The 5-2-1 is a 3 wire start assist. The main difference between a 3 wire and a 2 wire like a Supco is that the 2 wire uses what I will basically call a heat switch to remove the cap from the start circuit. It switches off when it reaches a certain temperature, usually in a couple of tenths of a second. A three wire senses when the compressor reaches about 75 percent of run speed and then cuts out.
When running an RV A/C on a smaller generator, the rpm and voltage dip at startup but the start cap cuts out before the generator can get the compressor up to speed. This usually is the cause of a no start. A three wire stays in circuit until the compressor reaches 75 percent of full speed and usually will start the A/C much better than a 2 wire.
Some people will say that it is bad for the windings for the cap to stay in circuit for a longer time, but there are trade off on everything and with the limited wattage of a small generator that has not likely.
My Yamaha 2400 starts my 13.5 every time at up to 7000 MSL using a 5-2-1 three wire start assist.
