cat5 or cat6 - voltage drop

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cat5 or cat6 - voltage drop

Postby ksu » 2018-04-11 23:38

hello,

I need to feed 12V over an unused cat5/6 cable - it needs to reach 2A to satisfy the specs - all this because the UPS is far away from the junction box and I need to feed 12V@2A from an UPS to a router/switch.
The length is unknown, I would say it should not exceed 100 feet ( ca 30m )
knowing this is an AWG24 cable, I can lower the loss by using two pairs ea. for - and + resulting in 4 pairs being used for power only.
What would be the >>max<< amperage i can push to get 12V ( or really any other "low voltage" ) over a standard cat5/6 cable? - more than just a "guess" of voltage loss would be more than I hope for.

- I am not looking for a great mathematician or physics teacher - although it would be cool.
- I can expect voltage drop of ca. 0.4-0.2V considering the distance - this is my guess based on my questionable abilities at this moment.
- I do not want to fry the cabling.

As always, Thank you All,

Mike
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Re: cat5 or cat6 - voltage drop

Postby jibberjabber » 2018-04-12 00:50

Why not try this ?
What would be the max amperage i can push to get 12V or really any other low voltage over a standard cat5/6 cable?
In the results, there are some charts that show the ratings for various cables,
What would be the >>max<< amperage i can push to get 12V ( or really any other "low voltage" ) over a standard cat5/6 cable?

Obviously, you would not be able to exceed the amperage that the cable is rated for, since I don't want to promote any particular site, and there are plenty in the search results, for example one says:
The chart below will tell you the maximum recommended current handling capacity (Amps) for a given wire gauge in both Chassis Wiring and Power Transmission Wiring versions. Remember to choose the Power Transmission Wiring value for most cables purchased through us and other cable vendors (bundled wire).
The chart included Cat6 and Cat5 cables,.....
Oh , and what does this have to do with Debian ?
N5RLX > "Jibber jabber jibber jabber,all day and all night, jibber jabber jibber jabber"

Then there is Papa Parrot
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Re: cat5 or cat6 - voltage drop

Postby ksu » 2018-04-12 01:15

Please do not ask me what this has to do with Debian - you are not any help anyway - why do you respond?

I can just change the forum and feed this problem to another community - unlike a Debian issue with bind9 that I posted earlier - seems like nobody cares here anyway, but someone should...
so, do not ask me.

At the end I had to bump up the voltage to 15V to have 12 at the other end - I was not expecting it - looks good, although it does not match my "calculations".

Have a good night,

Mike
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Re: cat5 or cat6 - voltage drop

Postby jibberjabber » 2018-04-12 01:42

Well I can't help but to ask, seeing that the board says:
General Discussion
Here you can discuss every aspect of Debian. Note: not for support requests!

Have a good day or night, as well :mrgreen:
N5RLX > "Jibber jabber jibber jabber,all day and all night, jibber jabber jibber jabber"

Then there is Papa Parrot
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Re: cat5 or cat6 - voltage drop

Postby steve_v » 2018-04-12 05:29

ksu wrote:I need to feed 12V over an unused cat5/6 cable - it needs to reach 2A to satisfy the specs - all this because the UPS is far away from the junction box and I need to feed 12V@2A from an UPS to a router/switch.
The length is unknown, I would say it should not exceed 100 feet ( ca 30m )
Sorry I'm late, but I'll put this here anyway in case it benefits someone.

1: Find the resistance / length value for the cable size you are using, either from the manufacturer or a generic table. The first table I found says 0.02567 ohm / foot for 24AWG copper.
2: Calculate total resistance for your run. In this case, 0.02567/2 (using two conductors in parallel, halve the value) *200 (there and back again) = 2.567 ohm.
3: Use ohms law and your known current to derive voltage: V=IxR, V=2x2.567, V=5.134.
4: Get a bigger cable or run at a higher voltage (=lower current), you're going to drop 5.134V at 2A over 100 feet. You'll get not 12V but 6.866V at the other end.

See here for a lazy man's method.

Disclaimer: I am an industrial electrician.

ksu wrote:At the end I had to bump up the voltage to 15V to have 12 at the other end - I was not expecting it - looks good, although it does not match my "calculations".

Sounds like your load isn't actually 2A, more like ~1.2. That's around what I'd expect for a device rated at 2A and doing not much. Make sure it performs under load.
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Re: cat5 or cat6 - voltage drop

Postby ksu » 2018-04-12 13:16

great! - thank you steve_v
the connection is stable, it was up in the morning :)
I will revisit the calculations again later today, and finish the wiring part "properly".

also, truth is, this is not Debian related, and I need to agree and apologize for posting it in the wrong section.

it was a long day yesterday (...) thank you all!

Mike
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