Mike Dolt Enterprises
Mike Dolt Enterprises
User: mikecentola, Location: Rochester NY
Asked on: Thursday, December 5, 2024 at 5:49 pm EST
Answered on: Thursday, March 6, 2025 at 5:57 am EST
Question: 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!
Okay, let's break this down and address your GEC questions for your 600A service.
Understanding the Code and Best Practices 250.24(C) Grounding Separately Derived Systems
While not exactly a separately derived system, the principle applies that only ONE point in your service is bonded. That's now at your 600A disconnect.
250.50 Grounding Electrode System
This section outlines what you need to do.
250.52 Grounding Electrodes
Describes the acceptable grounding electrodes. Your water pipe, building steel, and ground rods all fall under this.
250.53 Grounding Electrode Installation
This section covers the installation requirements for things like ground rods.
250.66 Size of Alternating-Current Grounding Electrode Conductor
This is where you find the sizing table for your GEC. It's based on the size of your service entrance conductors. For parallel 600 kcmil aluminum, you are correct that you need 2/0 copper.
250.64(F) Installation to Grounding Electrode(s)
This allows you to use a smaller conductor to connect to supplementary electrodes as long as the main GEC is sized correctly to the service entrance conductors.
Answers to Your Specific Questions1. "Does it make sense to do all three GEC to make for a good system?" Yes, with a caveat. NEC 250.50 states that all available electrodes in 250.52(A)(1) through (A)(7) shall be bonded together to form the grounding electrode system. So if a metallic water pipe and building steel are present, they need to be utilized. The ground rods are supplementary and can further improve the system. A robust grounding system provides multiple paths for fault current and helps ensure equipotential bonding.
2. "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?" No, the wire to the ground rods does NOT need to be increased to 2/0. This is a critical point. You can run a single 2/0 copper conductor from your 600A disconnect to a central grounding point. From that point, you can split off smaller conductors (e.g., #6 AWG copper) to the ground rods. This is allowed by 250.64(F). In this case, the 2/0 runs unbroken to the first electrode (water pipe or building steel), and 6awg is permitted for the ground rods. It is NOT permitted to connect to ground rods first and run a 2/0 to building steel or water pipe.
Practical Implementation Recommendations1. Run a 2/0 Copper GEC from the 600A Disconnect
This is your main GEC and must be sized according to 250.66 based on your 600 kcmil aluminum service entrance conductors.
2.
Establish a Central Grounding Point (If Needed)
If it's easier to connect everything at a single point, install a grounding busbar inside your disconnect enclosure or very close to it. This acts as a convenient connection point.3.
Connect to Building Steel
If the building steel is readily accessible and structurally significant (metal beams and a metal deck roof certainly qualify), connect the 2/0 GEC to it using an approved method (exothermic weld, listed clamp, etc.).4.
Connect to Water Pipe
Ensure you connect to the metallic water pipe within 5 feet of its entrance to the building and before any unions or insulating fittings. Use a listed clamp for grounding water pipes.5.
Connect to Ground Rods
From your grounding busbar or the main GEC (after it connects to steel and/or water pipe), run #6 AWG copper to each of your ground rods. Drive them at least 8 feet into the ground and 6 feet apart.
Important Considerations Exothermic Welds
For connections to building steel or other large metal surfaces, exothermic welds (Cadwelds) offer the most reliable and long-lasting connection. Consult with a qualified electrician experienced in exothermic welding.
Ground Clamps
Use ONLY listed ground clamps specifically designed for grounding.
Protection from Physical Damage
Protect your GEC from physical damage (e.g., running it through conduit where exposed).
Continuity
Ensure ALL connections are tight and secure. Periodic inspections are recommended.
Local Codes
Always check with your local electrical inspector for any specific requirements or interpretations in your area. They have the final say.
Professional Consultation
If you are not entirely comfortable with this process, it is always best to consult with a qualified and licensed electrician. Grounding is critical for safety, and a mistake can have serious consequences.
In summary: Run the 2/0 copper to your building steel (or water pipe), then tap off with #6 AWG to your ground rods. Using all three available electrodes creates a robust grounding system. Follow all applicable NEC requirements and local codes, and don't hesitate to consult with a professional electrician if you have any doubts. Flag for review