IMG_3400.JPG Sub Panel
You say its been that way for 10 years. Hard to believe it was installed and wired a qualified licensed electrician. I don’t think it was done by an licensed electrician.
How far is the sub panel from the main electrical service panel? Just a guess next to it or fairly close.
From the photo of the panel you provided the feeder wire from the main panel is only 3 wire. 2 black hot, (ungrounded), conductors that feed the 2 pole 100A main breaker in the panel and a bare stranded conductor for the neutral as well is being used for the equipment grounding conductor.
The sub panel should have been fed with 4 conductors.
Two hot (ungrounded conductors)
One white insulated neutral (grounded) conductor.
One equipment grounding conductor. (Note: If a steel conduit is used between the main panel and sub panel that meets NEC for use as an equipment grounding conductor.)
I assume the sub panel was install because the main panel was full. Three 120V circuits were moved from the main panel to the new sub panel. I assume, hopefully, to free up two breaker spaces in the main panel for a 2 pole breaker to feed the sub panel. Also just a guess two breakers spaces were needed for what ever the 2 pole 50 amp breaker in the sub panel is feeding.
(Would need a photo of the inside of the main panel to verify how the new panel was fed. The guy may have tapped the service entrance conductors ahead of the main breaker to feed the new panel. If that is the case then the panel would not be a sub panel.)
You might want to have a licensed electrician look at what you have and straighten it out... Make it right.
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