ATX Version 3 Multi Rail Desktop Platform Power Supply

Design Guide

ID 336521
Date 11/01/2023
Document Table of Contents

Modular Power Supply Connectors

A modular power supply, with multiple detachable cable options, is recommended to provide the greatest flexibility to the end user. This approach reduces the chassis volume consumed by unused power cables which improves both cable routing and cooling.

The dedicated 12V CPU connectors on the motherboard are either a single 8 pin (2x4) connector, or one or two 4 pin (2x2) connectors, detailed in Section +12 V Power Connector. These are often referenced as EPS12V connectors. PCI-Express (PCIe*) Add-in Card Connector details three cable/connector options that deliver +12 V power rails to a PCIe* Add-in Card. While each of the three connectors provides a 12V rail to power the chassis component, they use different pin locations and mechanical keying, and are not directly interchangeable. Therefore, a modular design is an option to support multiple end use configurations.

For example, the end user might require a power supply to support a system with a lower-power or non-overclocked CPU and multiple higher-power graphics cards and thus populate the PSU outputs with multiple power cables configured for the PCIe* graphics cards. Alternatively, a higher power overclocked CPU system mounting a single, lower power graphics card may require more 12V CPU power and a single plug for PCIe* power. A modular power supply allows connectors on the power supply to provide 12V power and then the end user can select the appropriately configured cable/plug to provide 12V power in their system with no change to the pinout of the PSU itself.

Three examples of modular designs are shown below. The orange box in each picture identifies the connectors on the power supply that provide 12V power rails.


18 AWG wire is typically used to meet the 6-8 Amp/pin requirements of most chassis components. (An important exception is the 12V-2x6 connector, which requires thicker, 16 AWG wires.) Based on this example of 6-8 Amp/pin, the following recommendation applies to how much power/current may be supported by each connector determined by the number of +12V pins included in that connector. Using Table 2-1, the number of pins and connectors for motherboard 12V CPU (EPS12V) connectors can be calculated.

  • 12-16 A support for 2x2 (4pin) connector
  • 18-24 A support for 2x3 (6pin) connector
  • 24-32 A support for 2x4 (8pin) connector

A distinction must be made between the current per pin available from the PSU through a connector pin and 18 AWG wire vs. the maximum demand for current from the connected chassis component such as a PCIe* Card or motherboard 12V CPU connector(s).

For example, a standard 2x3 PCIe* power connector supporting a graphics card will draw no more than 6.75 Amps total through its three power pins and two ground pins. Similarly, the 2x4 PCIe* power connector will draw no more than 13.5 Amps total through three power pins and three ground pins.

It is possible to reduce the number of conductors consumed at the PSU by providing the 12V to a 2x4 PCIe* card power connector through a 2x2 modular connector at the power supply, for example. Before reducing the conductor count, the PSU designer should also consider the copper losses and the resulting voltage drop incurred by the two cable connectors and a length of the 18 AWG conductor.

This recommendation is based on common design practice. The PSU and system designer may deviate from this guidance but remains responsible for designing the PSU to meet all electrical, thermal, safety and reliability requirements based on the application of the PSU.

It is important to recognize that the 600W 12V-2x6 cable/plug described in PCIe* CEM Revision 5.1 (detailed in Section Section ), requires 16 AWG wire and a per-pin current capacity of 9.2 A. Modular PSU designs supporting the 12V-2x6 cable/connector may benefit from implementing a corresponding 12V-2x6 header on the PSU chassis that would accept a symmetric “double-ended” 12V-2x6 cable/connector harness, instead of a proprietary connector. This simplifies management of the four required sideband conductors.