How can it be dangerous to feed a water heater from two separate panels?

In a recent question, a user asked "Can a water heater be installed between two apartments and wired to both apartments' breaker boxes, effectively splitting the bill between the two?". One of the answers suggested that it was okay, and the poster of the answer went on to ask in a comment "How is running a hot, neutral & ground from each box to a subpanel that's circuit breakered & running a single totally code line to the water heater wrong or dangerous?".

Rather than have a discussion on comments, I thought I'd pose this question to the community. How can supplying a 240 volt electric water heater from two separate electrical panels be dangerous?

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If the goal is to split the bill why not install an electrical meter and split the bill there. – vasin1987 Jan 21 at 17:07

WARNING:

This answer describes a fictional installation, and is NOT meant to be a solution to a problem. This setup should NEVER be implemented in the field, as it is unsafe, and violates countless codes. These codes are intentionally ignored in this answer, in an attempt to keep the answer short and to the point. There's likely no way to make this setup safe, and this answer makes no attempt to do so (and neither should you). There are obviously other flaws with a circuit like this, though this answer only seeks to explain one particular flaw. Please stay safe, and don't ever try this at home.

Assuming that both panels are fed by the same 120/240V single split-phase distribution transformer (which may be a terrible assumption). The wiring would look something like this.

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That's two panels, wired to a single transformer. I've eliminated the grounding conductors, to keep the diagrams cleaner. I've also added a 120 volt load to each panel, to make the installation a bit more realistic. The whole idea of a secondary panel has also been eliminated, again just to make the diagram cleaner.

Ignoring any obvious code violations, this idea seems to work. The heater works, and everybody is happy!

At some point in the future, the tenant in `Unit 1` has to have some electrical work done. Before beginning, the Electrician turns off the main breaker in the panel.

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Luckily, The Electrician immediately notices dimly glowing lights. They grab a meter, and find that the panel is still energized. The confounded Electrician begins to investigate, charging \$150 per hour while they do so.

If you follow the circuit, you can see why the Electrician was almost electrocuted.

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Electricity flows into the building on `Leg B` (red line), and into the panel in `Unit 2`. From there it goes to the water heater, through the heating element, and over to `Leg A` (blue line) of `Unit 1`s panel. Since the main breaker is open, the electricity can't follow the normal path along `Leg A` back to the transformer. Instead it takes a detour through a lamp, and ends up on the "neutral" line in `Unit 1`s panel. From there it flows along the service neutral, all the way back to the transformer.

The panel in `Unit 1` is being backfed through the water heater, causing an unsafe, potentially deadly situation.

To make sure this answer appeases all involved, and so that it's not "ABSOLUTE DRIVEL!". I've added a secondary panel to the diagram, to show that it still has the same problem. Unfortunately, I wasn't sure how to wire up the "neutral" for the panel, so somebody will have to fill me in on how that should work.

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This is a good illustration. Many people will assume the danger is that turning off one of the panels wouldn't cut power to the water heater so working on the heater would be a danger - easily fixed with a cut off switch at the heater similar to whats required for an A/C compressor. This shows the danger is much greater than that and can back-feed power to the entire second panel. – JPhi1618 Jan 20 at 15:20
Your stated assumption of "Assuming that both panels are fed by the same 120/240V single split-phase distribution transformer." is a fairly big assumption, especially within a commercial / rental environment. There's a non-trivial chance that you'll have separate phases feeding the two panels which will create a host of other problems. – GlenH7 Jan 20 at 18:30
@GlenH7 I agree, that's why I put that right up front. – Tester101 Jan 20 at 18:48
Oh, and by the way, if either panel has a line-to-line load on it, the whole system is now energized. – ThreePhaseEel Jan 21 at 1:08
If the main panel cutoff also disconnects the neutral then the electrician won't notice the dimly glowing light. But the Blue Hot would remain back-fed and he wouldn't notice it unless he double checks with a proper tester. Worse the time that it is live would not be immediately after he shut off the main breaker but only when the thermostat in the heater is cold which would activate the heating element. – ratchet freak Jan 21 at 11:07

This would be a violation of the Code.

The panels that the conductors originate from will be ferrous metal and you will have inductive heating passing in and out of the panels. That is why the Code prohibits single conductors except under very specific circumstances.

300.3(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

I don't think you can make this qualify for any of (B)(1) through (B)(4).

Edit: I forget to add that the metering equipment would also be inductively heated and with a 3 KW to 4.5 KW element this would generate a lot of heat. So, the utility company and the inspector would never allow this.

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He didn't ask whether it'd be legal, but how it'd be unsafe. While "is this safe" and "is this to Code" are by no means unrelated questions, they're not actually the same question, nor is "is this unsafe" the same question as "how is this unsafe". Twice over, this answer fails to address the question actually asked. – Matthew Najmon Jan 21 at 4:41
On the contrary "The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity." This is a Code violation because it is dangerous. It is dangerous because it causes inductive heating. This is in addition to the point Tester makes about backfeeding. – ArchonOSX Jan 21 at 7:58
Someone writing a code claiming their intention is to make things safer does not automatically establish a perfect 1:1 correlation between being safe and being to code. Having actually read some of those codes, safety is certainly a part of it, but alterior motives are pretty blatant. However, even taking them at their word that safety was the only point of the code, my previous comment still holds up: asking "is it a Code violation" is not the same question as asking "how is it unsafe". – Matthew Najmon Jan 21 at 16:53

If your house burns down and the inspectors find this code violation, you run the risk of them using that as a reason to not pay the claim. If an electrician later works on this and electrocutes themselves because they didn't know about the unusual configuration, you may also be liable. Not worth the risk. If saving money is the objective simply buy a more efficient water heater or go solar.

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It is always wrong and unsafe to connect a single load (hw heater in this case) to power via two breakers.

As already mentioned, this has the potential for live power where unexpected.

Given multiple meters and power circuits, it would be difficult to get the two supplies in proper phase. That would be the first unexpected result with the potential for destroyed breakers, smoking wire, etc.

Secondly, this proposal has the potential of causing a fire because the available current is doubled. That problem can't be solved because the water heater must continue to work, even when power is turned off for one apartment. Hence, each supply circuit must be able to sustain the whole heater load. The water heater and local circuits will have twice the rated current available.

Thirdly, this proposal won't achieve the desired results. For the load billing to be fairly split, the proposal assumes that current will flow equally on both circuits. That is highly unlikely in the real world. First, the load from each apartment will be different based on other use of electricity such as cooking, running hair dryers, etc. That load difference, w/o the cross-connect in this discussion, would result in different voltages on each apartment's circuits.

What will happen is that the apartment with the lowest other load will suddenly start supplementing the electric supply in the higher load apartment, even when the water heater isn't heating. So the apartment with lower overall usage will pay more than their share including for the free electricity they provide to the higher usage apartment.

If this imbalance exceeds the water heater rating, then one or both breakers will trip. Otherwise, the result is likely to be two nearly identical power bills.

The other potential issue is that breakers are designed for power flow from supply to load. I don't know if the backflow I've described would be benign, damage the reverse direction breaker or perhaps not be detected as an overload.

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