In regards to your question, the main power supply connection of the Intel® Server Board S5520UR uses an SSI-compliant 2x12 pin connector (J2K1). In addition, there are additional power related connectors:
One SSI-compliant 2x4 pin power connector (J3K1), which provides 12 V power to the CPU Voltage Regulators and Memory.
One SSI-compliant 1x5 pin connector (J1K2), which provides I2C monitoring of the power supply
You can find more detailed information on the technical support specification page 92. We recommend installing a power supply that meet the requirements listed on this document.