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Ethernet cable resistance

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Stonent:
Anyone know off the top of their head what the maximum resistance should be on about 100M of a Cat5e Ethernet cable pair?

Or I guess even better would be if I put a 9V battery across one pair, what kind of voltage drop should I expect on the other side? (I guess technically in that case it would be 200M since it would be in a loop.) Testing resistance on installed cable might be nearly impossible directly but sending a small voltage down a pair and testing it would probably be easier.

AndyC_772:
It's normally of the order of 1 Ohm for every 10 metres, so your 100m cable will be about 10 Ohms end-to-end (20 Ohms round trip). The voltage dropped across the cable will depend on the current you draw, of course.

Dave:

--- Quote from: Stonent on September 15, 2013, 09:53:23 pm ---Testing resistance on installed cable might be nearly impossible directly but sending a small voltage down a pair and testing it would probably be easier.

--- End quote ---
You could short a pair at one end and hook up a power supply and two multimeters to the other. You send a constant current through that pair, and measure both the voltage drop and the exact current. Apply Ohm's law and you have the resistance. Don't forget to divide it by 2 to get the resistance of a single wire. :)

Bertho:

--- Quote from: Stonent on September 15, 2013, 09:53:23 pm ---Anyone know off the top of their head what the maximum resistance should be on about 100M of a Cat5e Ethernet cable pair?

Or I guess even better would be if I put a 9V battery across one pair, what kind of voltage drop should I expect on the other side? (I guess technically in that case it would be 200M since it would be in a loop.) Testing resistance on installed cable might be nearly impossible directly but sending a small voltage down a pair and testing it would probably be easier.

--- End quote ---
Cat 5e has a loop-resistance of less than 0.188 Ohm/m. So, pulling 1A through one pair for a cable of 1m drops ~0.188V (2m -> ~0.376V, etc). If it is only DC, then this will hold. If you put AC or AC superpositioned on DC, then the AC part will see the impedance of the cable (depending frequency). For frequencies from ~100kHz you need to take account for the characteristic impedance (and that means proper termination). If you put a digital signal on it, you will see the impedance almost immediately (square-wave contains higher harmonic frequencies).

See also: https://en.wikipedia.org/wiki/CAT_5#Characteristics

digsys:
I had that exact situation last week. 12R drop on 120M of cable, one way. Matched exactly the specs of the reel.
NOTE: If you have a S/Mode at the other end though ie POE, it may not "start up", if it isn't a soft start type.
Had that too. A 1,000uF cap fixed it, messy but had to happen.

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