Power over cat5. What is the max current?

[Ed.Kloonk]

Hi.

Been asked to run a remote router with wifi to an attic. Would like to use cat5, 2 pairs for network and the remaining 2 pairs to power the router remotely. 12 volt dc. Wanting to do this because running separate power may be difficult. Very old house, existing wiring is extremely fragile. 

Wanting to know an very approx expected maximum amperage that can be pulled through a strand of cat5 over say, 1m 10m and 50m lengths. What kind of volt drop can I expect for those loads?

Or should I just run a beefy figure 8 up there?

[wraper]

You would rather ask what resistance to expect. As voltage drop depends on the cable resistance and current (which you didn't say) you pass through the wire. As for resistance, it depends what cable do you use. Many Cat 5E cables are cheating and useing thinner conductors or crap CCA (copper clad aluminium) wires. Considering that normally there should be AWG 24 wire, nominal resistance should be 0.084 ohm/m per single conductor. Datasheet of some random Belden cable specifies 9.38 Ohm/100m = 0.098 Ohm/m maximum resistance. You can calculate the voltage drop yourself (U=I*R).

[KedasProbe]

You can use this, although I don't know how accurate it is, it will give you a good guess.
http://www.powerstream.com/Wire_Size.htm

[Rasz]

just a router? no problem
voltage drop will be fine if its powered from 12V, router has internal buck dropping it down to 3.3 anyway

[Ed.Kloonk]

just a router? no problem
voltage drop will be fine if its powered from 12V, router has internal buck dropping it down to 3.3 anyway

Thanks.

[wraper]

just a router? no problem
voltage drop will be fine if its powered from 12V, router has internal buck dropping it down to 3.3 anyway

Depends on the router and wire length. Some of them consume a high enough power that you would'n want to run them over more than 10-20m of cable. Also because of the DC/DC converter, current consumption will rise with voltage drop. Therefore even if the router still works on the lower voltage (with increased stress on the DC/DC converter), power supply might be overloaded.

[PeterFW]

Over 20m of old and cheap CAT5 i was able to get a little over 1A at 12V without that much loss, i can not remember how much but it worked.

If you do not want to use POE you could put a 24V laptop brick at one end and a cheap buck regulator on the other side.
Ohms law will let you calculate the voltage drop, you will have to know the conductor cross section of course.

[AndyC_772]

The resistance of Cat 5 cable is in the order of 1 Ohm for every 10 metres. Remember to double this for the round trip.

A good start if you're interested in how much power you can get over a Cat 5 cable is to look at the Power over Ethernet specs. 802.3af specifies a max of 13W @ 48V. 802.3at allows more.

[wraper]

Over 20m of old and cheap CAT5 i was able to get a little over 1A at 12V without that much loss, i can not remember how much but it worked.

Should be about 1.5-2V voltage drop if running 2 wires in parallel for each polarity.