OK, but who would do that and why? Your electrical system is installed as a complete system designed to work together. Some here have raised the possibility your wires might have been overspecced from the beginning, allowing a 40A breaker. The breaker won't trip though because it's rated for 40A. Run the circuit at 39A and your wires might melt or catch fire, given enough time and bad luck. Now more current can flow than your wiring can accommodate, which generates heat in that wiring. Installing a 40A breaker raises the ceiling on the allowed current. That 30A breaker protects you from excess current flowing through the system. And hope you never get peak demand charges.Breakers are matched to wiring, in the same way that brakes and tires are matched to the vehicles they are installed on. Load calculation, possibly followed by an increase in your electric service, is necessary. Then someone gets stuck in traffic for hours (happened to quite a few people in Virginia recently) and come home in the middle of the night and take a nice hot 120A shower.and suddenly everyone is in the dark. Hmmm, I'll charge the EV at night only (easy) and use the hot water during the day. This is a truly major change in your electricity usage and, unlike the other common change, electric vehicle charging, harder to manage. Or you may have 200A service but actually only use 80A because it is a small house with some gas appliances and no bitcoin mining or grow lights, in which case a net gain of 80A to a total of 160A will, just barely, work. ![]() You'll trip your main breaker occasionally and your utility will complain too. You may have 200A service but already be using on the order of 150A, in which case a net gain of (optimistically) 80A (i.e., adding 120A but subtracting an existing 40A from the tank heater that is being replaced with tankless) just won't work. You also need to have large enough electrical service to support 125A of water heating. Having enough spaces - whether for 3 x 40A or 1 x 125A - is only one of the parts of the answer. ![]() To answer the unasked question: Do I have enough capacity in my panel? Doesn't matter in a new house but often goes in a retrofit. Once you do that, splitting the supply makes sense.įinally, wrestling 125A capable cable around in a basement is a LOT harder than 40A cable. Allows for partial heating, smaller controls, etc. In addition, the design may be easier to use 3 heating elements instead of 1 large heating element. Unless you're totally full, in which case you likely can't spare 120A of current anyway. Going for 3x40 works on almost all panels. 15, 20, 30, 40 and 50 amp breakers are all typically around the same price. Even when they are, they often cost a lot more. But not always, and not for the manufacturer.ġ25A breakers are not as readily available as 40A breakers for some panels. However a second subpanel for general use might be a good plan. It's using nearly 120A of the 120A capacity of 1/0Al feeder, and there's no room for anything else. I wish I could say the water heater sub will solve your "out of spaces" problem, but no. The money savings using 1/0Al for part of the distance instead of #8Cu pays for the subpanel, and then some. Aluminum feeder is perfectly reliable at these large sizes. Do we want to run three #8 copper cables? Or one 1/0 aluminum cable? The 1/0 is cheaper by the foot than even one of the #8, so that's a no-brainer. So you run one fat feeder to the sub, and then your three 40A circuits in the sub to the heater hopefully nearby. ![]() The answer to “not enough spaces" is a subpanel. As such, the heater is probably not near the panel. Better off crossing the distance with wire than pipe. Because they are compact and don't need a vent, and being closer cuts down on the wait for hot water. ![]() With tankless, the heater should be as near as practicable to the point of use. Because tankless heater manufacturers aren't even slightly concerned with breaker spaces, and you are sweating what's actually not a big problem.
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