Open two prebuilt PCs and you'll often find one with a tidy bundle of cables and one with a tangle stuffed behind the drive cage. A big reason for that difference is how the power supply attaches its cables. That single design choice affects airflow, build time, and how easy the machine is to service later.
CompTIA A+ Core 1 (220-1201) Objective 3.6 covers power supplies, and it expects you to recognize modular, semi-modular, and non-modular designs, along with the practical trade-offs of each. In exam terms, you should be able to look at a scenario and say which cabling style fits, and explain why. On the job, this knowledge saves you time during builds and helps you troubleshoot power problems without fighting a mess of unused cables.
Modularity describes how cables attach, not how much power you get
A power supply's modularity refers to whether its output cables are permanently attached or plug into sockets on the unit. It has nothing to do with wattage, efficiency rating, or voltage. A 500-watt supply and a 1000-watt supply can both be fully modular, and modularity by itself never makes a PSU more powerful.
What modularity does control is which cables are present in the case. A power supply provides several cable types: the 24-pin main board connector, the 4+4-pin CPU (EPS) connector, PCIe power for graphics cards, SATA power for drives, and older Molex connectors for fans and legacy devices. Depending on the build, many of those cables go unused. Modularity decides whether those unused cables are hanging inside the case or sitting in a spare parts bag.
That's the core idea to hold onto. When you compare the three types below, you're really comparing how much cable clutter you're forced to keep and how much flexibility you have to remove what you don't need.
The three cabling types differ by which cables you can remove
There are three designs you need to recognize. The difference between them is simple: how many cables are hardwired versus detachable.
| Type | Hardwired cables | Detachable cables |
|---|---|---|
| Non-modular | All cables | None |
| Semi-modular | 24-pin and CPU | PCIe, SATA, Molex |
| Fully modular | None | All cables |
A non-modular power supply has every cable permanently attached at the factory.