Is a 10-gauge extension cord at 75 feet fine for a 15-amp portable air conditioner?

Yes, a 10-gauge cord at 75 feet is more than adequate and safer than going lighter. Voltage drop is about 3.6%, well under the 5% threshold, while a 12-gauge cord would drop nearly 6%. The 10-gauge cord also handles the AC compressor’s startup surge. Just ensure it’s outdoor-rated, grounded, and not coiled while running.