Is 100 amp service enough for tankless water heater?

This is a common question that homeowners face when installing a tankless water heater. Tankless water heaters, also known as on-demand or instantaneous water heaters, heat water as it flows through the unit rather than keeping a tank of water hot all the time. This results in more efficient water heating and potentially lower energy bills. However, tankless water heaters have higher power requirements than traditional tank water heaters. So you need to consider your home’s electrical system when installing one.

How much electricity does a tankless water heater use?

The power consumption of a tankless water heater depends on the flow rate, rise in temperature required, and efficiency rating of the unit. But generally, tankless water heaters require a power input of 15 to 150 amps at 240 volts. Here are some more details:

  • Small unit for a single faucet: Requires about 15-20 amps
  • Whole house unit: Requires 60-100 amps
  • Large high-end unit: Can require up to 150 amps

So while gas tankless heaters have higher heating capacity, electric tankless heaters have much higher electrical requirements. To size your tankless water heater, you need to determine the peak demand, which is the maximum flow rate required by your household. Some key factors that determine peak demand are:

  • Number of hot water outlets (faucets, showers etc.) that might run simultaneously
  • Flow rate of each outlet
  • Required temperature rise
  • Climate and ground water temperature

Once you know the peak demand, you can select a tankless heater model that meets that demand. The power rating of that model will determine the amperage load.

What is a 100 amp electrical service?

The amperage rating of your home’s electrical service indicates the maximum power that can be delivered to run all your home appliances and equipment. A standard 100 amp breaker panel found in many older homes can deliver 24,000 watts (100 amps x 240 volts) continuously and safely.

Here are some key points about 100 amp service:

  • Can support about 24 circuits with 15-20 amp breakers
  • Can handle low to medium power appliances and lighting for an average sized home
  • Has some room for larger appliances but can get overloaded if too many are running at once
  • Is no longer considered adequate for newer large homes with more high power devices and appliances

Is a 100 amp service sufficient for a tankless water heater?

Whether a 100 amp electrical service is adequate depends on the size and power requirements of the tankless water heater model. Here are some general guidelines:

  • A small 15-20 amp unit for a single water outlet like a bathroom sink can work with 100 amp service.
  • A whole house unit will likely overload a 100 amp panel, trip the main breaker and leave you without hot water.
  • Even a medium 50-60 amp tankless unit might be borderline and trip the main breaker if other large appliances are also running.

However, here are some options to allow installing a tankless unit on 100 amp service:

Install a medium sized unit

A medium tankless water heater that draws around 50-60 amps may work if you don’t use other high power appliances at the same time. Prioritize your circuits with the tankless heater taking one of the first few slots on the main breaker. Avoid running the clothes dryer, oven, AC or other large loads along with the water heater. Also, consider gas tankless as an option since it uses less power.

Upgrade your electrical service

The best solution is to upgrade your 100 amp service to 200 amps. This will provide enough power capacity for a whole house tankless unit and other large appliances without overloading. Upgrading the electrical service includes replacing the main breaker panel and rewiring done by a professional electrician. Though not inexpensive, it improves the safety and functionality of your home’s electrical system.

Install load shedding device

A demand management or load shedding device can allow a large tankless unit on a 100 amp service. It works by temporarily shutting off non-critical circuits like the AC when the tankless heater activates. This prevents tripping the main breaker. These devices add cost and complexity so are not an ideal solution.

In summary:

  • Tankless heaters have high electrical requirements, from 15 amps for small units up to 150 amps for whole house.
  • A 100 amp service may only support a small unit for one water outlet.
  • Larger tankless units will likely overload a 100 amp panel when combined with other appliances.
  • To install tankless, the best option is upgrading to a 200 amp service if possible.
  • With careful use, a medium 50-60 amp tankless may work. Gas tankless is also an option.
  • Load shedding devices can allow larger units but are complex and not ideal.

Installing a tankless water heater provides the benefits of continuous hot water and energy efficiency. But the high power needs mean you have to carefully consider the capacity of your electrical system. Although possible with 100 amp service in some cases, upgrading to 200 amps provides the best results and should be factored into the decision to go tankless.

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