First you must understand how a residential 240/120 volts circuit breaker panel is wired, and more importantly how to balance the load on the 120 volts branch circuits, so that for every 120 volts load on a branch circuit using hot wire L1 to neutral, you should try to place an equal 120 volts load on a separate branch circuit using hot wire L2 to neutral. Because a 240 volts branch circuit use no neutral, the current flowing through hot wire L1 and hot wire L2 is always equal under a 240 volts load. You are aware that a 240 volts circuit use a double pole circuit breaker connected to L1 on one half of the breaker and L2 on the other half of the breaker. While its true that the current flowing in each individual ballast power cord is only half the amount of amps on 240 volts, lets look at an example of using even numbers (2,4,6,8, ect.) of ballasts of the same wattage. Using your example of four 600 watts @ 240 volts, 2.5 + 2.5 + 2.5 + 2.5 amps equal a total of 10 amps flowing on hot wire L1 and hot wire L2. Using four 600 watts @ 120 volts, with two 600 watts loads placed across hot wire L1 and neutral on one branch circuit and two 600 watts loads placed across hot wire L2 and neutral on a separate branch circuit, 5 + 5 amps equal a total of 10 amps flowing on hot wire L1, and 5 +5 amps equal a total of 10 amps flowing on hot wire L2.
