Can I replace a fried 60A tankless water heater wired to a double pole 60A breaker on 6AWG with a 40A tankless water heater?

Question

Original tankless is a Eemax 13 Kw 60 A and the replacement is a EcoSmart 8 Kw requiring a 40A DP. Currently in our electrical box is a 60A DP. Do I need to swap this for a 40A DP or will the 60 A DP be ok? Reason for downsizing is that the Eemax causes lights to flicker and just stopped working....thinking it is oversized for the wire/breaker combo. Previous owner replaced a tankless that had dedicated junction box in attic with the max wired directly to electrical box on the 60 A DP. Any guidance is very much appreciated.

Answer 1

Too large a heater doesn't cause those symptoms. You have another problem.

Your presumption "Therefore the heater must be too big" doesn't line up with the symptoms. For that I would expect main breaker trips. I would expect slight brownouts visible mainly on cheap capacitive-dropper based LEDs. I would not expect to see flickering and circuits going dead. That corresponds to a quite different problem, a failing wire connection typically due to wrong breaker or improper screw torque on the terminals (which is why NEC 110.14 calls out using a torque driver as of 2014).

So I would start my troubleshooting by assuming the heater is good, the Load Calculation is correct GIANT ASTERISK -- and assume wires have failed enroute.

I'd be looking at all wire terminations on the water heater circuit, the breaker bus stabs where the breaker clips onto the bus, and all that stuff on the subpanel it's served out of, if any. This is the crux of my answer. The rest is footnotes.

The problem is, if you downsize from 60A to 40A you will also be getting tepid water unless you add flow restrictors to reduce flow. But we'll have to come back to that concern.

That Load Calculation, though.

NEC Article 220 spells out the procedure for doing a proper Load Calculation for acceptable loads in the panel (or to be more precise, the service size required to support a list of loads, but it ends up working in reverse, "what can your service handle").

The very concept "enlarge heater until power system fails destructively, then downsize" indicates you are using a non-approved method of Load Calculation I call the "Thunder Run" (a Ukrainian tactic of advancing in light vehicles at insane speed, carrying mostly radios and Ukrainian flags, and see who shoots at you.) It's more formally called "Reconaissance by FIRE" which gives you a hint why I don't like it in electrical.

So you should crack open a reliable source on Article 220 Load Calculations, and figure out if your house's service can handle this thing or not. If it can, then absolutely positively you have a Different Problem, and downsizing is the wrong thing to do. If the Load Calc can't support a 60A tankless I'll bet it won't support a 40A either, and we'll be having a different conversation. We need to know this. Still doesn't address the original problem though.

The tankless problem

I love tankless heaters and are very optimistic for their future. "Smart Panels" which can dynamically shed loads (the killer app is ludicrous-speed EV charging), will sharply benefit tankless heaters.

However, failed tankless installations are ugly. People experience water that is tepid, to put it gracefully. This might be corrected except the failure galvanizes the occupants of the house against tankless, so the unit is given the unceremonious heave-ho, and they badmouth the technology at every opportunity. As such, Don't Mess Up Sizing of tankless, or there will be hell to pay.

The reality is tankless electric takes extreme quantities of power. My rule of thumb is "40A per GPM you want". People are often "out of touch" with this, and it results in the above project failures. However you seem to be confident and adjusted to your 60A unit. The problem is, a 40A unit will heat the water 1/3 less. (either 1/3 less heat at same flow which means 'tepid', or 1/3 less flow at same heat). This is how you lose the political war. That is why it's so important to troubleshoot it properly. You probably need to stay at 60A for this project to work.

If your Load Calculation is telling you that you don't have room for any tankless at all, then you go with a tanked heater (4500W/19A with standard elements, 25A breaker......... or 3800W/16A with Mobile Home elements, 20A breaker).

Answer 2

Need to swap the breaker? No. Ny definition you can always draw less current than the breaker is rated for.

Should swap the breaker? I'd seriously consider it. The less excess current the breaker permits, the faster and more reliably it will protect you if something goes wrong. Since in this case you know exactly how much current should be needed (at most) by that circuit, there's no reason to allow more, unless you reasonably expect to upgrade that water heater or use the same circuit to power something else. At least, no reason beyond saving a bit of money/time on the new breaker and the work to switch over.

Answer 3

6 AWG copper can be fine for 60A, depends on a few factors. Assuming it is OK (75C rated connections, individual wires not NM), the problem was not the wire and breaker.

A new 40A heater will require a smaller breaker but can use the same (which is now oversize, but that's perfectly fine) wire.

Instead, I suspect the problem was that the heater used too much power for the entire service. This is where load calculations come into play. If you are on 100A service, pulling 54A (13kW/240V) is going to have serious impact on the rest of your house. If you have 200A service and are not running too many other things at the same time (EV charging, electric heat, dryer, oven, etc.) then you should be fine. But that's what load calculations are for. Assuming that's the problem, downsizing to 40A has a good chance of solving the problem.

That being said, you may want to consider going down to 30A and a traditional tanked water heater. The EcoSmart 8 (and presumably similar models on 40A circuits) is rated at 0.8 to 1.9 GPM. So basically one shower at a time. If that works for you, great. If you need more, a traditional tanked water heater may be a better solution.

Answer 4

You need to swap to the correct size/amperage breaker for the device and wiring.

You should also inspect the wiring for damage possibly caused by being run at greater than specified amperage, and replace if any is evident. If copper, it was on the edge, and could be considered acceptable via rounding (rated 55A @ 60°C, so 60A breaker OK given no 55A breakers are made, if only provisioned for 55A) so this is NOT "must replace, severely overloaded" it's check, and probably more likely to find a bad junction you should fix (since lights flickering, stopped working) than melted insulation.

A 13KW water heater should draw 54.1A, but that means it was not properly derated for continuous loading, where it should be provisioned at 67.7A, (multiply by 1.25 or divide by 0.80) for the continuous loading. So the wire and breaker were technically overloaded, somewhat.

Answer 5

The wire size and breaker you have for the Eemax is, for all practical purposes, appropriate for a 13 kw heater. You will want to downsize the 60 amp breaker to a 40 amp breaker. The #6 AWG should fit in the forty amp breaker. If it doesn't, get some large wire nuts and pigtail some #8 to the #6 and connect the #8 to the forty amp breaker.