This is Turtle.
the Amp draw you will have on any given furnace would be very much different. a small 2 ton 65K BTU heat would take about 5 amps. . Then you take 5 ton cooling and 125K BTU heat with electronic filter, 3/4 horse Power Blower Motor, and humidifier would take about 12 amps.
1] Gas furnaces should have a single circuit to it for amp ability to the furnace. i like to have a #12 -2 with groud and a 20 amp breaker for the larger units and the 12-2 with ground with a 15 amp breaker for the small units.
2] All gas furnaces should have it's own circuit ! You can rob a circuit but it is your call. now it might come to this one day. You plug in your computor and when the gas furnaces comes on it throws the breaker. Then you can start running another circuit then when it is knee deep in snow.
TURTLE
TURTLE