Solitary-pole 120V breakers only have to have 1 slot about the panel. Double-pole 240V breakers have to have two slots. Tandem breakers join two 120V circuits in a single slot, which is useful for compact electric powered panels. changing an electrical panel, look at the sort, amperage and circuit breakers https://dallasbtlbt.full-design.com/electrician-fundamentals-explained-75702064