electrical
I currently have a 125A sub panel in my steel shop ran from my house 200A service panel. Existing shop sub panel is grounded from house panel to isolated ground bus and also is grounded to shop ground rod and bonded to shop frame.
Going to be running #8 to new panel 40 ft away for a welder and convenience outlets. New panel is bonded to building frame so is it required to run a grounding conductor to new sub and is a ground rod required?
I think you may be over-thinking this. You can make a connection on either bus without reaching across an ungrounded bus in that panel.
The ground and neutral bars are so located as to give you free access to them without have to reach around or near an energized bus. You could stand to the right or left to make your connections without reaching across the phase bus.
In a larger piece of switchgear, the phase conductor bus should be located further in and the ground and neutral bars closer to the access panel not the other way around.
I think that is the intent of the code.
Here's a diagram covering the grounding possibilities. There are new clarifications about lighting protection and grounding in the NEC 2017 code. By the way NEC 2017 becomes law in this state today.
Hope this helps. Good luck.
Best Answer
I'm assuming this new panel is in the same outbuilding shop as the 125A panel.
Any sub panels fed from the "main" panel for a structure (in this case the existing 125A panel is the main panel for your shop) should be grounded with a dedicated grounding conductor in addition to the hot and neutral conductors. So yes, you must run a ground wire to the new panel.
Note that connecting your panels to other metallic things like the metal building (or copper pipes, etc.) does NOT count as grounding. From a safety standpoint they are irrelevant. Your metal building is probably grounded to your existing panel/ground, not the other way around.