How to Install a Cheap Hard Start Capacitor

If you boondock or dry camp with a generator, you’ve probably experienced this frustrating problem: You turn on the rooftop AC, and the big startup surge causes your generator to sag for a second. Lights flicker, and your TVs (and other sensitive electronics) shut off briefly until the generator catches up.

I had the exact same issue with my Dometic 15,000 BTU rooftop AC. The fix? A simple hard start capacitor kit — specifically the Supco SPP6 (or equivalent). It costs under $15–$18 at places like Walmart or Amazon, takes about 10–15 minutes to install, and dramatically reduces the compressor startup surge.

Why a Hard Start Kit Helps (Especially When Boondocking)

A standard RV AC compressor draws a huge spike of current when it first kicks on. On a generator — especially smaller inverter models — this causes a brief voltage drop (brownout). That’s what resets your TVs.

The Supco SPP6 adds a temporary boost of extra starting torque (up to 500% more). The compressor spins up faster and with less overall inrush current, so the voltage sag is much smaller or eliminated.

Perfect for boondockers and generator users because:

You can run your AC more reliably on smaller or older generators without constant overloads.
TVs, lights, and other electronics stay on during startup.
It’s far cheaper and simpler than a full soft start kit ($150+).
Many RV owners report it makes a noticeable difference in hot weather when the compressor is harder to start.

What You’ll Need

Supco SPP6 Hard Start Kit (or compatible 2-wire hard start capacitor) — currently around $15–$18.
Phillips screwdriver
Zip ties (for securing the kit)
Ladder or safe roof access

Safety First: Turn off all power to the AC at the breaker and disconnect shore power or generator. Capacitors can hold a charge — discharge the existing one safely by shorting the terminals with an insulated screwdriver before touching anything.

Step-by-Step Installation on a Dometic 15K Rooftop AC

Remove the outer shroud/cover of your rooftop AC (if not already off). It’s held by several screws around the edge.
Open the electrical access panel. On most Dometic 15K units (Brisk, Brisk II, Brisk Air, etc.), there’s a small removable side panel on the electrical/control box
Locate the factory run capacitor. It’s usually a shiny oval or round metal can with 2–3 terminals labeled:
- C (Common)
- Herm (Hermetic / Compressor)
- F (Fan)
Install the SPP6 hard start kit:
- The SPP6 has two wires with quick-connect spade terminals.
- Connect one wire to the C (Common) terminal.
- Connect the other wire to the Herm (Hermetic) terminal.
- Do not connect anything to the F (Fan) terminal.
- Polarity does not matter — either wire can go to either terminal.
Secure the kit. Use a zip tie to fasten the SPP6 so it doesn’t rattle around. On tighter Brisk II models, you may need to trim a tiny bit of plastic cowling for clearance.

Reassemble the access panel and outer shroud.

That’s it! The whole job is basically “piggybacking” two wires onto your existing run capacitor.

Before and After Testing

After installation, restore power and test the AC on your generator. You should notice:

Smoother startups with less voltage dip
TVs and lights staying on without resetting
Easier operation in hot weather or when the generator is lightly loaded

Final Thoughts

For less than $15 and 15 minutes of work, this is one of the best bang-for-your-buck upgrades for any RV owner who runs their AC on a generator while boondocking. It won’t turn a tiny generator into a powerhouse, but it often makes the difference between constant frustration and reliable cooling.

Have you tried a hard start kit on your RV AC? Drop a comment below with your results or any tips — especially if you have a different Dometic model.

Stay cool out there!

How to Install a Cheap Hard Start Capacitor