See this table:
You will note that they give 2 current ratings for 20 gauge wire: one for a bundled wire (poor air circulation) which is about 1.5 A, another for a single conductor on a chassis (good air circulation) which is 11 A!
The 'bundled' rating is EXTREMELY conservative. The National Electrical Code allows
20 A in a 12 gauge 2 wire cable, rather than the 'bundled' rating of about half that. Your layout wires, hung in the open air under the table, are well-cooled, so I would say
3 A for the 20 gauge wire is fine. This is enough to run about 3 HO locomotives; your usual cheapie HO power pack delivers up to 1 amp.
Now, since the resistance of this 20 ga copper is about 10.15 ohms per thousand feet, we can use Ohm's Law to calculate the voltage drop at this rated current:
E = IR
E = 3 A * 10.15 Ohms = 30.5 volts per thousand feet.
It will take this potential to push a 3A current through a 20 gauge wire 1000 feet long.
With an 8 x 12 layout, the longest needed bus would have to go all the way around the table perimeter, and be 2*8 + 2*12 = 40 feet long. Using the above voltage drop, you get a 1.22 volt drop at the farthest point; half that if your bus is a continuous loop and the current only has to go halfway around.
Wire of 14 AWG has about a quarter the resistance of 20 AWG and is rated by the NEC for 15 amps in a cable. Using 14 AWG, your voltage drop as calculated above would be around .31 volts at the farthest point, instead of 1.22.
Since it's easy to get 14 gauge solid wire (type THHN, at your hardware store) it's probably best to use it. It will probably be cheaper than the 20 gauge, anyway. Suitcase connectors, to my mind, aren't really necessary for model railroading. Just twist the wires together tightly and tape. To connect to the bus, wrap and tape. Solder if you can - it's cheaper than any solderless connector. If you can't, just buy a soldering gun and learn how; it's an easy and very useful skill.
Cordially yours: Gerard P.