Objective 3.5: Given a scenario, install the appropriate power supply

Cert: CompTIA A+ Core 1 (220-1201) V15 Domain: 3.0 Hardware Weight: ~25% of Core 1 (domain total) Depth: Given a scenario, install the appropriate. Match a PSU to a build's needs.

What this objective tests

You should be able to look at a PC build and pick or evaluate a power supply that fits: enough total wattage, the right connectors, the right input voltage for the region, and the right efficiency rating. You also need to recognize the difference between modular and non-modular and know when redundant power supplies belong.

Key facts

Input voltage:

• 110-120 VAC. North America standard.
• 220-240 VAC. Most of Europe, Asia, and much of the rest of the world.
• Most modern PSUs are auto-sensing and accept the full range. Older PSUs had a manual switch on the back. Setting that switch to the wrong region can destroy the PSU and connected components.

Output rails:

• A PSU converts AC input into multiple DC output rails: 3.3V, 5V, and 12V.
• 12V powers the CPU, GPU, fans, and most modern components. It is the highest-current rail and the most important for capacity planning.
• 5V powers some chipset components, USB ports, and legacy peripherals.
• 3.3V powers some chipset and memory components.
• A PSU spec sheet lists how many amps each rail can deliver.

Main motherboard connector:

• 20+4 pin. The main ATX connector to the motherboard. Modern motherboards take 24 pins. Some older boards took only 20. The 4-pin section is detachable so the same cable fits both. Always seat the full 24 pins for modern boards.

Additional connectors a PSU provides:

• EPS (4 or 8-pin, sometimes 8+4). Dedicated CPU power. Required separately from the 24-pin.
• PCIe (6 or 8-pin, or the newer 16-pin 12VHPWR). Supplementary power for graphics cards.
• SATA power. Powers SATA drives.
• Molex. Older 4-pin connector still used by some fans, lights, and legacy drives.

Modular vs non-modular:

• Non-modular. All cables are permanently attached to the PSU. Cheaper. Unused cables become clutter inside the case.
• Semi-modular. Main 24-pin and CPU EPS are fixed; everything else is detachable.
• Fully modular. Every cable detaches. Cleanest builds, best airflow, slightly higher cost.

Wattage rating:

• Total power the PSU can sustain. Quality PSUs deliver their rated wattage continuously without sagging.
• Calculate the system's worst-case draw and add headroom (usually 20-30%) for efficiency, transients, and future GPU upgrades.
• An undersized PSU causes random restarts, especially under load. Oversize is generally fine but adds cost and may slightly hurt low-load efficiency.

Energy efficiency (80 PLUS):

• 80 PLUS certifications rate how much input wattage becomes useful output wattage.

• Higher efficiency means less wasted power as heat and lower electricity cost over time.

Redundant power supplies:

• Two (or more) PSUs in the same chassis. If one fails, the other takes over without downtime.
• Standard in servers, NAS appliances, and other systems that must not lose power even briefly.
• Each PSU is typically connected to a different power circuit or different UPS for true redundancy.

Common gotchas

• Wrong voltage switch on legacy PSUs. A PSU set to 115V plugged into 230V draws too much current and pops fuses or destroys components. A PSU set to 230V plugged into 115V just refuses to deliver enough power. Modern auto-sensing PSUs eliminate this risk.
• Forgetting the CPU EPS connector. The main 24-pin alone is not enough. The system will not post without the 4 or 8-pin EPS connected as well.
• Cheap "1000W" PSUs. Total watt claims on noname PSUs are often peak ratings, not sustained. Pay for a known brand with a real 80 PLUS rating.
• Under-spec PSU for GPU upgrades. A new GPU draws significantly more power than the old one. If the PSU is not upgraded too, the system may crash under graphics load.
• PCIe power adapters. Using Molex-to-PCIe or SATA-to-PCIe adapters for high-end GPUs is risky. The original connectors are not rated for the current modern GPUs pull. Use a PSU that has native PCIe connectors.
• Modular cable mixing. Modular PSU cables are NOT interchangeable between brands or even between models from the same brand. Mixing them can short circuit and burn the system.

Real-world context

For most office and SMB desktops, a quality 500-650 watt 80 PLUS Bronze or Gold PSU is more than enough. For gaming or workstation builds with mid-to-high-end GPUs, plan for 750-1000 watts depending on the GPU's TDP.

For servers, redundant PSUs are usually a basic requirement. Even small businesses running a single physical server should specify redundant power if uptime matters.

A failing PSU usually shows up as random restarts, system freezes under load, or refusal to power on at all. PSUs are also one of the few components where buying a known brand at a known efficiency rating reliably saves money in the long run (better warranties, lower energy bills, fewer dead components from power problems).

Sources

• [CompTIA A+ 220-1201 Exam Objectives Version 4.0, Section 3.6](../../../../../../30-RevyTechJourney-Build/../30-RevyTechJourney/CompTIA%20A%2B%20220-1201%20Exam%20Objectives%20%284.0%29.pdf)
• [Wikipedia: Power supply unit (computer)](https://en.wikipedia.org/wiki/Power_supply_unit_(computer))
• [Wikipedia: 80 Plus](https://en.wikipedia.org/wiki/80_Plus)
• [Wikipedia: ATX](https://en.wikipedia.org/wiki/ATX)