What Size AC Capacitor Do I Need? A Complete Sizing Guide
¿Prefiere leer esto en español? Vea esta guía en español →
Capacitors are one of the most commonly replaced AC parts — and one of the most commonly mis-ordered. Grab the wrong microfarad rating or voltage and your system won’t run right, or the new capacitor will fail prematurely. Here’s how to figure out exactly what size you need.
Understanding Capacitor Ratings
Every AC capacitor has two key ratings printed on its label: microfarads (MFD or µF) and voltage (VAC). The microfarad rating tells you how much electrical charge the capacitor stores — this must match what your motor expects. The voltage rating is the maximum the capacitor can handle — you can go higher on voltage but never lower.
For example, if your system calls for a 45/5 MFD 370V dual run capacitor, you could safely use a 45/5 MFD 440V capacitor (higher voltage is fine), but you cannot use a 40/5 MFD or a 45/5 MFD 250V.
Single vs. Dual Run Capacitors
A single run capacitor has two terminals and powers one motor — either the compressor or the fan motor. A dual run capacitor has three terminals (labeled C, FAN, and HERM) and powers both the compressor and the condenser fan motor from one unit. Most residential AC systems use a dual run capacitor in the outdoor condensing unit.
The dual capacitor’s rating is written as two numbers separated by a slash — like 45/5 MFD. The first, larger number is for the compressor (HERM terminal). The second, smaller number is for the fan motor (FAN terminal). C is the common terminal shared by both.
How to Find Your Capacitor Size
Option 1 — Read the old capacitor: The easiest method. Turn off power to the unit, remove the access panel, and look at the label on your existing capacitor. It will clearly show the MFD and voltage ratings. Write both numbers down or take a photo.
Option 2 — Check the unit’s nameplate: Your condenser’s data plate (usually on the side panel) lists the required capacitor specs for both the compressor and fan motor. Look for lines like “Comp. Cap: 45 MFD” and “Fan Cap: 5 MFD.” If you have the model number, we can look up the specs at the counter too.
Option 3 — Bring it in: If the label is worn off or you’re not sure, pull the old capacitor and bring it to our store. We’ll test it on the spot and match the replacement for you.
Common Capacitor Sizes by System Type
While you should always verify against your specific unit, most residential systems in the Dallas area fall into predictable ranges. Smaller 1.5 to 2-ton units typically use 25/5 or 30/5 MFD dual capacitors. Mid-range 2.5 to 3-ton systems commonly need 35/5 or 40/5. Larger 3.5 to 5-ton units usually call for 45/5, 50/5, or 55/5. Single-stage fan motors typically need a 5 MFD capacitor, while some larger fan motors use 7.5 or 10 MFD.
What About 370V vs. 440V?
Many older systems spec’d 370V capacitors. You can always upgrade to a 440V capacitor with the same MFD rating — the higher voltage rating means it handles electrical stress better and often lasts longer. Most of the Titan HD capacitors we stock are 440V, which makes them compatible with both 370V and 440V applications.
Check our capacitor cross-reference chart to find the right match, or bring your old one in and we’ll swap it out in minutes. Capacitors run $10 to $30 at our counter — a fraction of the $150 to $250 a service tech charges for the same part.
Open To Public HVAC Parts — 10226 Plano Rd, Suite 104, Dallas, TX 75238. Call or text (214) 340-9421. Open Monday–Friday 10–7, Saturday 10–3.
