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Thread: Maximum voltage and current over cat 5e

  1. #1
    Automated Home Legend Paul_B's Avatar
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    Default Maximum voltage and current over cat 5e

    Evening all,


    I've been doing a bit of interweb searching and I think it would be fine to use Cat 5E cable to send 12V and 1 Amp (12W), as I understand it PoE is around 50V and up to 1 Amp so 12W should be fine. What I'm not sure about is if this is three strands or just one strand. In normal IdraNET wiring we have a spare pair can I use this spare pair to power a LED light so long as it is below 12W (12V & 1 Amp)?

    Cheers

    Paul

  2. #2
    Automated Home Legend paulockenden's Avatar
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    PoE is 350mA. Remember, it's current that decides how hot wires get, not power. That's why fuses are rated in Amps rather than Watts.

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    Automated Home Legend Karam's Avatar
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    Quote Originally Posted by paulockenden View Post
    PoE is 350mA. Remember, it's current that decides how hot wires get, not power. That's why fuses are rated in Amps rather than Watts.
    I can understand why, other prameters being constant, it may seem so. However it is energy dissipated by the resistance which heats the conductor up. So it is power dissipation that determnes whether the wire will melt or not. This means it is not a straightforward answer because it will also depend on how quickly the conductor can exchange heat with its environment. This is why if you have a rolled up 13A extension lead it should always be unrolled if used at that capacity (e.g for a 3KW heater at 230Vac. I have seen the effects of not doing so .

    Cat5e typically has a conductor resistance of 10ohm/100m or perhaps a bit less. If you have 12V across a pair at one end of a 10m length (1ohm per each conductor) and a 1A load at the other, then it would mean a 2V drop (V=IR) in total across the 10m. So first thing to bear in mind is that the load wil be seeing 10V not 12V. The conductors themselves will have 1A flowing through them and the power dissipation will be IxIxR=1W each. So essentially you are considering 2W across a 10m length of cable. I think that is not a lot of power to have to get rid of over a 10m run notwithstanding the multiple sleeving insulation but it will of course depend on the cable surroundings (and what the other 6 conductors are doing).
    Last edited by Karam; 28th April 2018 at 11:24 PM. Reason: typo

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    Automated Home Legend Paul_B's Avatar
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    Thanks both for the response. The run I'm thinking about is about 2.5m. The cable is already in position and for about 75% of the run in the ceiling void the last 25% is behind drywall. Looking at some LED strips of around 30 cm they seem to be well under 1Amp and this would be to add some colour to a cabinet located in a restroom so not on for very long.

    Unfortunately I didn't have the foresight to run mains cable.

    Paul

  5. #5
    Automated Home Legend chris_j_hunter's Avatar
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    this approach seems to have been mainstream for a year or two ...

    ie: doing LED lighting via PoE Cat-something cable has been around for a few years, and has been / is being used by a number of companies in a number of commercial projects ...

    eg: LuxSpace PoE from Philips :

    and to quote from their data-sheet ...

    ---

    LuxSpace PoE ... with Power-over-Ethernet (PoE) technology, LuxSpace PoE receives power and data over a single standard Ethernet cable, eliminating the need for separate power cabling. With the simple click of a connector, LuxSpace PoE luminaires become part of a complete, integrated connected lighting system, delivering extraordinary illumination experiences and value beyond illumination. A built-in lighting and control system gives office users personal control over their preferred light settings via a specially designed smartphone app. With integrated sensors, LuxSpace PoE luminaires can track activity patterns, daylight levels, and in the near future humidity, CO2, temperature, or other data. This data allows facility managers to gain deep insight into building operations, helping them optimize the delivery of resources, enhance the experience and performance of occupants, and support improved asset management.

    Benefits ...

    • LED lighting and control system with breakthrough Total Cost of Ownership, reducing installation costs by around 25% and commissioning costs by around 50%

    • Software applications optimize working processes, comfort, and asset utilization and management

    • Clear customer benefits in both the construction and usage phases of a building

    Features ...

    • Data gathering on building usage via luminaire sensors
    • Personal control of lighting and temperature settings
    • Simpler and cheaper installation and lighting commissioning

    Application ...

    • General lighting for office buildings

    ---

    must say, the approach would seem to have huge potential ... our house has Node0 patched Cat-5e & switched Cat-6 run in multiples to many many locations, so there would be lots of possibilities ...

    and maybe not only with lighting, if the approach catches-on, going beyond existing security / camera devices ... which it seems certain to do, in the commercial world - and hopefully, too, in the domestic world ...

    Chris
    Last edited by chris_j_hunter; 2nd May 2018 at 05:48 PM.

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    Automated Home Legend Paul_B's Avatar
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    Thanks for sharing the Philips LuxSpace solution. I like the idea of using potentially spare Cat5e runs for low voltage applications such as lighting. At the moment it seems a little industrial being aimed at the commercial market but definitely something to keep an eye on for the future

    Paul

  7. #7
    Automated Home Legend chris_j_hunter's Avatar
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    yes, I thought also it gave some confidence that the approach was likely to work ... various possibilities for interference of one sort of another don't seem to be significant ...

    so, and do let us know how your install turns out - not least because we have a few situations where doing something of the sort might be helpful ...

  8. #8
    Automated Home Legend chris_j_hunter's Avatar
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    thought these might be of interest :

    https://www.amazon.co.uk/gp/product/...SGXW39ZT&psc=1

    https://www.amazon.co.uk/gp/product/...SGXW39ZT&psc=1

    https://www.amazon.co.uk/Wire-Connec.../dp/B071H7L3P3

    for T-ing off Cat-5e cable, to allow add-on PIR sensors to be powered, or after-thought lights to be added ...

    not tried them, yet ... has anyone done so, per chance ?

    Chris
    Last edited by chris_j_hunter; 7th September 2018 at 10:42 AM. Reason: added a third ...

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