Fences are permitted in this district in accordance with the following limitations and conditions:
(a) If an SP-INS zoned lot is not adjacent to a lot in a Residential district, fences may not exceed six ft. in height, except that fences surrounding athletic fields may be up to 15 ft. in height.
(b) If an SP-INS-zoned lot is adjacent to a lot in a Residential district, fences may not exceed a height equal to one hundred twenty-five percent (125%) of the maximum fence height permitted by the most restrictive adjacent Residential district.
(c) Any portion of a retaining wall that exists above the higher level grade shall comply with the requirements for fences. Where a fence is located on top of a retaining wall, the combined height of the retaining wall that exists above the higher level grade and fence shall not exceed the maximum height permitted for a fence. This subsection shall not apply to any retaining wall required to meet alternative standards by any provision of The Philadelphia Code, any provision of state or federal law, or any regulations adopted to implement the code or those laws.