When sizing a ground wire for a 200-amp electrical service, the term "ground" actually refers to two entirely different conductors. The direct answer depends on which one you are installing:
- Grounding Electrode Conductor (GEC): Connects your main panel to the earth (ground rod, ufer, or metal water pipe). For a standard 200A service using 2/0 AWG copper or 4/0 AWG aluminum service entrance conductors, the minimum GEC size is 4 AWG copper or 2 AWG aluminum.
- Equipment Grounding Conductor (EGC): Runs with your circuit wires to appliances and subpanels. For a 200-amp overcurrent breaker, the minimum EGC size is 6 AWG copper or 4 AWG aluminum.
Sizing Matrix: GEC vs. EGC for 200A Services
DIYers frequently confuse the wire that goes into the dirt (GEC) with the wire that goes to your outlets (EGC). Use this decision tree to ensure you pull the correct wire from your spool.
| Conductor Type | Purpose | Copper Size | Aluminum Size | NEC Reference |
|---|---|---|---|---|
| GEC (Electrode) | Panel to earth ground rod/pipe | 4 AWG | 2 AWG | Table 250.66 |
| EGC (Equipment) | Panel to 200A feeder/subpanel | 6 AWG | 4 AWG | Table 250.122 |
Note on GEC Sizing: Table 250.66 sizes the GEC based on the physical size of your ungrounded service entrance conductors, not the breaker. A standard 200A residential service uses 2/0 AWG copper or 4/0 AWG aluminum. If your utility required you to upsize to 3/0 AWG copper for voltage drop, your GEC must also increase to 2 AWG copper.
The Hazard: What Happens When Grounding Fails
To understand why you cannot simply throw a 10 AWG wire on a 200-amp service and call it a day, you have to look at the physics of a ground fault.
The primary hazard of an undersized or missing equipment ground is chassis energization and fatal shock. If a hot wire inside your 200A subpanel or a heavy appliance (like a 240V welder) frays and touches the metal enclosure, the enclosure becomes energized at 120V or 240V.
The ground wire's sole job is to provide a low-impedance path back to the source to force the breaker to trip. Standard thermal-magnetic breakers rely on a magnetic trip mechanism for instantaneous clearing during a dead short. This magnetic trip typically requires 10 to 20 times the breaker's rated current. For a 200A breaker, you need 2,000 to 4,000 amps of fault current to trip it in under 0.1 seconds.
According to Ohm's Law ($I = V/R$), if your ground wire is too small, its resistance ($R$) is too high. If the impedance of the fault loop is just 0.5 ohms, a 240V fault will only push 480 amps. The breaker's magnetic trip will not engage. The metal chassis will sit at lethal voltage waiting for a human to complete the circuit, or the undersized ground wire will melt and start an electrical fire inside the wall cavity.
Ground vs. Neutral vs. Bond: Clearing Up the Confusion
Miswiring these three conductors is the most common cause of failed inspections and dangerous stray currents. Here is the functional distinction:
- Neutral (Grounded Conductor): The white wire. It carries the normal, everyday return current back to the transformer. It is a current-carrying conductor.
- Ground (Equipment Grounding Conductor): The bare or green wire. It carries zero current under normal operation. It only carries current during a fault.
- Bond (Main Bonding Jumper): A physical bridge (usually a green screw or a copper strap) that connects the neutral bus bar to the ground bus bar.
The Golden Rule: The neutral and ground must be bonded together at exactly one point in the system: the main service disconnect. If you bond them in a subpanel, normal return current will split and travel back to the main panel on both the neutral wire and the ground wire. This puts 120V return current on your metal conduit, appliance chassis, and coaxial cable shields, creating a severe shock and fire hazard.
How to Verify Your Grounding Path
You cannot verify a ground simply by looking at the wire. You must test the integrity of the path. Here is a numbered procedure for verifying a grounding system using standard bench and jobsite tools.
- Visual Bond Inspection: With the main breaker OFF, remove the panel cover. Verify the main bonding jumper (the green screw or strap) is installed and torqued to the manufacturer's spec (usually 40-50 in-lbs). Verify the neutral and ground bus bars are physically connected.
- Continuity Check (De-energized): Set your multimeter to continuity/resistance. Place one probe on the ground bus bar and the other on the metal water pipe or ground rod clamp outside. You should read less than 1.0 ohm. If it reads OL (open loop), your GEC is broken or disconnected.
- Neutral-to-Ground Voltage Drop (Energized): Turn the power ON. Set your multimeter to AC Volts. Go to the furthest receptacle on a 200A feeder (or a standard 15A branch circuit). Plug a high-draw load (like a hairdryer or space heater) into the bottom outlet. Measure the voltage between the Neutral slot and the Ground slot on the top outlet. A healthy ground path will read less than 2.0V. If you read 5V to 10V+ under load, your ground path has high impedance (likely a loose lug or undersized wire).
Advanced Testing: True earth ground impedance (measuring how well your ground rod dissipates lightning or utility surge energy into the dirt) requires a specialized 3-point fall-of-potential ground tester. Standard multimeters cannot measure soil resistivity.
When a Licensed Electrician is Required
While replacing a breaker or adding a branch circuit is well within the DIY scope, service entrance work crosses into heavily regulated territory. You must hire a licensed electrician and pull an AHJ permit if your project involves:
- The Utility Side: Any work on the service drop (overhead wires from the pole) or service lateral (underground wires from the transformer).
- The Meter Base: Swapping a meter socket, installing a meter-main combo, or upgrading from a 100A to a 200A meter pan.
- Service Mast and Weatherhead: Modifying the conduit that exits the roof or attaches to the fascia where the utility splices their lines.
Utilities will physically cut their tamper seal and pull the meter to allow you to work on the main panel, but they will not reconnect it without a passed inspection card from your local building department. Attempting to pull a meter yourself under load can result in a fatal arc flash.
Frequently Asked Questions
Can I use a 6 AWG ground wire for a 200 amp service entrance?
No. A 6 AWG copper wire is the minimum size for an Equipment Grounding Conductor (EGC) protecting a 200A circuit or subpanel feeder. The Grounding Electrode Conductor (GEC) that connects your main panel to the physical earth (ground rod or ufer) must be sized based on the service entrance conductors. For standard 2/0 AWG copper service wires, NEC Table 250.66 requires a minimum 4 AWG copper GEC. Using 6 AWG for the main earth connection will fail inspection.
What size ground do I need for a 200 amp subpanel?
If you are running a 200-amp feeder from your main panel to a subpanel (for a detached garage or workshop), you need a 6 AWG copper or 4 AWG aluminum Equipment Grounding Conductor. Additionally, if the subpanel is in a detached building, NEC Article 250.32 requires you to install a separate grounding electrode system (like two ground rods) at the detached building, and the neutral bus bar in the subpanel must remain completely isolated from the ground bus bar.
Does the ground wire need to be in the same conduit as the service conductors?
Yes. NEC Section 250.128 and general electromagnetic principles require the Equipment Grounding Conductor to be routed in the same raceway, cable, or trench as the circuit conductors. If you run a 6 AWG ground wire in a separate conduit away from the hot and neutral wires, the magnetic field generated by a massive short-circuit fault will induce a choking effect (high inductive reactance) in the isolated ground wire. This artificially spikes the impedance of the fault loop, preventing the 200A breaker from tripping in time.
