How can it be dangerous to feed a water heater from two separate panels?
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 1s 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 &quot;neutral&quot; line in Unit 1s 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 &quot;ABSOLUTE DRIVEL!&quot;. 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 &quot;neutral&quot; for the panel, so somebody will have to fill me in on how that should work.

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