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Electrical Wiring

Wire Tankless Electric from Two Sub-panels?

5 years ago

last modified: 5 years ago

Am considering upgrading my vacation home's old, giant, hot water heater to a tankless. Why heat water when I'm rarely there, right?

The water heater models I'm considering generally require 120 amps total, and are powered by three separate 40 amp circuits (each on its own dual pole 240v breaker with 8/2 grounded wire). This is within the capacity of the home's electrical service (upgraded by the prior owner for, um, indoor agricultural use). However, there's not enough room in either subpanel for 3 240v circuits. They're right next to each other, though, and there's enough empty space to use 2 40A circuits from 1 panel, and 1 40A circuit from the other.

Is there any reason why I shouldn't use the two sub-panels in this fashion? To my mind, the two-subpanel set up side by side is like having one large subpanel, but I am not a professional and wouldn't move forward with this without being 100% certain I'm OK.

Featured Answer

Ron Natalie

5 years ago

I assume that you're talking about the Rheem. Nothing really precludes you from putting the breakers in different panels. You just have to make sure you're not overloading either one individually.

One of those panels (or possibly both) are going to be fed by a breaker somewhere else (the simplest has the subpanel fed by a breaker in the main panel next to it). You need to assure yourself that you're not going to trip the feeder breaker with the circuits you have in the subpanel.

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Comments (9)

greg_2015
5 years ago
If your only concern is the waste of energy (and $) of heating the water while you aren't there, then just turn your tank down (or off) when you leave.
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Thanks for the info guys...I suspected as much. I am very capable of doing this myself. There's nothing mysterious about electrical circuits, current flow, grounding, bonding, etc. But sometimes I'm not sure of the codes involved, or the proper procedures. (Maybe this means I'm NOT capable!) This pool is definitive proof that not all professionals are what they are cracked up to be. It may be safe, but none of you guys here would have ever done amything like this. This guy should be ashamed of himself. Must be the same guy that installed my neighbor's satellite dish... I am no fool and know my limits. Where all new conduit must be run to the pool (you were right...it's all in 1/2 inch conduit), I will probably just hire an electrician to run the conduit and install the sub panel and connect the equipment. I do have one other question...the chlorinator control panel we installed needs to be connected to the bonding circuit. We did that by running the bare copper lead from the box and connecting it to the bonding circuit with an approved lug. Will this suffice? Or should we have actually cut the bonding circuit and run it to the panel and then back to the other side of the cut bonding wire (and put the panel in series with the circuit)?
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A really, really, good time to ask about this stuff is before you start the work. Older code versions allow you to use a 3 wire feed from the main disconnect on the pole, but you have a 4 wire feed there you're not using. You should hook up the unused #2 and use it as the neutral and the #4 as the ground, both hooked to the same buss at the pole disconnect. Then at the shed the neutral buss should be isolated from the panel and the #2 and any neutrals attached to it and the #4 attached to a ground buss, installed extra if needed. The conduit from the shed to the cabin should have another #2 installed for a neutral (assuming aluminum here)and the #4 again used as a ground . There needs to be a ground rod or 2 at each panel (one panel per structure assumed). The neutrals and grounds need to be separated in the panels. Prior practice would allow you to use the neutral as the ground as well, and you probably could here but it doesn't make much sense as the 4th wire sounds like it is present in most of the installation. The whole idea is to get the metal housing of the panel out of the current carrying business, which it does in a 3 wire system. A neutral failure in this configuration puts line voltage on the metal housing where a 4 wire doesn't.
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You obviously meant buy a ground bar (rather than a neutral). In addition to the additional bar, you need to remove the bonding between the existing bar and the case (and obviously move the ground and neutral feeder conductors as well as the ones for the branch circuits to the right bars). In addition to Joe's questions as to whether they properly ran a ground as well as the neutral to this panel, the rest of the answer depends on the manufacture of the panel. How this is accomplished depends on the panel. Some panels already have multiple bars that are bonded together and you just remove the bond and use the one electrically isolated from the case as the neutral bar. In others, you install an additional bar as the ground and remove the bonding (often it's just a screw) to isolate the neutral bar from the case (and ground).
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I suggest you try a medical forum (perhaps one that deals with alternative facts). There's nothing we can do to help with your neuroses in an electrical forum. The PANEL is not going to be a source: 1. It's pretty well shielded. 2. It's a small radiating area. The issue is the wiring. These are almost always unshielded in most areas (type NM or the like) and they are long and provide a much bigger radiating area (especially necessary when you're talking something extremely low frequency like household current). Any electrician can tell you the coupling/radiation from a long wire is pretty easily detectible. Inside a panel (even with the cover off) not so much. So if you are really in a panic, use metal conduit or MC cabling.
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Paul A.
Original Author
5 years ago
last modified: 5 years ago
Thanks, Ron. They are separately fed from the main and I would make sure they can each handle their load.

Way to not answer the question Greg. I need to replace it anyways and am looking at options. If you're not going to contribute to the topic of discussion please don't bother replying.
greg_2015
5 years ago
Wow. Your specific question was already answered, so I just thought I'd give you something else to consider that maybe you hadn't.

If it's not helpful, then you don't have to do it. I was just trying to being helpful.
Paul A.
Original Author
5 years ago
It's not helpful at all as it didn't address my issue. It's just obnoxious and irrelevant.
weedmeister
5 years ago
Not really irrelevant. I was going to suggest if you didn't have propane, an electric POU heater is an energy hog. Depending on your water temperature, it may not heat as well as you want while your meter is whirling away (only so much temperature rise through the unit). The less expensive alternative might be to use a smaller electric tank that you turn off and drain when not there.

I had a friend with a vacation home with two full baths and laundry. She only had a 20gl tank that was only active when occupied.
Ron Natalie
5 years ago
last modified: 5 years ago
This is not a POINT OF USE heater (those have much smaller power consumptions). He's talking about the 24 or 27 KW whole house unit. Anyway POU or not, the thing is still likely a win over a tanked heater. It takes about 18KWh to take 50 gallons of water up to temp. This thing will flow 50 gallons in about 15 minutes, and only using 7 KWh to do it.

The biggest gripe with these things is making sure you have the elecrical capacity both in Amps and in pole positions on your panel to run it. Paul says he has enough...
kudzu9
5 years ago
greg-

For what it's worth, I thought your comment was entirely appropriate. When asking for advice, an OP can always choose to ignore a comment without getting huffy about it not conforming to their precise mindset.
weedmeister
5 years ago
R: sorry. I tend to use POU generically to cover all the tankless units out there. I'll be more explicit in the future.

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