Adding a circuit to back up load center post-install

[charlesj]

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@charlesj I could have wired via the 90A sub-panel, but a) that wiring path would have been very painful, and b) violating the spirit, but not the letter of electrical requirements isn't really my style.

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I would think that the 90A breaker in the sub panel would take care of any overload on that branch and in the calcs that subpanel would have been properly calculated even if it wasn't maxed out and would handle the load.
And yes, I fully understand the difficulty of rewiring a house for one additional appliance, nightmare, even for a ethernet cable.
I was asked and my answer was no so it was taken care of another way.