Why is there a voltage drop in a twisted pair wires of an UTP cat.5 cable?
Summary


It sounds highly likely that you have got a sample of 'known to exist' Asian sourced knock-off clones of genuine produts, which use variable conductor sizes and resistances within the same cable and copper clad aluminum. 
Applied brand names and labelling can be the same as genuine products. 
Look at the wires closely - if you have copper aluminum it's a cheap fake. 
Measure wire by wire resistances - see below. All should be the same and all should be under 20 ohms per 100m loop resistance. (For exact value - see below). 




But first - just in case: 


I'd measure end to end resistance of each wire. As will be seen below, you may get 6:1 (or worse of different) resistances within the same cable. 
You don't say what load you are placing on the cable when getting this result - either in terms of load resistance or current drawn.

If you measure with a voltmeter and with no load the result should be essentially 12 volts. If not then you are doing something wrong or something starange is happening. Voltage drop is caused by current flowiing in a resistance - a voltmeter alone draws ~= zero current so there will be ~= zero voltage drop. 


With a say 10 ohm loop resistance (see below), to get 5V out from 12v in the load resistor would have to be 10 * 5/(12-5) =~7 ohms. Less if your loop resistance was less. If your load is > 7 ohms them summat aglae. Read on ... 

What you should see:

Cat 5 cable can be 


Westpenn 20, 22 or 24 gauge. Loop resistances per 100 metres are 7 / 11 / 18 ohms 
AMP Cat5e 24 gauge, 9.4 ohms/100m
Wikipedia 19 ohms.


BUT

Low quality clones


Uh Oh !!! Electronicspoint 20 + 40 + 120 ohms, 


Whilst doing some dc loop resistance tests on some 'certified' cat5e cable I
found that green and orange pairs had 2x the expected resistance, and the blue
and brown pairs had 6x the expected resistance.
Close inspection revealed cooper clad aluminium with the copper thickness
varying depending on the pair....



And other people comment


Section 5.6.6.1 of this document
http://www.commsalliance.com.au/__da.../S008_2010.pdf
gives the Australian requirements for approved telecommunications cables. Other
than coax, which is permitted to use a copper-clad aluminium centre conductor -
provided the centre conductor is at least 2mm in diameter, all other cables must
be either plain copper or plated (tin) copper. Table 3 gives the resistance
requirements.


and 


That cable is definitely a Chinese knock-off and is not UL certified.


NETCONNECT is a proprietary trade name of AMP (Tyco). Here is the data sheet for their genuine Cat 5e cable. Note this is UL certified with the same number as on your Chinese cable.
http://www.ampnetconnect.com/documen..._Cut_Sheet.PDF