Power Supply Voltage Tolerances

The power supply in a PC supplies various voltages to internal devices in a computer via power connectors. These voltages don't have to be exact but they can only vary up or down by a certain amount, called a tolerance.

If a power supply is providing the parts of a computer with a particular voltage outside of this tolerance, the device(s) being powered may not work properly... or at all.

Below is a table listing the tolerances for each power supply voltage rail according to Version 2.2 of the ATX Specification (PDF).

Power Supply Voltage Tolerances (ATX v2.2)

Voltage Rail	Tolerance	Minimum Voltage	Maximum Voltage
+3.3VDC	± 5%	+3.135 VDC	+3.465 VDC
+5VDC	± 5%	+4.750 VDC	+5.250 VDC
+5VSB	± 5%	+4.750 VDC	+5.250 VDC
-5VDC (if used)	± 10%	-4.500 VDC	-5.500 VDC
+12VDC	± 5%	+11.400 VDC	+12.600 VDC
-12VDC	± 10%	-10.800 VDC	- 13.200 VDC

Note: To help when testing a power supply, I've also calculated the minimum and maximum voltages using the tolerances listed. You can reference my ATX Power Supply Pinout Tables list for details on which power connector pins supply which voltage.

Power Good Delay

Power Good Delay (PG Delay) is the amount of time it takes a power supply to start up completely and begin delivering the proper voltages to the connected devices.

According to the Power Supply Design Guide for Desktop Platform Form Factors (PDF), Power Good Delay, referred to as PWR_OK delay in the linked document, should be 100 ms to 500 ms.

Power Good Delay is also sometimes called PG Delay or PWR_OK Delay.