Any recommendations for a 50 foot 110v 3 prong extension cord plugged into a 20 amp outlet that will still provide 3-4 mph of charging?
For my charger kit I opted for this 20A 50ft extension cord: Amazon Smile Link for about $40 shipped and then I added this adapter: Amazon Smile Link for about $16. This will allow me to plug into a 20A 120-volt (what you call 110v) outlet and extend 50ft as you requested. If you have a 15A outlet & need to extend it 50ft you can simply plug in that little adapter and then plug the extension cord into the other tend. This gives you the most flexibility and if the extension is heavy enough to handle 20A at 120v then 15A at 120v will obviously be less of an issue. Best of both worlds for around $50 shipped IMHO.
The main thing is the actual thickness of the wire. Just because a cord says "HEAVY DUTY" doesn't mean squat. That's usually referring to the rubber insulation around it being tough and thick and resistant to getting cut or scraped, but it might have really thin metal wire inside it. I had two beefy looking extension cords at home, but when I went to check them for potentially charging the car, they were both 16 gauge! That's way too thin for long sustained current like this. So yes, I would say at least 12 gauge, like the one @Ostrichsak linked to.
Great point that I left out as I was just trying to give the 100-mile-view. I did the math and determined that I was ONLY comfortable with a 50-foot extension cord that used 12-gauge wire inside. Anything less and there would be more resistance which creates more heat and this is a bad thing when you're talking sustained longer charges @ 80% capacity that our Teslas draw. I searched high and low to find a wire that specifically used 12/2 (12 is for 12-gauge, and in this case they call it 12/3 but it should be called 12/2 as the ground wire isn't normally counted when you see that number as it's just the conductors... not really important here just wanted to point that out to avoid confusion for others reading) and didn't cost a fortune and this is the one I came up with. While you don't have to buy that one specifically make sure you get one that has at least 12-gauge wire inside and remember that the higher the number the smaller the wire inside. If you see one that says 14-gauge it doesn't mean the wire is thicker inside but is actually thinner regardless of what thickness insulation jacket they use on the outside to make it look bigger. The next step up in wire size inside would be 10-gauge and if you find something like that it will be cost prohibitive due to being an odd bird and the fact that the metal used to make the wires is quite expensive and any jump in diameter usually equals a notable jump in cost. Also, anything more than 12-gauge for this draw over this length of run would be overkill and a waste of money. It looks like the one I linked to is out of stock but I paid about $43 shipped for it looking back through my orders years ago. Shouldn't be difficult finding something similar at that price point. As a comparison, I saw a 10-gauge 50-foot extension cord that was over $20 more. Not only is that a waste of money but it will weigh a lot more, be more difficult to handle (especially during colder temps) and the benefits will never be noticed since you'll be running at 80% (or less) of capacity at all times.
The one I got was called the Yellow Jacket. https://smile.amazon.com/gp/product/B0009ON12G?pf_rd_r=04T6DP8EWC62TP96FH0Q&pf_rd_p=edaba0ee-c2fe-4124-9f5d-b31d6b1bfbee&th=1 It doesn't have the sideways blade of a 5-20 plug like yours does--just the normal 5-15 plugs. But if I want to use it with the Tesla 5-20 plug adapter, to be able to draw 16A, I bought a little 6 inch adapter cord to turn one end into the 5-20 receptacle type. Like this one: https://smile.amazon.com/AKJIA-listed-1-Foot-12AWG-Adapter/dp/B075J9N6RJ/ref=sr_1_7?dchild=1&keywords=5-15+to+5-20+adapter&qid=1602776464&s=hi&sr=1-7
Go big or go home. I rarely use it, but the 50' 10/3 version is in my frunk "just in case". (All wrapped neatly on a cheap holder) /edit. Here's a pre-molded 20amp version also: https://www.amazon.com/Yellow-Jacket-2992-Generator-100-Feet/dp/B000HEC2DW/ref=sr_1_2?dchild=1&keywords=10/3+20amp&qid=1603860424&s=hi&sr=1-2&th=1
I have a 40 feet 12 gauge extension in my frunk for any situation that may require it. I wouldn't consider anything with thinner wire both for the safety factor as well as the voltage droop costing charging speed.
Southwire seems to be a good brand. I just got this today for 120 volt, 12AMP charging. https://www.amazon.com/gp/product/B00004SQF4/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1