Great question — answering as the UphillTech team:
Put a peak load on the circuit and measure the voltage. If the voltage droops more than the permitted droop of 5% (that's 6 volts on 120V) -- less is preferable for incandescent lights and motors -- then you definitely have excessive resistance in the circuit wiring. And then have to check that the wires are properly rated for the load and the length of run and also have to inspect every junction box on that circuit and check each connection -- clean the wires, replace the connection device (wire nut, etc.), and retighten them preferably with a torque driver.
Full answer by Dan D.: https://diy.stackexchange.com/a/107663 (CC BY-SA 3.0).
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