NEC 220.87
Can My Panel Handle a Tankless Water Heater?
An electric tankless is one of the largest loads in a house, 46 to 112A, so this is the check that decides whether the job needs a service upgrade. Here is the fit threshold by unit size and service, and the gas alternative that avoids the question.
Will it fit? By unit size and service
Each cell is the highest existing metered demand your service can already be carrying and still take the tankless. “Upgrade / manage” means the unit alone fills the service, so no existing draw is small enough.
| The load | Adds | 100A service | 125A service | 150A service | 200A service |
|---|---|---|---|---|---|
| 11 kW tankless | 46A | ≤ 43.2A | ≤ 63.2A | ≤ 83.2A | ≤ 123.2A |
| 18 kW tankless | 75A | ≤ 20A | ≤ 40A | ≤ 60A | ≤ 100A |
| 24 kW tankless | 100A | upgrade / manage | ≤ 20A | ≤ 40A | ≤ 80A |
| 27 kW tankless | 112A | upgrade / manage | ≤ 10.4A | ≤ 30.4A | ≤ 70.4A |
Each cell is the highest 12-month metered demand (NEC 220.87) your service can already be carrying and still take the new load: it fits when your metered peak is at or under that number. “Upgrade / manage” means the new load alone fills the service, so it needs demand management or a service upgrade regardless of the existing draw. Computed at 125% of the metered demand per NEC 220.87; the nameplate rating of your unit governs the “adds” figure.
Why it is the load that forces the upgrade
A storage-tank water heater cycles a 4500W element and draws about 19A. An electric tankless heats on demand, so it pulls its full rating the whole time hot water runs: an 18 kW unit is 75A, a 24 kW unit is 100A, a 27 kW unit about 112A. That is comparable to the entire rest of a house. By NEC 220.87 the service must carry 125% of your existing metered demand plus that load, so a tankless is the appliance most likely to turn a conversion into a service upgrade. Size the circuits themselves on the tankless wire-size page, and check the whole panel on the load calculator.
Frequently Asked Questions
Can a 200A panel handle an electric tankless water heater?
Often, but not always, and it is the one to check. A common 18 kW whole-house unit draws about 75A, and by NEC 220.87 it fits a 200A service only if your 12-month metered peak is at or under 100A. A 24 kW unit (100A draw) needs your peak at or under 80A. Because these are large loads, an all-electric home with heat, a range, and a dryer can push a 200A service past its limit, so run the load calculation before committing.
Can a 100A or 150A panel handle a tankless water heater?
Usually not without a service upgrade. A whole-house electric tankless adds 46 to 112A, which is most or all of a 100A service on its own, so the 220.87 check comes up short in nearly every real home at that service size. This is the single most common reason a tankless conversion turns into a panel or service upgrade. A gas tankless avoids it entirely, and a smaller point-of-use electric unit for one sink is a much lighter load.
Why does an electric tankless water heater need a service upgrade so often?
Because it is one of the largest loads in a house. A storage tank heater cycles a 4500W element on and off and draws about 19A; an electric tankless heats water on demand and can pull 75 to 112A while running. That is comparable to the entire rest of the house. By NEC 220.87 the service has to carry 125% of your existing metered demand plus that new load, and on a 100A or 150A service there is rarely enough headroom.
Does a gas tankless water heater need a load calculation?
No. A gas tankless heats with a burner and needs only a small 120V circuit for its controls, igniter, and vent fan, typically a 15A circuit. It adds almost nothing to the electrical service, so there is no 220.87 concern. If your panel cannot take an electric tankless and you do not want a service upgrade, a gas unit or a heat pump water heater is usually the answer.
Related Calculators
Tankless Water Heater Wire Size
Amperage by kilowatt and the multi-circuit split, with per-circuit conductor sizing.
Dwelling Load Calculator
Run the full NEC 220.87 existing-load check against your service, every step shown.
Heat Pump Water Heater Circuit
The lighter-load alternative: a 240V hybrid, or a 120V plug-in that can share a circuit.
Can My Panel Handle It?
The same check for an EV charger, heat pump, hot tub, or induction range.