It depends on if you have transformers that give off heat. Try to get the electrical engineer to set them outside or hang them on a pole. If you have VFD's they can also give off heat. See the manufacturer's data for how much. If outside air is not too hot then you can ventilate with outside air to cool the space. Outside air will only get you down to 10 or 15 degrees above outside air temp, depending on how much air you move.
However, some of the electronic devices such as VFD's, TVSS panels or tranfer switches may not be able to handle much heat so you may need to air condition.
A good idea, should you have some extra general exhaust (not toilet or lab hood exhaust, of course)left over as a result of meeting ASHRAE 62 requirements, is to circulate a sufficient quantity of such exhaust through the switchgear room to maintain both humidity and temperature control. The room can be maintained at a negative pressure via a roof mounted or inline fan to allow the easy passage of air from either a plenum source or a ducted system. Maintaining good temp and humidity control in the switchgear room is a good idea for keeping the gear in good condition.