The correct size of ground for a 200 amp residential service is 4 AWG copper or 2 AWG aluminum. This applies specifically to the Grounding Electrode Conductor (GEC) that connects your main service panel to the grounding electrode system (ground rods, metal water pipe, or ufer ground). If you are running a feeder to a subpanel, the Equipment Grounding Conductor (EGC) required is 6 AWG copper or 4 AWG aluminum.
Confusing these two conductors or undersizing them is one of the most dangerous mistakes in residential electrical work. A 200-amp service typically utilizes 2/0 AWG copper or 4/0 AWG aluminum ungrounded (hot) conductors. The ground wire must be proportionally sized to handle massive fault currents without vaporizing. Below is the exact hazard breakdown, the physics of why this sizing matters, and the step-by-step decision path to terminate your system safely.
The Hazard: Why Undersizing the Ground Wire is Lethal
To understand the sizing requirements, you must separate three terms that DIYers routinely conflate:
- Neutral (Grounded Conductor): The white or gray wire that carries normal, balanced return current back to the transformer during everyday operation.
- Ground (Grounding Conductor): The bare or green wire that carries current only during a fault (a short circuit to the chassis). It is a safety shield, not a normal current carrier.
- Bond: The physical connection (usually a green screw or strap) inside the main panel that ties the neutral bus bar to the ground bus bar and the panel chassis. This bond is what allows fault current to travel back to the transformer to trip the breaker.
GEC vs. EGC: Sizing the Right 'Ground' for Your Panel
The phrase 'size of ground for 200 amp service' usually refers to the main Grounding Electrode Conductor (GEC). However, if you are installing a 200-amp subpanel for a detached garage or workshop, you are actually looking for the Equipment Grounding Conductor (EGC) size. The National Electrical Code (NEC) sizes these differently because their physical jobs are different.
The GEC connects your house to the earth (dirt). The earth is a terrible conductor of electricity; its primary job is to stabilize voltage from lightning and utility surges, not to trip breakers. Therefore, the GEC is sized based on the size of your main service entrance wires (NEC Table 250.66).
The EGC connects your main panel to a subpanel. Its job is to trip the breaker during a fault by providing a low-impedance metallic path back to the main bond. Therefore, the EGC is sized based on the rating of the breaker protecting the circuit (NEC Table 250.122).
Decision Path: Which Ground Wire Do You Actually Need?
Use this decision tree to select the exact wire gauge and material for your specific 200-amp application. Do not guess; follow the path that matches your physical installation.
| Installation Scenario | Conductor Type | NEC Reference | Copper Size | Aluminum Size |
|---|---|---|---|---|
| Main panel to ground rods, metal water pipe, or ufer ground | Grounding Electrode Conductor (GEC) | Table 250.66 | 4 AWG | 2 AWG |
| Main panel to a 200A subpanel (feeder circuit) | Equipment Grounding Conductor (EGC) | Table 250.122 | 6 AWG | 4 AWG |
| Main panel to a 100A subpanel (feeder circuit) | Equipment Grounding Conductor (EGC) | Table 250.122 | 8 AWG | 6 AWG |
Step-by-Step: Terminating and Routing the 4 AWG GEC
Once you have confirmed you need the 4 AWG copper GEC for your main service, follow these physical installation steps. This assumes a standard 200A residential panel with a 2/0 AWG copper service entrance.
- De-energize and Verify: Shut off the main 200A breaker. Use a non-contact voltage tester and a calibrated multimeter to verify 0V across the hot bus bars. Warning: The utility side of the main breaker (the lugs where the meter feed enters) remains lethal and is always live.
- Prepare the Conductor: Strip exactly the length of insulation required by the panel's ground bus bar lug (usually 3/4 inch to 1 inch for 4 AWG). Do not nick the copper strands; a compromised strand reduces the wire's fault-current capacity.
- Terminate at the Panel: Insert the bare 4 AWG copper into the main ground bus bar. Tighten the setscrew using a calibrated torque screwdriver. Most modern panels (like Square D HOM200 or Eaton BR200) require between 35 and 45 inch-pounds of torque for a 4 AWG conductor. Check the label inside the panel door for the exact spec.
- Route Continuously: The GEC must be run in one continuous, unspliced length from the panel to the grounding electrode. If you must splice it, you can only use an irreversible compression connector (like a Burndy tap) or an exothermic weld (Cadweld). Standard wire nuts or crimp sleeves are illegal and dangerous here.
- Physical Protection: If the 4 AWG wire is run exposed along a surface where it could be subject to physical damage (e.g., near a driveway or walkway), it must be protected by rigid metal conduit (RMC), intermediate metal conduit (IMC), or Schedule 80 PVC. If you run it through metallic conduit, you must bond the conduit to the ground wire at both ends to prevent inductive choking during a fault.
- Terminate at the Electrode: Clamp the wire to your ground rod(s) using an acorn-style clamp listed for direct burial. If using two 8-foot ground rods (required if the first rod fails a 25-ohm resistance test), you can run the 4 AWG wire from the panel to the first rod, clamp it, and continue the same unbroken wire to the second rod.
Verification: How to Test Your Grounding Path
Visual inspection is not enough. You must verify the electrical integrity of the ground path before energizing the panel and calling for inspection.
1. The Multimeter Voltage Test (Basic Verification)
With the panel energized and under a moderate load (turn on some lights and appliances), use a digital multimeter to measure voltage:
- Hot to Neutral: Should read ~120V (acceptable range 114V - 126V).
- Hot to Ground: Should read identical to Hot to Neutral (~120V). If this reads significantly lower, your ground path has high resistance.
- Neutral to Ground: Should read very close to 0V (typically 0.5V to 2.0V under load). If this reads high (e.g., 5V+), you have a loose neutral connection or an improperly sized/loose ground bond.
2. Ground Resistance Testing (Professional Verification)
To verify the ground rod system itself, professionals use a ground resistance tester to perform a 'fall-of-potential' test or a clamp-on ground loop test. According to standard electrical safety practices documented by Fluke Corporation, the NEC mandates that a single ground rod must have a resistance to earth of 25 ohms or less. If it exceeds 25 ohms, you must drive a second ground rod at least 6 feet away. Most modern installations simply drive two rods by default to bypass the testing requirement entirely.
Code Guidance and When to Call a Licensed Electrician
The sizing and routing rules outlined above are based on OSHA electrical safety standards and the National Electrical Code (NEC). However, the NEC is a model code. Your local Authority Having Jurisdiction (AHJ)—usually your city or county electrical inspector—has the final legal authority. Always treat NEC guidelines as the baseline minimum; your local AHJ may have amendments regarding ground rod depths, water pipe bonding requirements, or conduit types.
- Breaking the utility meter seal to pull the meter and de-energize the service entrance lugs.
- Upgrading the service entrance conductors from the utility pole/transformer to the meter.
- Working on the utility side of the main disconnect.
- Modifying the main service neutral bond in a way that requires utility coordination.
For the vast majority of 200-amp residential main panels, the definitive pick is 4 AWG bare copper for the Grounding Electrode Conductor. Route it continuously, torque the lugs to the manufacturer's exact inch-pound specification, protect it with Schedule 80 PVC if exposed to physical damage, and verify your neutral-to-ground voltage is under 2 volts under load before signing off on the job.
