Short answer: unplug everything, mount the unit with the fan facing the vent, connect the 24-pin, CPU and graphics card cables first, then drives. Use only the cables that came with the new power supply, and check every plug is fully seated before you switch on.
Before you start
- A Phillips screwdriver and the four screws from the power supply box.
- Cable ties or Velcro straps.
- Your case and motherboard manuals.
- A clear, well-lit space. Touch the bare metal of the case now and then to discharge static.
Removing an old power supply
- Shut down, switch off at the back of the power supply, and unplug the mains cable.
- Press the power button for a few seconds to drain any leftover charge.
- Photograph the cable layout so you remember what went where.
- Unplug every cable from the motherboard, graphics card and drives.
- Remove the screws at the back of the case and slide the unit out.
- Bag the old modular cables with the old unit. Never reuse them with the new one. See why.
Fitting the new one
- Plan the cables. With a modular unit, plug in only the cables you need before mounting it, if access at the back is tight.
- Mount it. Fan down if your case has a filtered vent below, fan up if not. See PSU fan up or down. Fix it with four screws.
- 24-pin motherboard cable. Route it through the nearest grommet and push it in until the clip clicks.
- CPU cable. Route it along the top of the case to the 8-pin socket near the processor. Use a second one if your board has two sockets and a high-power chip.
- Graphics card cable. Push the 16-pin plug fully home, or use one separate cable per 8-pin socket. Avoid sharp bends near the plug. See the ATX 3.1 guide.
- Drives and accessories. Connect SATA cables to any SATA drives and fan or lighting hubs.
- Tidy up. Strap the cables behind the motherboard tray, away from fans.
Checks before switching on
| Check | Why |
|---|---|
| Every plug clicked home | A loose plug can stop the PC starting, or overheat |
| Graphics card plug flush, with no gap | The 16-pin plug must be fully seated |
| No cables touching fans | Avoids rattles and damage |
| Only the new unit’s cables used | Old cables can be wired differently |
| Switch on the back set to on | A common reason a new build won’t start |
First power on
Plug in, switch on at the back, and press the power button. If the fans spin and you reach the BIOS, you’re done. If nothing happens, recheck the 24-pin and CPU cables and the front panel power switch connector.
- Easy to build with · Tier ACorsair RMx (2024)80 Plus Gold · Fully modular · ATX · 750/850/1000WFully modular with a native 16-pin cable.Check 850W price
- Easy in small cases · Tier ASeasonic Core GX ATX 380 Plus Gold · Fully modular · ATX · 650/750/850WOnly 140mm deep.Check 750W price
Questions
How long does it take to replace a power supply?
About 30 to 60 minutes for most people, most of it spent on cables.
Do I need to unplug the graphics card?
Only its power cables. The card can stay in its slot.
Which way up should the power supply go?
Fan down over a filtered vent, or fan up if there’s no vent. See the fan direction guide.
My new PC won’t turn on. What should I check?
The switch on the back, the 24-pin and CPU cables, and the front panel power switch connector on the motherboard.
Can I install a power supply without removing the motherboard?
Yes, in most cases. It’s easier with a modular unit.
Sources
- NZXT: PSU cable compatibility
- Tom's Hardware: why power connectors are melting, and what to do (June 2025)
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