A header assembly comprising: a first header connector having a first header housing having a first cavity configured to receive a first receptacle connector therein, the first header housing having a first at least one cartridge latch; a first signal header cartridge received in the first cavity and locked in the first header housing by the first at least one cartridge latch, the first signal header cartridge having a plurality of signal contacts extending between mating portions and mounting portions, the mating portions extending from the first signal header cartridge into the first cavity and defining separable mating interfaces; a first power header cartridge received in the first cavity and locked in the first header housing by the first at least one cartridge latch, the first power header cartridge having a plurality of power contacts extending between mating portions and mounting portions, the mating portions extending from the first power header cartridge into the first cavity and defining separable mating interfaces; wherein the first header housing comprises a mid-wall within the first cavity extending from sidewall to sidewall and dividing the first cavity into a front cavity for receiving the first receptacle connector and a rear cavity that receives the first signal and power header cartridges (132, 134), wherein the mid-wall includes a plurality of signal contact openings through which the signal contacts pass into the front cavity, and a power contact opening, characterized in that the power contact opening is an elongated slot that receives the power contacts, irrespective of the location of the power contacts along a mating edge of the first power header cartridge. a first header connector having a first header housing having a first cavity configured to receive a first receptacle connector therein, the first header housing having a first at least one cartridge latch; a first signal header cartridge received in the first cavity and locked in the first header housing by the first at least one cartridge latch, the first signal header cartridge having a plurality of signal contacts extending between mating portions and mounting portions, the mating portions extending from the first signal header cartridge into the first cavity and defining separable mating interfaces; a first power header cartridge received in the first cavity and locked in the first header housing by the first at least one cartridge latch, the first power header cartridge having a plurality of power contacts extending between mating portions and mounting portions, the mating portions extending from the first power header cartridge into the first cavity and defining separable mating interfaces; wherein the first header housing comprises a mid-wall within the first cavity extending from sidewall to sidewall and dividing the first cavity into a front cavity for receiving the first receptacle connector and a rear cavity that receives the first signal and power header cartridges (132, 134), wherein the mid-wall includes a plurality of signal contact openings through which the signal contacts pass into the front cavity, and a power contact opening, characterized in that the power contact opening is an elongated slot that receives the power contacts, irrespective of the location of the power contacts along a mating edge of the first power header cartridge.