The 300 v limit is a hard limit that should never be exceeded. Remember that in colder weather the voltage rises and you must be considering the open circuit voltage for the pump. In many places in the US, you can only use up to 6 modules in series. In warm-winter areas, you can go to 7 in series, but you have to run the low-temp coefficient numbers on the PV module to make sure that it does not ever exceed 300 VDC. You can however, use 2 parallel strings of 4 modules in series. it will be running the pump at max speed most of the day if the sun is out, but will optimize production on less than sunny days.
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