According to the updated NYSE Q2 circuit breaker levels, it will take a 4,350 point drop in the NYSE for an all day trading halt. Of course, if the DJIA tumbles by 30% intraday, whether to close the several hundred shares trading on the NYSE will be the last thing on people's minds.
From NYSE [5]:
The New York Stock Exchange will implement new circuit-breaker collar trigger levels for second-quarter 2013 effective Monday, April 1, 2013.
Circuit-breaker points represent the thresholds at which trading is halted marketwide for single-day declines in the Dow Jones Industrial Average (DJIA). Circuit-breaker levels are set quarterly as 10, 20 and 30 percent of the DJIA average closing values of the previous month, rounded to the nearest 50 points.
In second-quarter 2013, the 10-, 20- and 30-percent decline levels, respectively, in the DJIA will be as follows:
Level 1 Halt
A 1,450-point drop in the DJIA before 2 p.m. will halt trading for one hour; for 30 minutes if between 2 p.m. and 2:30 p.m.; and have no effect if at 2:30 p.m. or later unless there is a level 2 halt.
Level 2 Halt
A 2,900-point drop in the DJIA before 1:00 p.m. will halt trading for two hours; for one hour if between 1:00 p.m. and 2:00 p.m.; and for the remainder of the day if at 2:00 p.m. or later.
Level 3 Halt
A 4,350-point drop will halt trading for the remainder of the day regardless of when the decline occurs.