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.
| Tier | Efficiency at 50% load (115V) |
|---|---|
| 80 PLUS | 80% |
| Bronze | 82% |
| Silver | 85% |
| Gold | 87% |
| Platinum | 90% |
| Titanium | 92% |
- 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)