Most security system have a minimum of 9v DC, but I've seen up to 60v AC or DC, running to the sensors. The concern here is voltage drop as these are extremely low current devices. If you are using magnetic/reed switches, which it sounds like you are, they are unpowered digital switches (either on or off). Digital logic circuit typically require >50% of the feed voltage to trigger as high (on) and anything else is low (off); however, you always want more than the minimum so any interference and such doesn't affect you.
VD = Voltage drop (conductor temp of 75°C) in volts
L = One-way length of the circuit's feeder (in feet)
R = Resistance factor in ohm/kft
I = Load current (in amperes)
- 28ga copper = 64.898 ohm/kft
- 24ga copper = 25.669 ohm/kft
- 18ga copper = 6.3851 ohm/kft
Let's assume the wire is going to a back-lit keypad and draws .5A (trying for worst case scenario)
VD(100ft, 28ga) ~= 6.5V. 9v source means 2.5v at the end which won't work
VD(100ft, 24ga) ~= 2.5V. 9V source means 6.5v, a reed switch should work fine, a keypad or motion sensor may not.
VD(100ft, 18ga) ~= .64. 9v source means 8.36v, almost anything will work.
Now that is the worst case scenario (within reason). Most modern security systems are 24v so 28ga is pushing it for keypads/motion sensor (depending on what they actually need) but 24ga should be fine for everything. It really depends on what your trying to hookup for each stretch of wire, the length, and the input voltage.
If this breaks any Low-Voltage NEC, someone please add details.
*Source: http://en.wikipedia.org/wiki/Voltage_drop, http://amasci.com/tesla/wire1.html