Don't know the official line, but I am pretty sure I am entering more that 4X my equity when buys and sells are totalled. I do know that the IB system will not allow new position opening orders, and will automatically cancel standing orders to open new positions, if the account is undermargined. If maintenance level is violated, the system will automatically liquidate positions. (I have no idea how it decides which ones to blow out- never had to find out).