I have been considering adding a tankless point of use water heater in my kitchen so that I don't have to wait 10 minutes for hot water there. I started by looking at my electrical service to see if I have enough capacity to add the 50 Amp circuit required for the new heater. My main breaker is 225 Amps. When I add up the total of the house breakers, I get a whopping 605 Amps. This includes a 50 Amp air conditioner with 125 Amp electric heat (not a heat pump), 2x30 Amp water heater, 20 Amp washer, 30 Amp dryer, 20 Amp refrigerator, 20 Amp dishwasher, 2x50 Amp stove top and oven, 20 Amp well, and 8x20 Amp plug and light circuits for a 2300 sq ft house built in the 70's in Texas. There are a couple of additional 20 Amp breakers and one 30 Amp breaker that appear to be unused. Is the service sized correctly? I was under the impression that the main breaker should be 10-15% over the sum of the circuit breakers... is that incorrect? Will the service support another 50 Amp circuit (40 Amp actual draw)?

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I have been considering adding a tankless point of use water heater in my kitchen so that I don't have to wait 10 minutes for hot water there. I started by looking at my electrical service to see if I have enough capacity to add the 50 Amp circuit required for the new heater. My main breaker is 225 Amps. When I add up the total of the house breakers, I get a whopping 605 Amps. This includes a 50 Amp air conditioner with 125 Amp electric heat (not a heat pump), 2x30 Amp water heater, 20 Amp washer, 30 Amp dryer, 20 Amp refrigerator, 20 Amp dishwasher, 2x50 Amp stove top and oven, 20 Amp well, and 8x20 Amp plug and light circuits for a 2300 sq ft house built in the 70's in Texas. There are a couple of additional 20 Amp breakers and one 30 Amp breaker that appear to be unused. Is the service sized correctly? I was under the impression that the main breaker should be 10-15% over the sum of the circuit breakers... is that incorrect? Will the service support another 50 Amp circuit (40 Amp actual draw)?

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The 605 amps is called Total Connected Load. You never use all of it at once.

When you design an electrical service, you use Load Factors. For example, you never turn on all your lights at once, nor do you turn on both the heat and the air conditioning at the same time.
Many loads are intermittent, for example, the water heater and refrigerator cycle on and off, the range is typically used for about half an hour to make meals (the oven also comes on intermittently once it arrives at temperature)
Likewise, the washer and dryer are only used when actually doing laundry. The tankless water heater will only operate for a few minutes at a time, chances are your 225 amp service will be able to handle it. Have an electrician do a load calculation on your service if you want to be sure.

Adding the ratings of all circuit breakers in a panel means absolutely nothing, and certainly does not mean total connected load.

Each circuit breaker is rated higher than the actual load it serves, and each type of load has it's own rating of branch circuit.

All of this is outlined in detail in Article 220 of the National Electric Code.

Ohbob's advice of getting an electrician to do a load calculation is absolutely correct.

Your impression of a service rating should be 10-15% higher than the sum of the breakers is not correct, must be some new myth I never heard of.

Load calcs are also outlined in Article 220. You can gain access to the NEC through a Stick Note at the beginning of this Electrical Forum. I caution you, do not try to do the calcs yourself. It is not written for laypeople.

Before committing to a new tankless water heater, you must have a load calc done to be sure you have enough capacity to handle this new load.