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HPWC set to 64amps, daytime charging drops to 48.

During night time charging i almost always get 64amps sustained for as long as i need to charge (HPWC dip switches set to 64) but during the day if i flick a few things on like computer and lots of lights and/or run the AC in the house, it'll sometimes drop to 48amps and the car will display this warning on the dashboard: "Charge speed reduced -- Ext. Cord used or Bad Wiring". Question is, for added safety should i just set the DiP switches to 48 permanently or can i safely rely on the tesla charging system to monitor for what i assume is a voltage sag on the incoming electrical(?) and drop the amps itself? Alternately, could i go up to 80 amps and let the system take care of itself?

For what it's worth the fuses and wiring from the garage breaker box that feeds the HPWC is all rated for 100amps, but the breaker panel itself shares a 100amp line from the main house with a few other things connected, which is the reason i conservatively set the 64amp limit in the first place.
Just leave the DIP switches set to 64 Amps.

The current reduction algorithms in the Model S are not documented outside of Tesla Engineering. However, the 25% current reduction seems to be triggered by a Voltage drop from zero current to operating current which indicates a high resistance in the feed, and also seems to be triggered by Voltage transients. The latter appears to be attempting a primitive arc fault detector in software. Your operation of larger loads in the house like the air conditioner is probably inducing transients and causing the current reduction algorithm in the Tesla to trigger.

To look into this further, you can observe the zero current Voltage in the car as charging is about to start, the 64 Amp Voltage in the car, and the Voltage as you start the Air Conditioner in the house. If anything causes more than 5-8 Volts change as read in the car, you may want to have an electrician visit and check the wiring and connections to see if there is a particular "hot spot" anywhere. If the electrician sees that the Voltage feeding your house from the meter has a large drop, you may want to contact your electric company about the electrical feed to your house.

Good Luck!
Since you can reproduce it by adding load on other circuits, it would point to undersized service conductors to the home from the transformer, or an undersized transformer. You can either call an electrician, or call the power company; tell them the car is detecting a larger voltage drop than usual when you're charging and there are heavier loads, and ask them to look at your service.

Do not increase the DIP switches to 80A (100A circuit) unless you have wiring and an overcurrent protection device that is sized. It is likely that you'll trip your circuit breaker if you do.