Can I Replace a 40 5 Capacitor With a 45 5 Safely?
Can I replace a 40 5 capacitor with a 45 5? In a pinch, yes, and the unit will run. A 40+5 and a 45+5 are both dual run capacitors, and the swap only bumps the compressor side from 40 µF to 45 µF. That is a 12.5% jump. It sits just above the ±6% tolerance a run cap is built to. So it makes a solid emergency fix. It is not the part I would leave in for years. When you can, match the microfarads exactly.
Last updated: August 13, 2026. Corrected the bogus series/parallel math from the old version, added a spec table, laid out the voltage rules, and gave a plain bench verdict.
Can I Replace a 40 5 Capacitor With a 45 5? The Short Answer
Usually fine, with two conditions. The voltage must be equal or higher, and the part has to fit the bracket. Nail those and a 45+5 will start your compressor and spin your fan today. That leg just pulls a touch more current than it was tuned for, so it runs slightly warmer over time. For a permanent repair, order the exact 40+5. For a Saturday save, the 45+5 gets you cool again.
Here is what the old page got wrong. Those two numbers are not wired in series or parallel with each other. Skip the reciprocal math. Read each section alone.
What the 40+5 Numbers Actually Mean
The 40 and the 5 are two capacitance values in microfarads, not degrees. That typo travels everywhere online. The unit is µF.
The bigger figure feeds the compressor. The smaller one feeds the condenser fan motor. Three spade terminals poke out of the top: HERM, FAN, and C. HERM is the compressor leg. FAN drives the blower. C is common, shared by both. Standard motor-run wiring, explained plainly on Wikipedia’s motor capacitor page.
Going from 40+5 to 45+5 changes exactly one thing. The fan section holds at 5 µF. The compressor section climbs from 40 to 45. Everything rides on that single leg.
40+5 vs 45+5: The Spec Table
Here is the whole comparison on one screen. This is what I check before handing anyone a substitute.
| Spec | 40+5 µF cap | 45+5 µF cap | Does it matter? |
|---|---|---|---|
| Compressor (HERM) section | 40 µF | 45 µF | Yes, up 12.5% |
| Fan (FAN) section | 5 µF | 5 µF | No, identical |
| Typical tolerance | ±6% (37.6 to 42.4 µF) | ±6% (42.3 to 47.7 µF) | 45 clears 40’s ceiling |
| Voltage rating | 370 or 440 VAC | Equal or higher | Yes, never lower |
| Terminals | HERM, FAN, C | HERM, FAN, C | Same wiring |
Notice the tolerance rows. A healthy 40 µF part may read 37.6 to 42.4 and still pass. The 45 lands above that. So it is close, not identical. I dug into why that window exists in a separate piece on whether capacitance tolerance matters.
The Voltage Rating Rule You Cannot Break
This one has no give. Your replacement must be rated equal to or above the original. Run caps arrive in two flavors: 370 VAC and 440 VAC.
A 440 can stand in for a 370. Higher is safe. Backward is dangerous. Drop a 370 where a 440 lived and it can cook, bulge, then pop. I have yanked plenty of swollen units that died this exact way. If the logic feels backward, this guide on fitting a higher voltage capacitor spells it out.
The Decision Tree: When the Swap Is Fine
Walk these branches before you install anything.
- Voltage lower than the original? Stop. Do not use it. The one hard fail.
- Voltage equal or higher, and it fits? You are clear for a temporary swap.
- System under warranty? Ring the installer first. An off-spec part can void coverage.
- Compressor old, weak, or hard-starting? The extra draw can shove it over the edge. Match the 40 instead.
- Permanent repair? Buy the 40+5. Do it right once.
The rhythm is simple. Voltage and fit are the gates. Microfarad is the preference. Clear the gates and that 12.5% gap becomes something you tolerate for a while, not forever.
How I Choose a Replacement on the Bench
Here is my order of operations. In my experience it catches mistakes before they cost a compressor.
- Read the failed part. Note both µF values and the voltage.
- Match voltage first. Equal or higher, never below.
- Match microfarads if the exact cap is in the van. Otherwise a 45+5 bridges a 40+5.
- Check the can diameter and height against the strap. It must clamp tight.
- Discharge the old one through a resistor before touching terminals. A charged cap bites.
- Wire HERM to HERM, FAN to FAN, C to C. Photograph the leads first.
Five minutes, no drama. If the motor still hums and refuses to spin, you may be chasing a start issue, and testing a hard start capacitor is the next move.
What Goes Wrong With a High Value
A slightly high cap will not blow up your air conditioner. The effects are quieter, and they build over months:
- Higher amp draw through the compressor winding
- The compressor and condenser running hotter
- Shorter motor life, because heat wrecks windings
- The cap itself wearing out sooner
None of it happens overnight. A 45+5 might live for years in a 40+5 slot. It might also shave time off the compressor. That gamble is why I treat it as a bridge. For the wider view, here is what happens when you use the wrong size capacitor, plus the lifespan you can expect from these parts when matched right.
An old tech I trained under said it best. “Match the microfarads, respect the volts, and the cap will outlast the compressor.” That line has held up on every call since.
Myth: You Add or Multiply the Two Numbers
You do not. This is the top mistake I see online, and the previous version of this page repeated it. A dual run capacitor does not fold 40 and 5 into one total through a series or parallel formula. The two sections live separately inside the shell. One does the compressor job. The other does the fan job. For the raw physics, lean on the Wikipedia capacitor article and the farad definition.
Frequently Asked Questions
Is it safe to use a 45+5 instead of a 40+5?
Yes, for a short fix, so long as the voltage is equal or higher and the part fits. The compressor pulls roughly 12.5% more, so it warms up. Install the exact 40+5 when you can.
Can I replace a 40 5 capacitor with a 45 5 permanently?
I would not. It cools fine, but the compressor sees more current than designed, year after year. For a lasting repair, match the microfarads and buy the correct 40+5.
Can I use a lower voltage capacitor to save money?
No. Never drop below the original. A 370 V unit in a 440 V slot can overheat and fail fast. Equal or higher only, no exceptions.
Does the fan side change when I go from 40+5 to 45+5?
No. The 5 µF fan section is identical in both parts. Only the compressor leg moves, 40 up to 45. Your blower never notices.
Should I call a pro before changing the capacitor?
If the system is under warranty, yes, since an off-spec part can void it. These caps also store a charge that can shock you. Unsure how to discharge one? Get a technician.
What if my capacitor keeps failing after replacement?
A cap that dies again quickly points to a deeper fault, like a struggling compressor or high head pressure. Here is why an AC capacitor keeps blowing and what to rule out first.
Bottom Line
Can I replace a 40 5 capacitor with a 45 5? For an emergency, yes, if the voltage is equal or higher and it fits the bracket. The fan side stays put, and the compressor side runs about 12.5% richer, which is fine for a bridge but not ideal for years. Match the exact 40+5 for the permanent fix, honor the voltage rating above all, and skip the series and parallel math, because a dual run cap holds two independent sections, not one combined value.
