For a standard 200-amp residential service using 2/0 AWG copper service entrance conductors, the required Grounding Electrode Conductor (GEC) to your ground rods is 4 AWG copper. If running a 200-amp feeder to a subpanel, the Equipment Grounding Conductor (EGC) must be 6 AWG copper.
- Material: Copper ( Aluminum alternatives noted in FAQ)
- Temperature Column: 75°C (Standard for modern terminals and THHN/THWN-2 wire)
- Ambient Temperature: 30°C (86°F) baseline; no high-heat derating applied
- Conduit Type: PVC Schedule 80 or standard EMT; no more than 3 current-carrying conductors bundled
- Code Reference: NEC-style guidance (2020/2023 cycles); your local AHJ has final authority
The Two 'Ground' Wires: GEC vs. EGC
When DIYers and even some journeymen ask 'what size ground wire do I need,' they are usually conflating two entirely different conductors governed by different NEC tables. To size correctly, you must identify which wire you are actually pulling.
| Feature | Grounding Electrode Conductor (GEC) | Equipment Grounding Conductor (EGC) |
|---|---|---|
| Purpose | Connects panel ground bus to the earth (ground rods, Ufer, water pipe). | Runs with circuit conductors to subpanels or appliances to clear internal faults. |
| NEC Table | Table 250.66 (Based on largest ungrounded service conductor) | Table 250.122 (Based on overcurrent device rating) |
| 200A Size (Cu) | 4 AWG (Assuming 2/0 Cu service wires) | 6 AWG (Based on 200A breaker) |
| Normal Current | Zero (only carries lightning/surge energy) | Zero (only carries fault current during a short) |
If you are hooking up the main service panel to the ground rods outside, you are installing a GEC. If you are feeding a detached garage or a 200-amp subpanel inside the house, you are pulling an EGC. According to the National Fire Protection Association (NFPA 70), mixing these up leads to either wasted money on oversized wire or a dangerous code violation on undersized wire.
Why These Sizes? (And Why Not One Smaller)
It is tempting to look at a 6 AWG EGC and think it looks too small to handle a 200-amp fault. Why not just run 2 AWG for peace of mind? Conversely, why not use 8 AWG to save money?
The sizing in NEC Table 250.122 is based on thermal withstand, not continuous ampacity. A ground wire does not carry load current; it only carries current during a dead short (e.g., a hot wire touches the metal casing of a dryer). During a bolted fault, current spikes to thousands of amps. The EGC must remain physically intact long enough for the magnetic trip inside the 200-amp breaker to clear the fault—typically within 1 to 3 cycles (16 to 50 milliseconds).
If you install an 8 AWG EGC on a 200A breaker, the available fault current can literally vaporize the copper wire before the breaker's mechanical parts have time to physically separate the contacts. This leaves the appliance chassis energized at 120V/240V with no path to trip the breaker, creating a lethal shock hazard. Always follow the minimums in Table 250.122.
What Changes the Answer: Length, Bundling, and Voltage Drop
The baseline sizes (4 AWG GEC / 6 AWG EGC) assume a standard installation. However, real-world jobsite conditions frequently force you to upsize. Here is the decision matrix for when the baseline changes.
| Condition | Impact on GEC (To Earth) | Impact on EGC (To Subpanel) |
|---|---|---|
| Long Distance (Voltage Drop) | No change. Length does not affect GEC sizing. | Must upsize proportionally if hot wires are upsized for VD (NEC 250.122(B)). |
| Conduit Bundling (>3 wires) | No change. | Must upsize proportionally if hot wires are upsized to compensate for ampacity derating. |
| Switching to Aluminum | Changes to 2 AWG Aluminum. | Changes to 4 AWG Aluminum. |
The Voltage Drop Upsizing Math (150 ft Feeder Example)
Suppose you are running a 200-amp feeder to a detached garage 150 feet away. You are using 2/0 AWG copper for the hot legs. At 200 amps, a 150-foot run of 2/0 copper yields a voltage drop of roughly 3.8% (using the standard 12.9 ohms-cmil/ft formula). The NEC recommends keeping feeder voltage drop under 3% for reasonable efficiency.
To fix this, you upsize your ungrounded (hot) conductors from 2/0 AWG to 3/0 AWG. Here is where most DIYers fail: NEC 250.122(B) states that if you upsize ungrounded conductors to compensate for voltage drop, you must proportionally upsize the EGC based on the circular mil area ratio.
- Calculate the ratio: 3/0 AWG (167,800 cmil) / 2/0 AWG (133,100 cmil) = 1.26 ratio.
- Apply to baseline EGC: 6 AWG baseline (26,240 cmil) × 1.26 = 33,062 cmil required.
- Select new EGC: 4 AWG copper is 41,740 cmil, which safely exceeds the 33,062 cmil requirement.
Therefore, your 150-foot 200A feeder requires 3/0 AWG copper hots and a 4 AWG copper EGC, not the standard 6 AWG.
When an Engineer or AHJ Must Confirm
While the NEC tables cover 95% of residential and light commercial work, edge cases require professional stamping or explicit Authority Having Jurisdiction (AHJ) approval. You must pull an engineer into the loop if:
- Available Fault Current Exceeds 10kA: Standard residential breakers are rated for 10,000 Amps Interrupting Capacity (AIC). If your utility transformer is pad-mounted right outside your house, the available fault current might be 22kA or 42kA. You will need higher AIC breakers, and the thermal stress on the EGC may require upsizing beyond Table 250.122 to prevent busbar melting.
- Parallel Conductor Runs: For services over 400A (or 200A runs with massive voltage drop mitigated via parallel feeds), NEC 250.122(C) requires an EGC in each parallel raceway, sized based on the overcurrent device, not divided among the pipes.
- Ufer Grounds with High Soil Resistivity: If you are using a concrete-encased electrode (Ufer) in extremely dry or rocky soil, an engineer may need to calculate the required ground ring supplement to achieve the NEC-mandated 25-ohm threshold.
Frequently Asked Questions
Can I use aluminum instead of copper for my 200-amp ground wire?
Yes, but you must follow the aluminum column in the NEC tables. For a 200-amp service with 4/0 AWG aluminum service entrance wires, your GEC (to the ground rod) must be 2 AWG aluminum. For a 200-amp feeder EGC, you must use 4 AWG aluminum. When terminating aluminum ground wires to a copper ground bus or brass ground rod clamp, you must use an anti-oxidant compound (like Noalox) and ensure the lugs are explicitly rated (AL/CU) for aluminum to prevent galvanic corrosion and high-resistance failures over time.
Does the exterior ground wire need to be in a conduit?
NEC 250.64(B) requires the GEC to be protected from physical damage. If the 4 AWG copper wire is run down the exterior of a house where it could be struck by a weed whacker, lawn equipment, or animals, it must be protected. You can run it inside PVC, EMT, or rigid conduit. However, if you use metallic conduit (like EMT) to protect the GEC, the conduit itself acts as an inductor. During a lightning surge, the magnetic field will choke the current flow unless the metallic conduit is electrically bonded to the GEC at both the top and bottom ends. To avoid this 'choke effect' headache, most electricians simply use PVC conduit or schedule 80 PVC for the physical protection of the GEC.
What if I have a 200-amp panel but only a 150-amp main breaker?
This is a common scenario when a homeowner upgrades their panelboard to 200-amp spaces but the utility service drop or the main breaker itself is limited to 150 amps. The sizing rules split here: The GEC (to the earth) is still sized based on the physical service entrance wires coming from the meter. If those are 2/0 Cu, the GEC remains 4 AWG Cu. However, the EGC (if this panel is actually acting as a subpanel fed from an outside main disconnect) is sized based on the overcurrent device protecting it. A 150A breaker requires a 6 AWG Cu EGC per Table 250.122. (Note: If the 150A breaker is inside the main service panel, there is no EGC, only the GEC to earth and the bonding jumper).
