I bought a new surge protector and the included specification material says I need 30 feet between the outlet and the service panel. My situation is probably about 5 feet. The outlet I want to protect is on the first floor and almost directly over the service panel in the basement. The surge protector itself has a 6-foot cord.
Can I add a heavy gauge grounded extension cord (let’s say 25 feet of the same gauge as the house wiring) to satisfy this requirement?
No! Never coil up an extension cord and put power through it! This happens. I know you didn't say "Imma coil it up" but what else were you going to do with it? Also most extension cords are a size or two smaller than in-wall wiring, which makes this heating problem more acute.
They want that length of wire so they can use the resistance of the wire to help tamp down surges. That means the device needs to shunt (short) far fewer amps to get voltage within limits which is easier on the surge suppressor.
But honestly, rather than run a bunch of wire in a straight line, I would run it in a tight coil around an iron core. Not me personally; I'd buy a transformer pre-made. Nothing suppresses surges like a transformer! They are tuned for 60 Hz and attenuate other frequencies, like the high frequencies seen in surges and spikes.
If you wish to follow good practices for wiring, and meet the spec, it's quite trivial to manage.
Run 30 feet of building cable (NM, AC, MC) (not coiled up, not an extension cord) out and back along the basement ceiling (properly attached) or floor joists or wall on your way from the service panel to the outlet 5 feet away. Basically take a 12-1/2 foot detour out and back again, or out 10, over 5, back 10 - whatever is convenient.
The smart move would also be to (additionally) install the type of surge suppressor that goes right in the panel itself and is actually intended to take lightning-related surges (though nothing really survives a direct strike...) as opposed to depending on a piddly one by itself. That is, a "Type 1 or Type 2" surge suppressor. The ones that drop into 2 adjacent breaker spaces are quite handy if your panel isn't full.
The risk of having the surge protector close to the service panel is less than the risks created by plugging it in to an extension cord. Use it as-is. It will be fine. The requirement for a minimum 30 foot distance is relatively new. There is a small risk created by the short wiring, it's true. However it has never been recommended to use a power strip or surge protector on an extension cord.
