In the US, a 15 amp circuit requires 14 gauge wire; a 20 amp circuit requires 12 gauge wire. Even with 100 amp service, there should be 220v coming to the house. In the breaker box, the phases are split into the "right" side and the "left" side. So, two breakers in the same horizontal "line" are on two different phases. To get 220v to a device, a breaker fits into two vertical slots, and thus has two phases (typically, red and black wires, one wire going to each phase).
If there are any open slots in the panel, you can take the cover off and see what I mean. It'll be harder to see if there are no open slots.
100 amps is not bad, assuming you have things like gas stove/cooktop, gas clothes dryer, no AC, etc. It's when you start loading the system with a bunch of high-demand electrical (stove, cooktop, clothes dryer, air conditioners, heaters), that it becomes an issue.