detached building subpanel - ground rod

10-4, exactly what I’m planning to do unless someone gives me a good reason not to.

quote:

---

I'm adding a 100 amp subpanel to a detached building to supply power for lights (1 15 amp circuit) and outlets (2 20 amp circuits). I'll be adding a double pole 60 amp breaker from my exterior combo meter box on the house and running 2224 aluminum wire to the subpanel. The subpanel will have a ground wire connecting subpanel and main panel ground bars. It seems that code requires two new ground rods separated by 6' at the detached building. I'm exempt from following code or inspections as it is a barn with the code exemption filed. I have wired the detached building using MC wire/conduit and all metal boxes are screwed directly to metal building frame - outlets have ground wires connected to the green outlet screw and outlets have firm contact to metal boxes. The subpanel will be mounted on wood, but I was planning to ensure good ground connections by screwing a ground wire into the metal building frame and linking to the subpanel ground bar. Here is my question - is it a safety risk to not install a new ground rod(s) at the subpanel. I have read that it is for lightning to have a quick ground pathway at the detached building instead of having to travel longer to the main panel ground rod. Sorry for the long post but wanted to provide context. Thanks in advance for any insight.

---

A grounding electrode system (GES) is required and all neutrals (grounded conductors) and equipment grounding conductors (EGC) should be isolated from one another inside the subpanel. The GES is connected to the EGC. I am not certain but I am not sure it has to be ground rods though.....thats the easiest and most cost effective way to do it but I think it can be accomplished by grounding the wire in the slab if there is one. I may be wrong about that.

You do not want to connect grounded conductors (neutrals) and equipment grounding conductors (bonds) anywhere except a common point at the main. Doing so will create a parallel path between the bond and the neutral. The neutral is a current carrying conductor - it is the path back to the source for current. The bond is there to carry any fault current to ground (current in an unwanted location) and to clear direct shorts to ground. Tying them together at any point other than the common point at the main means the bond would be a current carrying conductor during normal operation and there would not be a true path to ground for fault current. IF the bonds and neutrals are connected anywhere in the system beyond the common point at the main and a separate grounding electrode is connected there will a potential difference between the 2 points and that charge will be introduced into the system causing all manner of ill shite.

The system required at the detached structure is indeed meant to provide a path to ground for lightning. It is not a bond...or equipment grounding conductor. That would be the 4th wire in the cable ran from the main to the sub panel.

I installed new ground rod and ufer ground in my metal building. I did NOT bond the sub panel (ie neutral to ground), but I did bond the building frame itself to the ground.

quote:

---

Another question - if I install ground rod to subpanel ground bar would y’all still recommend grounding box to metal building frame via copper wire? On separated ground bar - ground rod, 4th wire connecting main feeder and subpanel, ground wire connecting building frame to ground bar, then grounds from all circuits?

---

Generally speaking all metallic objects not intended to be current conducting in a given structure be bonded together to prevent a potential difference between conductive properties. In the case of building steel it is certainly required. If the metallic can of the subpanel is bolted to the steel and is bonded that should be sufficient but there is nothing wrong with going beyond that. If the can is screwed to the steel with self tapping screws it certainly needs to be bonded with a ground clamp or even cad welded.