Trying to change my lower water heating element. Label reads 3380 for both upper and lower element. Went to go find a replacement and only saw 3500W and 4500W. I'm leaning towards 4500W...read around the net and saw that it relates to 3380W, by the voltage 240/208V. Also, read that it should be close in wattage...3380W and 3500W to not blow the fuse. Please let me know if I'm thinking about this correctly and should be getting the 4500W water heating element. thanks!

I’m installing a whole-house, tankless, hot water heater and need some expert advice. The model I selected has two circuits, each protected with a double pole, 240 volt, 60 amp breaker. The heating elements for each circuit are rated at 58 amps. The manufacturer’s web site recommends wiring each circuit with 6/2 copper cable with ground. While shopping for Romex 6/2 cable, I discovered that it’s only rated for 55 amps – 3 amps less than the heating elements. I discussed this with a representative of the manufacturer. He seemed surprised that Romex 6/2 was only rated at 55 amps and simply commented that they’ve installed many of these units [same model as mine] and have had no problems using 6/2 copper cable. I’m reluctant to proceed with the 6/2 cable from both a potential safety and compliance standpoint. An alternative would be to use a 4/2 copper cable with ground wire but I’m hesitant to do this because of the increased expense and the increased difficulty in working with a heavier cable. A friend suggested I use four, #6 thhn copper wires and one #10 copper ground wire in a gray plastic conduit. The #6 thhn wires have an amperage rating of 65 amps but I understand that when multiple conductors are in a conduit, the amperage rating would need to be derated. Does anyone know what the amperage rating would be for four, #6 thhn wires [plus one #10 ground wire] in a plastic conduit? Any other suggestions on how I might proceed would be greatly appreciated.