Wonder why it needs 3 40 amp breakers ? With 240 volts there’s two 110 volt legs and a neutral and a ground neither get a breaker. mind you it did say MAD in China
Because it is 24 kw, 8000 watts per breaker, and probably has 3 elements inside, each protected by it’s own circuit. Instant hot water takes a lot of power, more than anything else you will ever have in your house. Ours has a 220v 120 amp leg off the main box running to a sub panel with 3 220v 40amp breakers, 27kw total. Each element has it’s own independent circuit, and will turn on, off on it’s own as needed for flow. More flow needs more power.