National Electrical Code (Article 250.20 section A) defines "Low Voltage" as it applies to home distribution systems as less than 50 volts.


  (A) AC Systems of Less Than 50 V. Alternating-current systems that
  operate at less than 50 V are not required to be grounded unless:
      (1) The primary exceeds 150 V to ground
      (2) The primary is ungrounded


This includes systems such as cable television, network and telephone, doorbell, etc. Note that standard telephone wiring can typically "ring" at 48 volts and still provide an electric shock.

The reason the code makes the distinction for low voltage is so that some types of wiring can be exempt from additional safety requirements such as conduit usage, breakers, installation method, etc. Even so, there are still violations where installers confuse the issue (Top ten violations in low voltage systems PDF).

There are (to my knowledge) no current limits for data lines, except for the rating of the cable itself. Since most UTP network and telco cables use 24 or 26 gauge wire, this would theoretically be around 3.5 amperes, but you have to consider length, temperature, etc. to properly determine current load (discussion at allaboutcircuits).

EDIT

Guessing that you might be wanting to use data lines for higher power applications, I thought I'd add some additional commentary. If you are wanting to use Cat5 for power-over-ethernet applications, you'll probably want to stay under 1A or less per conductor. This is because of the voltage drop and resistance on longer distances. Keep in mind that higher voltages are more suitable for transmission (longer distances) as long as you stay under the 50V "low voltage" specification. Even though charts show that 24AWG can carry 3.5A, I wouldn't consider this safe unless working with a very short cable. Power-over-ethernet standards are confusing, but generally they are 48 volts or less at 600mA or less.

Electrical characteristics (at Wikipedia) for UTP Cat5E show a max current of 0.577A per conductor and a max voltage of 125V.

What are the current and voltage limits when determining low vs. high voltage wiring?
Because if it's not, it should be!
I'm trying to get 2 x 1GB PC2700 ECC Registered DDR333 DIMMs (Micron brand) to work in my computer, but it won't POST: the computer starts, fans and hard drive spin up, but the monitors don't power up.


Motherboard: Supermicro X5-DAE, E7505 chipset
CPU: (2) Xeon 2.4 GHz 
BIOS: 1.3b, the latest


Crucial recommends PC2700.  Corsair even recommends PC3200.  So I figured PC2700 would be ok.  Admittedly, the manual (section 2-5) states


  The X5DAE/X5DA8 supports up to 12 GB of ECC registered DDR-266/200 (PC2100/1600) memory.


but manufacturers are usually conservative.

So here's the Q: Is it supposed to work?  Can I prep the BIOS beforehand, change some timings or something, or is this type of thing just a no-go in ECC land?

If you can find an example of others using PC2700 with this board let me know.



Update:  I removed all cards, USB devices, cables and swapped the AGP video card out with a known good PCI just to mix things up.

Computer gives 1 beep on startup, which is normal.  No other beep codes.  Monitor still blank.  I've tried the PC2700 pair in all 3 banks, no change.  My good RAM is 2 x 512MB PC2100 ECC Crucial brand and works in all 3 banks.  I tried putting the PC2100 in bank 1 and PC2700 in bank 2 hoping bank 1 would force everything to PC2100.  Still nothing.  Anything else I miss?



This has been resolved with the vendor as a high density memory incompatibility issue.  Ebay has some literature on it:


  JEDEC standard/guideline specifies
  that 64Mx8 and 32Mx16 devices are to
  be used to construct a 1GB Unbuffered
  module. Any 1GB Unbuffered module
  constructed by using 128Mx4 device
  BREAKS all the JEDEC
  standard/guideline in which is
  supposed to be designed only for
  Registered module. Since JEDEC doesn't
  want the modules to be built that way,
  so the companies who make them (a lot
  are generic and 3rd parties), don't
  put their company label on the
  Unbuffered modules.

