It is important to note that when a battery is specified at 18 AH, that is for the current draw over a long period of time. For our battery, it was for 20 hours. When you draw more current, the battery has a lower capacity.
It's defintely worth your while to look at the battery's data sheet:
http://www2.usfirst.org/2005comp/Specs/batex.pdf
For our battery it is 18AH over 20hrs. Over 1 hour, it is only 10.8 hr. Looking at the discharge characteristics, At approximately 50 amps, it will last between 5 and 8 minutes. At 100-120 amps, you are probably getting close to the 2 minute 15 second match length.
Also note that the 120 amp circuit breaker doesn't trip as soon as it reaches 120 amps. The data sheet is here:
http://www2.usfirst.org/2004comp/Spe...it_Breaker.pdf
The circuit breaker will actually allow 200+ amps for 10-20 seconds. It will also allow at least 140 amps for the full 2 minute match.
With those two pieces of information, I have no doubt that it is possible to completely drain the battery in 2 minutes and 15 seconds without tripping the main 120 amp circuit breaker.