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I have a 27KW tankless electric Water Heater (WH) that is hooked up to a sub panel from the main panbel, with a max amp draw rating of 112.5 amps. Additionally, the 2.0 hp well pump is also wired to this sub panel. This sub panel coming from the main panel uses a 100 amp breaker. The WH has 3 x 40amp, double pole breakers at the sub panel hence the 112.5 amp rating. So the voltage across the sub-panel when the WH is Not on is 244 volts. When the WH is drawing max amps it drops to about 237 or 238 volts, which I believe is normal per NEC (~3% drop).

My questions:

  1. Is it normal that this voltage drop affects the other circuits on main panel? I have LED recess lights which flicker when hot water is turned on.

  2. Should the breaker that branches the sub panel be 100 amps? It seems like it should be greater than max amp rating for sub panel (WH plus Well Pump)?

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  • @crip659 I am measuring ~112 amps on 1 leg/wire of the sub panel when the water heater is drawing max amps and it does not trip the breaker, which lead to the question?
    – arias_JC
    Mar 28 at 14:31
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    @crip659 The current should be the same on both legs if load is balanced. There is usually some difference because load is rarely perfectly balanced. But this circuit is clearly overloaded.
    – JD74
    Mar 28 at 14:42
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    @jd74 the tankless is 240 and likely doesn't use neutral at all. Not a question of balance. Just huge load
    – manassehkatz-Moving 2 Codidact
    Mar 28 at 16:51
  • @manassehkatz-Moving2Codidact I agree, the way I interpret crip659's comment it sounds like he is saying the load is divided between the two hot legs to the neutral.
    – JD74
    Mar 28 at 16:57

3 Answers 3

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TL;DR Yes, this is normal, because you're using too much power relative to the entire house/utility feed.

Stop Using PART of This Water Heater - Now!

It turns out there is currently 3 AWG wire from the 100A breaker to the subpanel. That is OK for 100A. The problem is that the current load is 112.5A (for the water heater) + some amount for the 2 HP pump (Harper says NEC says 8A, a couple of sites I've checked (but not NEC) show around 12A). That total is somewhere > 120A, and in any case even 112.5A > 100A. The quick, temporary, solution as suggested by Harper is to disconnect one of the three water heater circuits (just not the one that powers the controls). That solves the breaker/wire issue for now (i.e., to make it safe) and will help somewhat with the voltage drop issue, but it doesn't solve the core problem - too much load - and it leaves you with limited hot water.

Since the problem is more than just the wire, everything needs to be considered:

  • Rated capacity of the subpanel. If this is a 100A-rated subpanel then it may literally be overloaded itself when the water heater turns on.
  • Wire - needs to be replaced - probably 1 AWG copper or 2/0 aluminum. And possibly a size up to allow for other stuff in the subpanel.
  • Breaker - needs to be replaced. Minimum 125A, but possibly larger (with wire to match) to allow for other stuff in the subpanel.
  • Main panel - LOAD CALCULATION to determine if this tankless water heater is even allowed based on (a) panel size and (b) utility service size.

Based on the symptoms, I suspect the utility service is not large enough to properly support this water heater together with additional ordinary electrical loads. In which case, my recommendation is:

  • Replace the tankless water heater with as large a tanked electric water heater as you can get, except probably not a heat pump water heater (more below). That will likely use a 30A breaker.
  • Replace the 100A subpanel feed breaker with a 60A or 70A breaker.

and then you're done.

As far as a heat pump water heater, there are two problems:

  • Speed - Due to the way they work, they use less power than a regular tanked water heater (which is great!) but the recovery rate is generally much lower, unless the heat pump water heater has a backup resistance element - but then the power savings mostly go away. See this GE web page for a sample comparison/explanation.
  • HVAC - A typical heat pump water heater, unlike a heat pump HVAC system, has everything in one big contraption that sits inside your house. Which means that it pulls heat from your inside air. Which means it cools your inside air as much as it heats the water. That's great in a hot climate as it is like adding a small heat pump air conditioner. It is horrible in a cold climate as now you have to use more energy to heat the air that got cold.
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  • I can replace the wires if need be, the current wire size is 6 AWG that goes to sub-panel. What is the appropriate size for the sub panel (112.5A + [Well Pump]A) x 1.2?
    – arias_JC
    Mar 28 at 14:35
  • So the issue with installing a tank heater, is this tankless is installed on the OUTSIDE wall of my house in a weather proof enclosure. I might have mispoken with the 6 AWG (this is the wire size to actual WH, not sub panel) but thank you I have to triple check. I think best bet is downsizing WH, ~18KW which would put me at just about 80 amp max including well pump in subpanel.
    – arias_JC
    Mar 28 at 18:40
  • You need to check the wires to the subpanel. 6 AWG from panel to water heater is OK because there are three sets of those wires, one for each 40A breaker. In fact, it looks like 8 AWG would be OK (even NM, barely).
    – manassehkatz-Moving 2 Codidact
    Mar 28 at 18:43
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    Gas tankless is just fine. Electric tankless... 27kW, that's just an insane load.
    – Nelson
    Mar 29 at 0:59
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    So subpanel wire gauge is 3 AWG which is rated at 100A at 75 C...
    – arias_JC
    Mar 29 at 1:44
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STOP RIGHT NOW and cut 2 of the breakers to the water heater so only 1 of the 3 breakers is switched on. This can be revisited soon after the Load Calcs are complete.

Your panel is screaming out a warning because it's trying to set your house on fire. This is what happens when you add random crud to a service without doing a NEC Article 220 Load Calculation. This is your new term of the day. You get to learn all about these and you'll be doing 2 of them - first on the house's entire service (all loads fed by that meter) to see whether all that stuff will fit on your service... and then another one on the subpanel loads to determine the required size for the subpanel and the feeder wires to it.

Load Calculations

So Load Calculation #1. All loads in the house. This is to determine whether which size of electric service you are required to have to support the loads you are running. If your service isn't big enough you'll need to remove loads, order a bigger service, or depend on a 3rd trick we'll cover at the end.

Load Calculation #2. All things which are on the subpanel. If the subpanel powers general receptacle and lighting loads, you count the square footage of the house which it is serving.

If the loads you describe are the only thing on the sub, then I get 112.5A x 100% (not a continuous load) + 8A (2hp motor per NEC) x 125% giving 122.5 amps.

That's a lucky break. It shaves just under 125A, which is a common breaker size, unfortunately it just misses 115A and 120A, which are the amps of #2 copper or #1/0 aluminum feeder. So you will need #1 copper or #2/0 aluminum feeder. Fortunately 2/0 aluminum is a commodity product widely sold. If you're inclined to throw money at safety, buy aluminum wire and a torque wrench - not copper wire. Panel lugs are made of aluminum and science proves torque matters, even on the small stuff.

For now, if your Load Calculation says you can run 40A of water heater, switch off all the breakers except one on your tankless. That will give you a 40A tankless and you will need to get a super low flow showerhead; because it won't support full-temperature hot water more than about 1 to 1.5 GPM (depending on water input temperature). This will require family education but will work you around the problem long enough to get your service upgrade ordered. If your Load Calc says you're good to 80A, then switch off 1 breaker making it an 80A tankless. If you are able to run permanently like that, then permanently disconnect the unused circuit so somebody doesn't turn the breaker back on!

Energy management systems to the rescue!

All this can fit on your existing 200A service, if you deploy some new technology to manage panel load. Unfortunately this stuff is just on the cusp of coming out, and isn't really commodified yet - you can do it with a SPAN panel if cost is no object, but cheap solutions are not really available yet - except for EV charging, they are. So the Hummer is safe lol.

Technology Connections just started a video series on that - let me cue up the part that is most relevant. The jokes at the beginning are worth winding back to the start, though.

Like I say, this technology is just on the cusp of consumerization. But the general idea is that a lot of loads are interruptible - they need to run, but they don't need to run right now. If a dryer gets paused for 20 minutes, that's fine. If a house heat pump or A/C gets paused for 20 minutes, your house will lose, what, 1 degree F in that time? Because effectively, your house is a thermal battery.

So we can solve panel overload problems like this just by interrupting loads which are interruptible. Tankless heaters and ranges are non-interruptible, then you have a pecking order of priorities: dryer > HVAC > tanked water heater > EV charging. Notice how all those are storage loads - they need to do it, but not right now. Drop loads off the bottom until the load is within service capacity.

When a load has this type of interruption or throttle-back, it disappears entirely from the Load Calculation.

The SPAN panel will do this right now if cost is no object. Otherwise, you'll need some load interrupting devices to knock out lower priority loads when the heater is on.

You still need to enlarge the feeder to the subpanel, and the subpanel, and the supply breaker to it.

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I would be concerned that your 100 amp breaker is defective. It should trip at 112 amps. If your well pump is running at the same time it will add about 6 more amps. A 100 amp circuit shouldn't draw more than 80 amps continuously. That circuit is way overloaded and is a fire hazard. Do you know what size conductor it is?

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  • I believe it is a 6 AWG, I will confirm
    – arias_JC
    Mar 28 at 14:45
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    @manassehkatz-Moving2Codidact If my daughter is taking a shower it's going to be on for a good 30 minutes:)
    – JD74
    Mar 28 at 15:17
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    When the wife takes a shower its definitely more than a few minutes, ~25min or so. The breaker hasn't tripped yet which leads to my confusion because the math says it should.
    – arias_JC
    Mar 28 at 15:23
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    And the typical trip curves says something like: "full capacity to 10% over, takes a very, very long time to trip" (which allows for manufacturing tolerance, etc.) and "20% takes a couple of minutes to trip" (which allows for motor startup and other short term legitimate stuff) and "50% takes a few seconds to trip" and "100% over (i.e., double) a second" and "200% over (triple) 1/30 of a second". Or something along those lines.
    – manassehkatz-Moving 2 Codidact
    Mar 28 at 16:11
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    @arias While the breaker's behavior is curious, it doesn't really matter because that breaker is leaving. Unless you're willing to shut off 2 of the 3 breakers to the water heater (one of the three also powers the water heater's controls; that's the one you need) ... which would make it a 40A heater. You would have to slow flow down to ~1 GPM for that to produce normal temperature, but it would work. That will get you by in the meantime while you sort out feeder and service upgrades.
    – Harper - Reinstate Monica
    Mar 28 at 18:58

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