Sizing GEC wire with multiple electrodes

mikecentola

mikecentola

Member
Location
Rochester NY
Occupation
Engineer
Hey all! First time posting as I've found tons of great information reading over the years. I have a question regarding the GEC for a 600A system for my building. Long story short, I needed to move my weatherheads to accommodate gutters, which involved POCO, which meant moving/relocating a CT cabinet, which turned into running all new wiring underground, new CT cabinet outside, adding a main disconnect (previously CT inside ran right into 600A MDP), Anyways, previous wiring was parallel 350mcm wire (560A capacity), so I'm running new Aluminum parallel 600mcm for cost and 616A capacity. Onto my actual question... I have confirmed with POCO that the CT cabinet gets its own ground rod, bonded to neutral and cabinet, and they run that ground into the new meter can.

From the CT cabinet, I have 4" PVC through two sets of Mogul LB's into my 600A fusible disco. Since the disconnect becomes my new service entrance, I know that neutral and ground gets bonded there and nowhere after that. Previously the only building's GEC was from the old CT to the water pipe. I have two 5/8" copper ground rods I was going to drive 6' apart either outside right on the other side of the wall or inside through the slab. I understand that wire doesn't need to be any bigger than 6AWG and some people typically just do 4AWG 7 strand bare. I also have the option to run to building steel (metal beams, metal deck roof, CMU walls) and home run up to the water supply pre or post meter. Per 250.66, the GEC to either of those would be 2/0 copper and land all back at the disco.

Does it make sense to do all three GEC to make for a good system? I don't have the option to do a Ufer. Second, if I do add the 2/0 to the steel and water pipe, does the wire to the ground rods have to be increased to 2/0 as well or does that stay 4 or 6AWG? I'll edit and attach a couple pics to help it make sense once I can.

Thanks in advance!
 
Does the building steel qualify as an electrode? If so then I would forget about the ground rods. Is there a water pipe electrode?
 
infinity said:
Does the building steel qualify as an electrode? If so then I would forget about the ground rods. Is there a water pipe electrode?
Click to expand...

I don’t think building steel would qualify as one by itself only because it doesn’t have vertical steel beams. They just sit on the CMU walls. Technically, they would be part of it since much of the conduit is hung from it I guess.

I do have water pipe that would qualify and would be about a 120’ run from disco to where the meter is.

I know my Haas CNC machines install docs sometimes call for ground rods at the machines but I think that’s for EMI or lightning/surge protection.
 
You only need a #6 to the rods regardless of what other electrodes are present (unless you also connect to other electrodes with that conductor).
Unfortunately if the water line qualifies as an electrode you must also connect to it within 5' of the pipes entry to the building.

If you steel is just trusses supporting the roof with no framework you don't need to address it.
 
augie47 said:
You only need a #6 to the rods regardless of what other electrodes are present (unless you also connect to other electrodes with that conductor).
Unfortunately if the water line qualifies as an electrode you must also connect to it within 5' of the pipes entry to the building.

If you steel is just trusses supporting the roof with no framework you don't need to address it.
Click to expand...

Ok that makes sense. The water line does so I’ll take that into account. Previously, they attached the GEC about 50’ away from entrance but I’m assuming it passed because of the exception in exposed copper line.
Regardless, I’ll do the home run to there with 2/0 THHN and make a decision about adding the ground rods or not.

One other question… I know the copper run to the rods is typically bare copper. Can I use something direct burial wire? It just weirds me out having bare copper inside the CT and disconnect with all the big mechanical lugs there. I guess I could zip tie it down.
 
To answer the short question about increasing the size for the rods, the answer is it depends on where you make the connections for the grounding electrode system. You can use #6 until they connect to the water pipe or the water pipe's grounding electrode conductor. You are not allowed to reduce the size for the water pipe to #6 (unless that makes sense using the table). They might have suggested #4 because of 250.64(B)(1) and it not needing to be protected by conduit where not exposed to damage.

My general rule of thumb is to always reconnect the old GEC system when installing a replacement service.

You should, if it is in an electrical room, install a ground bar and just connect all of that there. Then one from the bar to the preferred point to ground the service neutral.

Bonding building steel is 250.104. You should bond it if it is likely to become energized. You can bond that to the copper ground bar.

I didn't see the voltage but a ground bar can also make it easier for grounding dry types that are installed in the electrical room or near the electrical room.

Something like this:

1733517622764.png



P.S. I tried to just send the link from Grainger but the post would corrupt the link and made smilie faces in the link.
 
Elect117 said:
To answer the short question about increasing the size for the rods, the answer is it depends on where you make the connections for the grounding electrode system. You can use #6 until they connect to the water pipe or the water pipe's grounding electrode conductor. You are not allowed to reduce the size for the water pipe to #6 (unless that makes sense using the table). They might have suggested #4 because of 250.64(B)(1) and it not needing to be protected by conduit where not exposed to damage.

My general rule of thumb is to always reconnect the old GEC system when installing a replacement service.

You should, if it is in an electrical room, install a ground bar and just connect all of that there. Then one from the bar to the preferred point to ground the service neutral.

Bonding building steel is 250.104. You should bond it if it is likely to become energized. You can bond that to the copper ground bar.

I didn't see the voltage but a ground bar can also make it easier for grounding dry types that are installed in the electrical room or near the electrical room.

Something like this:

View attachment 2574671



P.S. I tried to just send the link from Grainger but the post would corrupt the link and made smilie faces in the link.
Click to expand...

That's a good idea. The MDP and disconnect are not in an enclosed room, but instead in the corner of our 4500 sq ft machineshop. Voltage is 240/120 4 wire delta with a wild leg. I like the idea of bringing everything to one of the copper ground bars, and I suppose I could do that inside of its own small enclosure next to the disconnect. I was actually planning on getting one of those style ground bars for my little server closet, where our network rack, alarm/fire panels, and BMS stuff are for grounding the shielded ethernet and all the equipment.
 
Hey guys I’m struggling a little with the best way to land some of these grounds and the MBJ. Here’s a pic with some lines I drew on it… does this seem the best way to do thi?

Color Legend:
Green - 2/0 GEC wire to busbar
Dark Green - 2/0 EGC wires from MDP
Fluorescent Green/Yellow - 2/0 wire from neutral bar through the ground bushings to mechanical lug on cabinet back
Orange - 4AWG from busbar to outside copper ground rods
Purple - 2/0 to roof trusses
Blue - 2/0 to copper water pipe

I think this doesn’t violate any codes and I think I remember a post by @infinity thar had a pic of using a ground busbar like this, but I’m just wondering if this is the best way to do it. Thanks!

IMG_6353.jpeg
 
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