So my home was built in 1960 and 15-20 years later another room was added on. There are panel boxes everywhere ha. I have mapped it all out and isolated almost everything. Anything that is unidentified must not be important or no longer existing because nothing seems to lose power when I flip these breakers off.

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The big question…

I have in my possession this electric tankless water heater:


It states that it has an amperage draw of 113amps and at the panel box It will need to have (3) independent 40 amp 2-pole breakers.

You will see in my layout that on Panel A left side I have a 70amp, 30amp, and 40amp breaker that are now obsolete. Can I use those 3 spots to accommodate the new heater?

From my understanding the amperage specifications for an appliance are normally 20% higher than what it actually draws and that most appliances may draw that amount at the initial start up but after that draw much less to maintain operation. I am having difficulty getting a straight answer out of technical support though.

Will I be pushing my limits?

Thanks for your help! Corbin

  • 1
    Hello, and welcome to Home Improvement. Wow: complex, but well-documented question. Good work. – Daniel Griscom Jan 23 at 21:17
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    I upvoted just for asking before buying. Then I read the question, and I regret that I have but one upvote to give. Also, Woooooo! Stab-lok!!!!! That last bit was ironic but it's true: This will be interesting! – Harper Jan 23 at 21:21
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    I'd say that in many cases the listed amps on a device can follow your understanding, except for the case of resistive heaters. They don't have startup, or different power settings or anything like that. They are on or off, and when on they normally use very close to what they say they do. – JPhi1618 Jan 23 at 21:25
  • What's your location, out of curiosity? Haven't seen a breaker panel like those before. – JPhi1618 Jan 23 at 21:29
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    Saw the stab lock and quit reading, don't do anything before reading about FPE or federal Pacific stab lock. I will only replace these panels. – Ed Beal Jan 23 at 23:33

Well, to simplify the maze somewhat, you have all the usual loads of an all-electric house.

  • 50A furnace
  • 50A range
  • 30A air conditioning pack
  • 30A dryer
  • Eleven 120V branch circuits, some MWBC, many on 30A breakers (WTH?)

Obviously we can oversubscribe this somewhat, e.g. You're not likely to run heater and A/C at the same time.

The fine art of panel subscription is not a core skill for me, but hopefully this summary will make it easier for someone else.

That panel, though

While we're here, we gotta talk about that panel. First, panel B top-tapping panel A before the main breaker is very dangerous unless there is another main breaker before this. The meter is not a fuse, and there are no fuses between the transformer and main breaker. This is considered acceptable for a very short hop from the meter to the main, however, in your case this totally unfused feeder crosses your whole house. Nothing prevents a short in the cable, or in this panel, or summed loads on this panel, from setting that cable on fire.

Second, all 4 panels are Federal Pacific "StabLok". It has serious issues with panel fires due to bus defects, and unreliable trip. When feasible, all four should go.

The three subs will be straightforward to DIY, the main will be more troublesome unless there's an outside main breaker. I would DIY install a 200A main service panel right next to it as a subpanel, reuse that 70A breaker to feed it, and move all the 120V circuits out of the main into the new sub. Then bring in an electrician to cut it over to be the new main.

I see many branch circuits on 30A breakers. Those should be 15 or 20 depending on wire gauge.

Gaps in the panel covers should be patched so curious fingers can't touch buses, except finding either breakers or blanks to cover those spots will be a pain.

  • It definitely looks like someone was "upgrading" breakers that were "nuisance" tripping, those should be reverted to what the wire sizes dictate. Having an electrician trace the unidentified wires would add some peace of mind of where they go and avoiding accidental backfeeding. – ratchet freak Jan 24 at 10:12
  • I did not even look at the breaker sizes after noticing it was a stab lock, good catch, only the family room is properly protected+. At least the subs would be easy to replace, but I would replace all 4 , I have seen several busses that were seriously burned, I found a FPE panel in the old section of my mill and removed it for grins tested some of the breakers, after the failures on a load test, I now toss these in the trash, I used to save every old breaker but not these. Not trying to be an alarmist but I won't touch stab lock or even replace fixtures on a stab lock protected branch circuit. – Ed Beal Jan 24 at 17:54

Wow! Well written question. I took a look at the installation pdf from the website. Looks like 3 runs of 8 awg/2 conductor cables(plus grounds) between panel A and the unit. My big,big concern that I see which is a big, big safety concern is that panel B is tapped off before the main breaker. That means that that wire has no protection. Big bang potential if you get my drift. Next suggestion is all the obsolete breakers and cables need to be removed from the panels. That is another potential hazzard. Then install as per instructions and area electrical codes. It turns on the necessary resistive heaters based on the flow rate. It is not an inductive load which have start-up currents.

  • I'm not sure if a tankless is even a wise solution to his problem... – ThreePhaseEel Jan 24 at 3:11
  • Threephaseeel, the question was if he could. Not if he should. The information about panel locations suggests a house not best suited for only 1 location of an on demand water heater. – user68386 Jan 25 at 15:07


The first order of business here is to rip every last ounce of Fire Protection Eliminated hardware out of your house and replace it with reputable panels and breakers, starting ASAP with the main panel here, as it simply cannot be relied upon to function.

I would start with a minimum of a 42-space, 200A, main breaker panel as the basis for this setup to provide plenty of space for Arc Fault breakers as well as expansion room, with a 54-space or 60-space panel preferable if space and budget allow. The feed to panel B should be rerouted to make it a proper subpanel of panel A at this point as well. Note that this work will definitely require help from the electric utility to pull the meter/shut power off to your house, and may require you to hire a professional, depending on local codes.

Once you're done with that, you can then replace the other panels at a bit more of a comfortable pace and in a more comfortable fashion as well. I would recommend 24-space if not 30-space, 125A, main lug panels as suitable replacements for panels B, C, and D, space permitting (panel B certainly can be replaced this way, while panels C and D may have to be smaller if there isn't a full-width stud bay there to fit the panel into).

Furthermore, while the breakers for major hardware seem to be correctly sized, the breakers for many of the branch circuits are improperly sized and cannot be used as guidance when selecting replacement branch breakers. This means that you will have to look at the size of the wiring on each branch circuit and use it as your guide when deciding between a 15A and a 20A branch breaker for these circuits.

Also, you may notice breaker trips after you do this due to the combination of improper breaker sizing to begin with and the noted tendency of FPE breakers to not do their one and only job. In that case, you will need to pull additional branch circuits and/or redistribute loads in order to stop the breaker tripping.

Send that tankless heater back the way it came, for it's probably not the answer to whatever your hot water woes are

This may sound strange since you already have it sitting in your house, but that tankless heater of yours is probably not the answer to whatever hot water problem prompted all this. Instead, given that your climate is reasonable for an all-electric house, you are better off using a clever device called a split-system heat pump water heater (often called an EcoCute after what they are called in Japan) that uses an indoor tank plumbed to an outdoor heat pump unit that heats the water and sends it back to the tank, where it can then be stored for use.

These are more efficient than both standard tank-type heaters and tankless heaters, and provide superior capacity for most applications (with the ability to bring water up to 180°F for storage, and then use a mixing valve to temper it down to a more reasonable temperature for use). In particular, with a tankless heater, whatever efficiency gains you get by removing storage losses in a tank from the equation are lost in the power company needing more spinning reserve running at all times in order to be able to power 112.5A of tankless heater whenever it turns on, instead of 12A of heat pump water heater or even 24A of electric tank heater. (Furthermore, modern electric resistance hot water heaters are very well insulated, with Energy Factor ratings upwards of 0.9.)

The one downside to split-system heat pump water heaters is that they require a bit more plumbing than the norm due to their split nature, but this is manageable in many installations, and is one of the keys to their efficiency -- EcoCutes do not have the "parasite load" problem that unitary heat pump water heaters do.

Other efficiency tips

If you want to get maximum performance out of what you put in for a water heating solution, there are two other things you can try. First and foremost is to insulate, or "lag", the hot water pipes in your house with at least R-4 (5/8" thick foam) or better insulation. This reduces standby losses from a well-insulated tank, and also means that the water in the pipes stays hot enough to satisfy short draws from the heater without having to wait for the heater to kick on.

The other upgrade you can make to your plumbing would be to add a drain water heat exchanger (DWHX) to your shower, dishwasher, washing machine, or other high-usage appliances. These can regain 30% of the heat lost in drain water, which is especially important if you do go tankless, to help it keep up with high-usage loads.


The real acceptable load of electric wires depends on factors like air temperature, number, load and distance of wires running parallel nearby, ventilated or enclosed space, insulation, to name a few. F.e., in an insulated wall, the maximal load may be derated to only 70% or less. Search words: electric cable load chart


In case of electric shocks or fire with fatalities caused by this installation, in many western countries it would be difficult nowadays to avoid a jail term for the responsible person(s). Also insurances would refuse to cover material or any other damages.

Main points: - Unreliable breakers - Metal parts with high voltage against ground not covered - Ground fault breakers seem to be missing - Some regions even demand Arc fault devices (should be considered in wooden houses/walls)

Proper earthing (of the water tubes if made of metal) will be needed as well with many water heaters. The conductivity of the local water must be in specific limits if the blank electric heating wire is in direct contact with the water. No (re-) connections of Neutral and Earth downstream of the GFCI.

Fire/fume detectors above all panels (and of course, in bedrooms) is a first non-expensive DIY safety improvement.

This installation was most likely designed with standard simultaneous load assumptions which are void if a very high load (27kW) is installed (some people prefer to get their blood circulation running with a hot 30 min shower in winter mornings).

A tankless heater could be combined with a 'countercurrent exchange' type of heat exchanger. Even among technicians it is not well known that the temperatures can be completely exchanged between the fresh shower and drain water - in theory. In practice some 30-60% can be recovered. Reduces the load to the wiring and breakers, saves energy and money. See DIY videos, f.e. Rob the plumber, heat exchanger.

Even without DIY plumbing work, a water stop valve should be also installed, which closes if there is an unintended waterflow.

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