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I'm trying to learn about PoE and a friend told me the 100 meter limitation on a PoE circuit is because of the DC voltage drop on the power channel, not the data. I had understood the opposite, that the limitation is due to the data signal degrading beyond that point too much to be reliable. He claims the data uses an "extremely low voltage" so it doesn't have the same problem as, say, a 48V power channel. I suspect he has things backwards and it is actually the lower voltage that would have more problems due to voltage drop.

In addition to this question, how would one stretch the limits if you needed a little more distance? My friend said he had cable runs of over 500 feet and never had trouble, but he relied on careful planning (avoiding EMI sources and keeping cables free from bends).

He also mentioned some kind of boosters you could put midspan to boost the signal if needed. What are these, and would they also work for the power channels on a PoE run?

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  • Because the ethernet UTP standards are limited to 100 meters, the PoE was engineered to match the ethernet UTP cable length.
    – Ron Maupin
    May 16 at 20:56
  • Because of the "E" in "PoE". For purely passive PoE, I've seen 500ft runs (to an LED light panel.) Higher levels of PoE power require some level of communication, so the 100m ethernet limit may still apply. The ethernet part is only guaranteed to work to 100m, beyond that is rolling the dice.
    – Ricky
    May 17 at 21:37

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IEEE PoE is an option for IEEE 802.3 twisted-pair Ethernet which is generally limited to 100m in total - up to 90m of rigid, solid-core cable and up to 10m of flexible, stranded cable.

If you use thinner cable or longer runs, the voltage drop may exceed the specifications and your PD might not work. For data, attenuation and crosstalk increasing with distance and exceeding the reach may cause increased error rates or total link loss.

There are many cases where you can exceed the reach without noticing problems but if anything happens you're on your own. If you need to rely on the link you should consider fiber for data and a more local power source.

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PoE requires copper wire. Otherwise, longer reaching spans of fiber optic cable would normally be used along with a media converter + PoE power injector at the far end. Another solution to distance limitations is wireless WiFi which can get up links up to 10+ miles. You can run fastEthernet equivalent throughput, 10Mbps, approximately 1 mile (1.6Km, or 5,000ft) over Cat 3 or 6 copper. Cisco's Long Reach Ethernet product line discusses:

Cisco Long Range Ethernet (LRE) technology dramatically extends Ethernet over existing Category 1/2/3 wiring at speeds from 5 to 15 Mbps (full duplex) and distances up to 5,000 feet. The Cisco LRE technology delivers broadband service on the same lines as Plain Old Telephone Service (POTS), digital telephone, and ISDN traffic.

PoE is supported over Cat3 as well however the distance limitation of the power is not well documented other than to say the standard 100 meters.

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    LRE is a VDSL variant - completely different from 10BASE-T or 100BASE-TX. The 100m maximum for those is defined in IEEE 802.3 clause 33 as is the maximum loop resistance.
    – Zac67
    May 16 at 22:07
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    As Zac said, LRE is not "ethernet"; it's not defined by 802.3. What was originally called LRE became VDSL. I don't know what Cisco is pushing for their proprietary LRE. (yes, those ports speak ethernet as well, but the LRE mode isn't.)
    – Ricky
    May 17 at 21:39
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    Power is a factor of wire size and length. The "cat" specs are for signal characteristics, as such, they mean nothing to DC power capability. I.e. a 24awg CAT3 cable can carry more power than a 26awg CAT6 cable, of the same length.
    – Ricky
    May 17 at 21:45

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