Electrical – Can a built-in dishwasher be plugged into a receptacle

If your circuit breaker isn't tripping when the old or new dishwasher ran a full cycle, then hardwiring won't change a single thing. Check that your drain hose is clear & run up to above the sink trap or underside of the countertop.

There should be quite a forceful stream of water visible in the garbage disposal or sink drain. If not & you have an Air Gap atop the sink, then take the cap off the Air gap (dishwasher off) & clear all debris. You'll need to snake something down the hose leading into the garbage disposal or sink drain until you can see it come out in either. Correct any hose under the sink to be as inclined as possible above it's connection so future flow-back is eliminated.

And, especially check that the bottom of the dishwasher's inside grate or strainer is clear...if clear glass ever broke in there you really won't notice it blocking the water exit. Also, if you wash food product jars without completely removing labels, they will gum up the works quite badly & quickly.

All this should get you back in tip top shape. A new dishwasher doesn't fix an old or worsening clog.

The best solution is to actually change the single gang box to a double gang box and install a second unswitched duplex outlet. This is not actually difficult for an experienced electrician, the DIY skill level is moderate. The total draw is only near max, if both dishwasher is on dry and hot water gets used, but insta-hots actually only run for a very short cycle if used. It shouldn’t trip if there’s nothing else on the circuit.

That said, I used this: https://www.homedepot.com/p/GE-6-Outlet-Grounded-In-Wall-Adapter-White-54947/203742151 or equivalent... specifically a model tho with a center screw, that’s critical for this application. If it’s the typical garbage disposal/dishwasher under sink plug that is a half switched duplex outlet, then the other two plugs on the switched row of the adapter are pretty useless. Regarding 15 vs 20 amp ratings, I think you won’t find 20 because it’s a NEMA 5-15 pin configuration.

(Footnote: I plan to change this when we remodel the kitchen and the cabinets are out to the 2-gang 2 duplex outlets described at the beginning, due to the location of the stud and the side they chose to nail the original to—mine was not the simple case of make a 1 gang into a 2 gang, mine actually has to change to the other side of the stud as well, I opted to use the adapter and have not had any problem.)