Troubleshot flickering lights and tripped breaker for the water heater. Tankless water heater has 3 40 amp breakers and one would trip. Load calculation as well as amperage readings were performed. Service load is 144 amps on a 200 amp service and should not be overloading the main. The homeowners are aware that since it is such a big power draw, the lights flickering is a common occurrence. The amp readings were right at 34 amps on each individual breaker. Everything on our end is working properly. New breakers were installed to eliminate the possibility of it being a breaker issue. All the taps in the house had hot water running and held fine on the new breakers. If issues persist, they were made aware to contact a plumber to do a further diagnosis on the water heater. 17 mismatched breakers were also replaced in the panel