For example, if the heaters add up to 4,000 watts and also you divide that by 240, you get just over sixteen amps. Divide the overall wattage of your heaters by 240 to find out how many amps of present you'll need. In this case, you'll need to put in two separate 20-amp circuits for the heaters, because you cannot safely draw greater than sixteen amps over 12-gauge cable.