop chargers are over 60W. The big ones are 90W. Also don't try this in your house! pushing 3.5A through that little pinner wire it could get hot and start a fire.
  - tardo99 10y ago
    Makes sense. Could you instead apply a DC bias to the AC power jack?
  - Scoundreller 10y ago
    Wouldn't there be 4 conductors, so resistance would be half when you pair them together?

sitharus 10y ago

Yes, but you'd be better off using a higher voltage and switching DC-DC converters at the end. The POTS lines would have ~50V on them in normal operation so they'd be insulated for 48V. However, V=I/R, and the lines have a constant resistance so if you can raise V and lower I you can get the same total wattage for a lower line loss. This is also why distribution networks are at 11kV and up. Modern switching converters are pretty efficient, and you can also tailor them for the device so you don't have to double convert.

Scoundreller 10y ago
Since you have 4 conductors to work with, you could run 12V for low-amperage applications and 48V for high-amperage stuff that justifies the switchgear.

dreamcompiler 10y ago

Yes, but only for very small amounts of power, as others have pointed out. A better idea would be to run DC on a select few runs of the existing 14-gauge romex from your breaker box to the rooms they serve. You'd be taking those 110v lines out of service and using them exclusively for DC, but you could pump a lot more power across them than you could across telephone wire. (You still wouldn't be able to run electric heating appliances or big motors or incandescent lights, but pretty much everything else in a modern house would work, modulo the need for switched DC-DC voltage converters.)