For two years I ran a Flex 6600 and Expert 1.3k-fa off a standard 120 V line because I could not find anyone to install the 240v line. About 1200W out and no problem. Now I run the Expert off 240V. Same 1200W out. No problems either way.
The Expert takes about 17 A peak but only 1/2 of this average in CW and SSB. So on average within specifications of a 120V line. The bad about the Experts is that they won't heat the room.
I would think that 240 is needed with 1500W+ amps but 20A is enough. Unless one really wants to win contests with bigger amps. Say OM Power 4000.
Ignacy NO9E