Sub panel bonding

Kopper said:
"because this might happen"
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The code only exists because this and/or that have happened. Of course there's going to be the lobby argument (possibly valid) but the reality is that code has been developed in response to problems. No electrical system is absolutely safe, the code is designed to make the system as safe as possible. That means designing out as many "what-ifs" as possible.
 
Joe.B said:
The code only exists because this and/or that have happened. Of course there's going to be the lobby argument (possibly valid) but the reality is that code has been developed in response to problems. No electrical system is absolutely safe, the code is designed to make the system as safe as possible. That means designing out as many "what-ifs" as possible.
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I understand and appreciate? that. It is FAR better and much more noble an ambition to try to understand what makes things work and why they work the way that they do and to design and prescribe accordingly - a proactive approach instead of a reactionary approach. I certainly hope and expect that is the approach that the code system takes. It is certainly the approach we try to take at THE Kopper Konnection LLC. Learning from mistakes is good but to be armed with a good understanding of what and why can help to eliminate many issues before they are issues. I do understand that sometimes trial and error uncovers things that we just miss and maybe couldn't have planned for but proactive is generally (maybe always?) superior to reactive!
 
There have been several good responses here that explain why. The system you provided as an example may have been code compliant at the time it was installed, and clearly has functioned just fine. If you were to bring it up to current code, it would be safer. That's the simple answer. How much safer? I don't know, but it would definitely be safer. The best way to explain that to most lay people is with an analogy. Try this one.

That old 70's pickup you've been driving around since high school has been working just fine, and probably will keep doing so as long as you keep up the maintenance. There's a chance that you might die in a wreck, but probably not. A new truck has a lot of safety features built into it that would make it safer. If you can afford to buy a new truck I strongly recommend it, you're less likely to die in a wreck in that new truck. How much less likely to die? That depends.
 
Think of this aspect: a subpanel fed by 3 wires is using the neutral as the equipment grounding conductor. Since it can carry current under normal conditions, it will have voltage drop. This means there will be a voltage gradient between the enclosures of the main panel and subpanel. That implies that the EGCs of equipment fed from the 2 different panels can have a voltage gradient between them. The level of this gradient depends on the resistance of the feeder neutral and its current (I^2R). A person in contact with 2 pieces of equipment could feel a shock if each is fed from a different panel. Whether this shock is of any consequence depends on many factors, the main one being the neutral current on the feeder. It could be substantial during a fault condition.

You may consider this just another rabbit hole, but it describes a reason the requirement was added to the NEC.
 
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