look at the rating stamped on the cable assembly. the thermostat wire i use and sell on the site is CMR/CLR2 rated - 300V
http://www.cerroretail.com/electrical/product-line/specialty-wire/thermostat/
voltage is a function of insulation
ampacity (i.e current carrying capacity) is a function of wire gauge, and is required to be reduced when 3 or more conductors are in a cable/conduit/etc. is not usually spec'd on cable because it varies how you use it (free air, conduit, buried, etc)
Note 8 to the ampacity tables in the NEC contains the requirement columns for derating ampacity because of adjacent current-carrying conductors. This note states that when the number of conductors in a raceway or cable exceeds 3, the ampacities are to be reduced by the appropriate percentage.
18 ga per NEC is 5.6A ampacity continuous. since there are only 2 conductors in t-stat cable it is rated for full 5.6A continuous. most of us run 2A or less so there is plenty of headroom.
http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/industrial_control_panels/BUS_Ele_Small_Conductor_Protection.pdf
for long runs always upsize your wires to reduce voltage drop. google "voltage drop calculator" theres tons of them online. if youre mounting a big high-current driver like an HLG 320 or 600 in another room, 12/2 romex is cheap and widely available
a 13A HLG-600-48 will lose 0.41V over 10 feet of 12 ga, but will lose 1.04V over 16GA - thats 17 watts thats going straight to heat, where it could be less than 6 with adequate wire sizing.
voltage drop is particularly important when youre running a driver near its voltage limit (like 50V cobs on a 48A, or 4 36V cobs on an 185-1400b