I have a 1700 sq ft basement on Long Island, NY. It is a less than 1 year old condo with a finished basement. The initial gas heating was supposed to be a separate HVAC zone, but with only one zone control, the builder tried to balance the main floor heat with the basement. It did not work and the builder's second attempt was a 12000BTU electric split unit on a 15 amp circuit hanging on a wall - but that didn't heat more than 2 degrees in winter, upon from 62 degrees. NOW they want to replace it with a 18000BTU unit on that 15 amp circuit. Is that BTU enough andis the 15 amp circuit sufficient?
18000 BTU = 5200W 5200W @240V= 21.67A
So, no, a 15A circuit is not sufficient. But, I've never heard of or seen a 15A/240V breaker. Seems to me they need would need to install a 30A/240V breaker and 10AWG copper circuit for an 18kBTU system... BUT, it is worse than that!
18kBTU isn't large enough for 1700 sq.ft. This site suggests 24kBTU for 1500 sq ft. (https://www.sogoodtobuy.com/blog/sizing-guide-for-a-mini-split-air-conditioner/). Seems like your builder has, and continues, to grossly under size the system for your needs.