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My gas water heater is very old, and I'd like to replace it with a heat pump electric water heater. The one I'm looking at is 19 amps, 240 volts. My breaker box is in a finished part of my basement, and it would be difficult to run a new circuit to the unfinished part of the basement, to where the water heater will be.

My oven circuit runs almost to the unfinished part, and the end that goes up to the oven is easily accessible, so I'd like to use that wiring to have a subpanel installed, then feed the oven and water heater from there. The oven breaker is 50 amps, and the wiring is also for 50 amps, and has 3 wires plus ground.

Would it be to code to power both of those through the subpanel? At some point in the next few years, I'm likely to get an EV, could a 30 or 40 amp car charger also be added to that subpanel?

My house is in the US (Michigan), and was built in the late 1990s, and I have a 150 amp main breaker. An online load calculator says with the water heater and car charger, I'm only at 137 amps, so I think I'm fine for the total house load.

Edit: I'm not concerned about tripping the breaker, we'll be able to set times for heating/charging to not draw too much current at once. I'm concerned about whether doing this will be to code.

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    First what does the oven use/rated for? If the oven calls for the full 50 amps, adding more will be difficult on a 50 amp circuit. Most times you might get away with it, but making a big family and friends dinner plus the water heater will trip breakers.
    – crip659
    Commented Jul 5 at 16:16
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    If you ever end up in a 'don't do that or it will pop the breaker' situation with hardwired appliances, the quick reference is here: it's not to code.
    – KMJ
    Commented Jul 5 at 17:28
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    Heck, even if the oven only calls for 40A, you won't be able to run a 19A water heater at the same time.
    – Huesmann
    Commented Jul 6 at 12:44
  • Load calc and load management issues are valid, but take a 40,,000 ft look: Wire costs money, builders just plain don't use wire any bigger than what is needed for the circuit they are building. Commented Jul 7 at 1:48

3 Answers 3

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Load Calculations are the key. And you've got some issues there.

An online load calculator says with the water heater and car charger, I'm only at 137 amps, so I think I'm fine for the total house load.

Some online calculators are better than others. A particular concern here is the 80% derate for continuous loads. In particular, EV charging is always a continuous load. Water heating may or may not be considered continuous, but that is an important thing to double-check. Magic number of 137A sounds great. Is that with 30A or 40A (you mentioned both) of EV charging? If it is with 30A then don't decide to upsize to 40A (147A is a bit too close to 150A for comfort, at least for me) without changing something else.

So I suspect with these major changes you may really be in line for a heavy-up to the current usual 200A. The cost for that, and the other possible benefits, depends a lot on your existing panel type and capacity. If it is a 200A rated panel and has empty spaces then you may be able to replace the feed (utility to meter, meter to panel) and change the 150A breaker to 200A breaker and be done. On the other hand, if the panel is full or has other issues or has a maximum rating of 150A then you will need a new panel.

But the bigger issue is the oven circuit/panel. There are ovens using anywhere from 20A to 50A. While I doubt your oven only needs 20A, it is quite possible that it can work on a 40A or 50A circuit. If that's the case then you could install a subpanel. However, you would then only have nominally 10A to spare when the oven is use. There is sometimes some additional wiggle room. But adding 19A (so that's a minimum 20A circuit, but quite possibly a 30A circuit may be required like a bog-standard electric tank water heater) for a water heater and 30A (or more!) for EV charging - I just don't see how you can do that on a 50A circuit that already has a 40A or 50A load (the oven). Yes, you can load shed so the water heater and EV turn off when the oven is on, but while this type of load shed can work extremely well at the entire service level (because most of the time you won't be using anywhere near 137A), doing that on a 50A panel doesn't make much sense to me.

One good piece is that EV charging for most people doesn't need 40A and probably not even 30A. So if everything else falls into place and you end up with only 20A for EV charging then you'll probably do just fine. But with the current setup I think even 20A is not going to work.

My recommendation:

  • Leave the oven as is. There just isn't the extra capacity to realistically use it as a subpanel feed.
  • Run a new line for a new subpanel, either conduit (so you can upgrade the wires later if needed) or cable with a rating of at least 60A. Yes, that will cost, but it is a one-time thing (especially if you use conduit - if you use cable then I'd even go for larger cable - e.g., aluminum 2 AWG to allow for up to 90A) to avoid having to replace it in the future.
  • Install a medium to large subpanel. Minimum 100A rating, minimum 12 spaces. But better is 20 spaces or more. Each 240V circuit will take 2 spaces, and you can't predict what other loads - e.g., convenience circuits for lights, tools, etc. - you may want to add in the future. The major cost of a subpanel is not the panel itself, it is running the feed.
  • Unless you really truly have sufficient capacity (triple-check the load calculation), consider a heavy-up - i.e., main panel replacement and increase service to 200A.
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Do a load calculation for just the subpanel.

However you have some options.

A smart EV charger can be made to sense the current being drawn by the entire subpanel and throttle down the current the car is allowed to draw to keep it within limits. Letting you discount it almost entirely from the load.

With a bit of additional smarts you can additionally turn off power to the water heater when the oven is in use.

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    I'm concerned about whether doing this will be to code. Is the subpanel load calculation sufficient to determine that?
    – BaddDadd
    Commented Jul 5 at 17:03
  • @BaddDadd yes, EVEMS is to code. It's always been to code because UL approved it. However, some inspectors refuse to recognize that NEC 110.2 delegates approval of new things to UL (didn't need a code change to support Wagos or blue/pink Romex). So, they made several successive edits in NEC 625.42 to try to make it more clear. More on EVEMS diy.stackexchange.com/questions/277803/… Commented Jul 6 at 20:12
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Normal 4500W water heaters are 18.75 amps. So a "heat pump water heater" that is 19 amps, is actually worse than a normal one LOL.

That's because it is a normal one. Either the heat pump is a lie, or it's an accessory system but it still relies on old-school heating. Well, that certainly complicates the circuit design.

Since it provisions the same as a tanked heater, its heat-pump superpower is irrelevant.

I'm just saying it's a damned shame that you don't have a 120V heat pump water heater that could plug into any random 120V circuit not already dedicated to a room or appliance, and disentangle it from the current (heh) situation. It would sacrifice the 240V heater, or have a simpler, slower 120V backup heater. Which will work. A normal water heater can be wired to a 120V circuit, it'll just take 4 times longer to recover.

The 3-wire problem

Unfortunately, as of the 1980s, it was still legal to run an UNGROUNDED 3-wire connection to a range. In 20/20 hindsight this is completely insane, but it's true. It MIGHT be possible to use this for a feeder to a subpanel, but something needs to be done to deliver ground.

  • If the cable used was "/3+ground" black-red-white-bare, you're all set.
  • If the cable is the odd "SEU" - 2 black conductors wrapped in a mesh neutral, then it's possible to use this, but you will need to retrofit ground to the subpanel. You will need to insulate the bare neutral so it can't hit grounded bits of the subpanel.
  • Your house is too new to use 1960s era "/3 no ground", but if so, see SEU.
  • If the cable used was "/2+ground" black-white-bare, this was illegal the day it was installed and cannot be used for the range. There is no way to get neutral out of this and the range needs it.* It can, however, be re-tasked to serve "240V-only loads that do not use neutral", and you get major brownie points if you already know EVs are one of those. For the range, you will need a completely new run of 6/3+gnd from panel to range, or, decide it's a fine time to switch to gas.

So the 3-wire problem will need to be dealt with, if present.

Location, location, location

Your idea of sticking a subpanel inline is a tricky one. You can't just cut the wire in a clever place and squeeze in the subpanel inline, unless you get very lucky with wire routing and have a nice tall panel with lugs from the breaker. Isn't going to happen in a run-of-the-mill 6 space panel, which you can't use anyway because the EVEMS equipment will take more room than that. Note that you CAN mount a panel sideways, but if you do, you can't use breaker positions where down is on.

Further, position of wires tends to be incompatible with subpanel Working Space rules, which require panels to be at a certain height, have a Working Space kept clear of ALL non-electrical equipment which is the width of the equipment (but at least 30"), 78" tall, on the flat not a stairway, and 36" of standback room from the panel face. That rectangular prism needs to be kept clear At All Times. This could be rather ungainly.

However, it may be possible to git-r-dun with load shed devices which are not subpanels and don't require subpanel working space.

"Not all at the same time"

What sweeps to our rescue is the topic of a pair of Technology Connections videos starting here. The idea is that the loads don't need the power at the same time. Now Technology Connections illustrates with the SPAN panel, but we can grab a simpler set of kit.

Required reading is my Q&A on EVEMS and dumb load sheds.

The water heater is a dumb load that needs to be knocked out when the range draws power. You can use relatively simple "load shed" kit like the SimpleSwitch or Blackbox here. The virtue here is simplicity and compatibility with dumb loads; the cost is actually fairly high. But this can happen without a subpanel; you are simply taking the line to the range and splitting it with a Simpleswitch etc. setup to detect current on the range circuit.

I believe the water heater branch coming off the load shed will require a 30A breaker or fuses to protect the water heater circuit. Fused disconnect?

The EV is capable of dynamic EVEMS. That's more versatile and cheaper kit. It has a current sensor on the supply, and figures (capacity - what is being used = available) and tells the car to stay within the "available" value. It does this on the fly in real time.

Since both the range and EVSE are fit for 50A breakers, it might be possible to avoid the subpanel, install a DIN Rail enclosure with the required power meter + a couple of lug splice DIN rail modules, (I'm presuming Wallbox Pulsar Plus here, since an Emporia would be a complete waste in this type of installation)... and not have any breakers involved at all.

After the EVEMS enclosure, you go onward to the dumb load shed that splits off water heater and range.

I think we're getting this done without anything being a "subpanel" that would require working space.



* For the oven light, if you can believe it. So people can use ordinary incandescent light bulbs like everyone uses in every fixture... great. (Clearly, ranges were not designed by HP or Epson. Also 120V is sometimes used for clock and controls. I could not possibly suggest to rewire the oven to use a 240V oven light, and install a UR-Recognized autotransformer to source 120V for clock/controls. Yeah, don't do that.

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  • I wonder why ovens didn't simply use a pair of sockets in series, and specify that bulb life will be adversely affected if the installed bulbs don't match? Or why, in more recent decades, they haven't switched to using a triac-based dimmer circuit?
    – supercat
    Commented Jul 7 at 19:49
  • @supercat Really, this could be solved in a year with a $100 excise tax on ovens that require 120/240V. Commented Jul 9 at 4:46
  • Since many electrical codes have for years required running 3-wire (plus ground) service, such ovens would only be advantageous in places that don't already have such service. In those situations, I would think having a UL listed autotransformer/circuit breaker assembly (which would cut 240V power in case the transformer wouwould be overloaded) would be better than grandfathering things. By question was about why the problem wasn't averted half a century ago.
    – supercat
    Commented Jul 9 at 14:39
  • @supercat but purely on a "what do people ask about online" basis, the 3-prong is still heavily in circulation if not dominant. Commented Jul 10 at 18:39
  • If the chicken and egg problem between manufacturers, consumers, and regulatory agencies were overcome, what downsides would you see to having a UL rated assembly that could fit in an oversized junction box that would combine an autotransformer, a dryer outlet, and a a thermal cutout that would disconnect both legs of the 220V supply, perhaps including an attachment for a Bowden cable which could in turn connect to an external actuating lever if the box itself would be in an inconvenient location? I would think in many installations that would be cheaper than running a new cable...
    – supercat
    Commented Jul 10 at 19:40

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