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Is PoE to everything a common layout? I didn't realize it could support anywhere near that much power.

by mNovak 6y ago

brtkdotse 6y ago
PoE is dark magic to me. How can you send any amperage at all through those hair thick leads?!
- CodeWriter23 6y ago
  It’s ok, the cable will act as a fuse when necessary.
  - 29083011397778 6y ago
    Aircraft fuses exist not to save the several hundred-thousand dollar avionics install, but specifically to protect the wire from melting, shorting, or starting fires where fire does not belong [1] (page 11-15, para. 11-51). This is relevant because fire also does not belong inside the walls of your home. I assume you're just having a laugh, but just in case others take you too seriously: wire is not a substitute for a fuse. [1] https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43.13-1B_w-chg1.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...
    - CodeWriter23 6y ago
      Correct on all points, sir or maam.
  - bleepblorp 6y ago
    Hey, hey, we're not talking about USB-PD here. :-) PoE actually works.
- hinkley 6y ago
  They split the amperage across multiple wires. I think PoE is always DC so the runs can’t be as long as with AC. Actually why can’t you run PoE as AC? Embedding a high frequency signal on a lower frequency carrier is older than dirt at this point. At 120 volts, 30 watts is only a quarter amp.
  - hinkley 6y ago
    Jesus, IEEE 802.3bt can do up to 100 watts PoE now? https://en.m.wikipedia.org/wiki/Power_over_Ethernet https://en.m.wikipedia.org/wiki/Power_over_Ethernet
  - taneq 6y ago
    Cable run length doesn't really have anything to do with DC vs. AC, it's all about the voltage. Power losses are proportional to current squared, so for a given power transmitted, they're proportional to the inverse square of the voltage. We're just used to power transmission being AC because with AC it's easier to switch large currents on and off, and easier to step voltage up/down.
  - miahi 6y ago
    The insulation for CAT5 and RJ45 is rated for 125V. You don't want to be that close to the rating for a cable that can be moved/bent/stretched (for comparison, an AC cable is rated at least 300V). And the RJ45 connectors are not designed for that kind of dangerous voltage.
    - hinkley 6y ago
      > And the RJ45 connectors are not designed for that kind of dangerous voltage. Right, there will be a voltage where the pins will arc, and a lower potential where a dirty connector will arc. We rented an office that had previously housed some sort of telecom equipment, so the whole place was wired for, I believe it's 40 volts rackmount equipment. That runs at lower current (thus watts) than computers and so we had to run new electrical and AC into the server room to use it for IT equipment. Cars are trying to switch to 48V because 40-50V is a range where touching a live wire gets your attention but won't do too much permanent damage (eg kill you). Or if you went with 20v you might be able to reuse equipment designed for USB power supplies.
- Gibbon1 6y ago
  I'm going off memory but you can run ~2amps over 22g. But voltage drop starts being a problem at low voltage. I think Ethernet cables are 24-26g.
- miahi 6y ago
  CAT5e and especially cat6/cat7 is not usually hair thick, it's usually 24 AWG (and in better cables 23 or even 22). The ampacity of 24AWG is around 3A (more if you accept a bit of heat). PoE is limited to less than 1A per pair, but it makes the power through voltage (usually between 48 and 57 volts), and by using more than one pair.
- mschuster91 6y ago
  PoE runs at 48V, which means even if you're running only half an amp per pair you'll end up with about 24x4=96W of capacity (the standard actually allows for 600 mA per pair).
monocasa 6y ago
Yeah, agreed. If I'm reading it right and they're putting the TV on PoE too, they're way out of the power budget which is only 30 watts per port. And I say this as someone who's currently going nuts with PoE going to home automation (speakers, cameras, soil monitor sensors, sprinkler relays, etc).
- Nick-Craver 6y ago
  Clarification there: I have a US-8 behind each TV, taking 802.11af loads to power. For the TV with an AP or something else PoE behind it, those switches can take in 802.11at and output 802.11af on port 8. The TVs themselves aren’t PoE...I’m crazy, just not that crazy :)
  - monocasa 6y ago
    Lol, word, that makes a lot more sense. Thanks for the clarification! : )
- Gibbon1 6y ago
  IEEE 802.3bt is supposed to be good for 50-90W.
sz4kerto 6y ago
Pretty much. I have a small flat compared to US standards, but there's plenty of Ethernet wiring and I have a small (passively cooled) 16-port Linksys PoE switch so I can run 2 wifi APs and 4 security cameras through PoE, which is nice because I don't need the 230V power or the bulky power adapters.

Is PoE to everything a common layout? I didn't realize it could support anywhere near that much power.

by mNovak 6y ago

Is PoE to everything a common layout? I didn't realize it could support anywhere near that much power.

brtkdotse 6y ago

PoE is dark magic to me. How can you send any amperage at all through those hair thick leads?!

CodeWriter23 6y ago
It’s ok, the cable will act as a fuse when necessary.
- 29083011397778 6y ago
  Aircraft fuses exist not to save the several hundred-thousand dollar avionics install, but specifically to protect the wire from melting, shorting, or starting fires where fire does not belong [1] (page 11-15, para. 11-51). This is relevant because fire also does not belong inside the walls of your home. I assume you're just having a laugh, but just in case others take you too seriously: wire is not a substitute for a fuse. [1] https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43.13-1B_w-chg1.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...
  - CodeWriter23 6y ago
    Correct on all points, sir or maam.
- bleepblorp 6y ago
  Hey, hey, we're not talking about USB-PD here. :-) PoE actually works.
hinkley 6y ago
They split the amperage across multiple wires. I think PoE is always DC so the runs can’t be as long as with AC. Actually why can’t you run PoE as AC? Embedding a high frequency signal on a lower frequency carrier is older than dirt at this point. At 120 volts, 30 watts is only a quarter amp.
- hinkley 6y ago
  Jesus, IEEE 802.3bt can do up to 100 watts PoE now? https://en.m.wikipedia.org/wiki/Power_over_Ethernet https://en.m.wikipedia.org/wiki/Power_over_Ethernet
- taneq 6y ago
  Cable run length doesn't really have anything to do with DC vs. AC, it's all about the voltage. Power losses are proportional to current squared, so for a given power transmitted, they're proportional to the inverse square of the voltage. We're just used to power transmission being AC because with AC it's easier to switch large currents on and off, and easier to step voltage up/down.
- miahi 6y ago
  The insulation for CAT5 and RJ45 is rated for 125V. You don't want to be that close to the rating for a cable that can be moved/bent/stretched (for comparison, an AC cable is rated at least 300V). And the RJ45 connectors are not designed for that kind of dangerous voltage.
  - hinkley 6y ago
    > And the RJ45 connectors are not designed for that kind of dangerous voltage. Right, there will be a voltage where the pins will arc, and a lower potential where a dirty connector will arc. We rented an office that had previously housed some sort of telecom equipment, so the whole place was wired for, I believe it's 40 volts rackmount equipment. That runs at lower current (thus watts) than computers and so we had to run new electrical and AC into the server room to use it for IT equipment. Cars are trying to switch to 48V because 40-50V is a range where touching a live wire gets your attention but won't do too much permanent damage (eg kill you). Or if you went with 20v you might be able to reuse equipment designed for USB power supplies.
Gibbon1 6y ago
I'm going off memory but you can run ~2amps over 22g. But voltage drop starts being a problem at low voltage. I think Ethernet cables are 24-26g.
miahi 6y ago
CAT5e and especially cat6/cat7 is not usually hair thick, it's usually 24 AWG (and in better cables 23 or even 22). The ampacity of 24AWG is around 3A (more if you accept a bit of heat). PoE is limited to less than 1A per pair, but it makes the power through voltage (usually between 48 and 57 volts), and by using more than one pair.
mschuster91 6y ago
PoE runs at 48V, which means even if you're running only half an amp per pair you'll end up with about 24x4=96W of capacity (the standard actually allows for 600 mA per pair).

monocasa 6y ago

Yeah, agreed. If I'm reading it right and they're putting the TV on PoE too, they're way out of the power budget which is only 30 watts per port. And I say this as someone who's currently going nuts with PoE going to home automation (speakers, cameras, soil monitor sensors, sprinkler relays, etc).

Nick-Craver 6y ago
Clarification there: I have a US-8 behind each TV, taking 802.11af loads to power. For the TV with an AP or something else PoE behind it, those switches can take in 802.11at and output 802.11af on port 8. The TVs themselves aren’t PoE...I’m crazy, just not that crazy :)
- monocasa 6y ago
  Lol, word, that makes a lot more sense. Thanks for the clarification! : )
Gibbon1 6y ago
IEEE 802.3bt is supposed to be good for 50-90W.

sz4kerto 6y ago

Pretty much. I have a small flat compared to US standards, but there's plenty of Ethernet wiring and I have a small (passively cooled) 16-port Linksys PoE switch so I can run 2 wifi APs and 4 security cameras through PoE, which is nice because I don't need the 230V power or the bulky power adapters.