Insight: When Smart Mods Go Silent — Why Your Raspberry Pi Refuses to Negotiate 5A Over USB-C

Insight: When Smart Mods Go Silent — Why Your Raspberry Pi Refuses to Negotiate 5A Over USB-C

The Custom Build That Should’ve Worked

Let’s say you’re designing a slick little Raspberry Pi enclosure—something you can switch on and off without yanking cables. You’ve got a panel-mount USB-C jack wired into the Pi’s power input. You run the 5V, GND, and CC lines to a toggle switch, so when you flip it on, the full power signal flows in cleanly. It’s neat. Minimal. Smart.

Then it happens: the Pi boots, but throws a warning. “This power supply can’t provide 5A to USB devices.” Which makes no sense—you’re using the official Raspberry Pi power adapter. No sketchy chargers, no bargain-bin cables. Just your own case mod. The same power supply works perfectly when plugged in directly. So what gives?

USB-C: The Silent Negotiator

Under the hood, USB-C is a full-blown diplomat. The moment you plug in a power source, it starts quietly negotiating things like:

• How many amps can be safely delivered
• Whether the cable supports higher current
• What voltage modes (5V, 9V, 15V...) are allowed
• Who’s in charge—host or device?

This negotiation happens across a handful of deceptively boring pins—CC1, CC2, and sometimes D+/D–, depending on how the power controller was designed. And the Pi? It’s picky. To unlock the full 5V at 5A profile, it wants a very specific handshake. If even one expected signal is missing—say, CC2 is floating, or the cable ID pin isn’t connected—it assumes you're using a low-end power brick and pulls back on current limits to avoid frying itself.

Why Your Mod Might Block the Handshake

So back to that custom case. If your switch only toggles the 5V and one CC line, you’re giving the Pi partial information. It might be enough to boot—but not enough to negotiate. And since negotiation happens before power fully flows, even a clever inline switch can accidentally sabotage your supply without ever showing an error on the bench.

This isn't a case of bad wiring. It’s just a classic USB-C trap: thinking 5V means power is guaranteed. With USB-C, the agreement matters just as much as the voltage.

How to Avoid the Power Limiting Surprise

If you’re building your own enclosures or inline power solutions:

• Use full USB-C breakout boards that preserve all signal lines, not just 5V and CC
• Avoid routing only one CC pin unless you know your Pi’s chip revision tolerates it
• Consider switching just the 5V rail, not CC, if you need to cut power
• Don’t assume the official Pi power supply will behave the same through custom wiring—it still expects to see certain signals to enter full-power mode
• If your mod adds a panel-mount USB-C jack, make sure it’s wired pin-for-pin, or clearly labeled if it's power-only

The Takeaway

The Raspberry Pi is famously versatile—but like a lot of powerful tools, it’s also a bit high-maintenance when it comes to power. USB-C makes it look like you can get away with quick hacks, but under the surface, it's running a tight diplomatic protocol.

If your power mod feels smart but behaves flaky, don’t assume your wiring is at fault. Sometimes, it’s just USB-C being USB-C—rigid, quiet, and extremely particular about who gets to draw 5 amps. 

Need Raspberry Pi Expertise?

We'd love to help you with your Raspberry Pi projects.  Feel free to reach out to us at info@pacificw.com.

Image: Gemini