Using a apple 20w usb-c power adapter with a i-pad pro 3rd gen. What stops power over load?

Your iPad will automatically regulate and optimally charge its battery from its Power Adapter. Your iPad will charge at its maximum supported charging rate when connected to a 20W, or higher rated, USB-C Power Adapter. Use of a higher rated USB-C PD (Power Delivery) compatible Power Adapter will cause no damage.

Most models of iPad require a USB Power Adapter rated at 12W (i.e., 2.4A @5V) or greater to reliably charge.

Having an iPad continuously powered for extended periods is not an issue - as this is common practice in POS (Point of Sale) and Kiosk applications. iPad will optimally manage battery charging - and may automatically change its charging strategy by reducing the maximum battery charge level to about 80% of maximum capacity. This strategy is intended to extend the life of the battery by reducing chemical ageing that occurs when the battery is continuously maintained 100% charge. When a “normal” charge/discharge pattern is resumed, charge to full capacity is restored.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

Hi Buffy, apologies if I misunderstand your question. You are asking what happens when the power brick has a higher wattage rating than the device? To the best of my understanding: the brick's 20W rating defines its maximum wattage. It is not a 20W or nothing scenario. The brick will provide no wattage through 20W and theoretically everything in between. The actual power draw from the brick will be based on what the device pulls from it.

Does that answer your question?