When wiring a 100-amp service or subpanel feeder, the minimum ground wire size is 8 AWG copper or 6 AWG aluminum. However, DIYers and even junior apprentices frequently confuse the Grounding Electrode Conductor (GEC) at the main service disconnect with the Equipment Grounding Conductor (EGC) running to a subpanel. While both default to 8 AWG copper for a standard 100-amp setup, the National Electrical Code (NEC) applies entirely different sizing rules—and upsizing traps—to each.

The Decision Path: Sizing Your 100-Amp Ground Wire

To pick the correct wire, you must first identify which 'ground' you are actually installing. Use this decision tree to terminate your search with a concrete pick.

Scenario Wire Function NEC Reference Minimum Copper Size Minimum Aluminum Size
Main Service Entrance (to ground rods/water pipe) Grounding Electrode Conductor (GEC) NEC Table 250.66 8 AWG 6 AWG
Standard 100A Subpanel Feeder (under 100 ft) Equipment Grounding Conductor (EGC) NEC Table 250.122 8 AWG 6 AWG
100A Subpanel Feeder (upsized for voltage drop) Equipment Grounding Conductor (EGC) NEC 250.122(B) Must upsize proportionally (see below) Must upsize proportionally
Main Service Neutral-to-Ground Bonding Jumper Main Bonding Jumper (MBJ) NEC Table 250.24(C)(1) 8 AWG 6 AWG
Bench Tip: For the EGC running inside a conduit with your THHN/THWN-2 hot and neutral wires, you can use a bare 8 AWG copper wire or an insulated green 8 AWG wire. For the GEC running from the panel to an exterior ground rod, the NEC requires it to be protected if exposed; many electricians use bare copper but run it through PVC conduit where it travels down a wall.

The Hazard: Why Undersized Ground Wires Cause Fatal Shocks

The ground wire does not carry current during normal operation. Its sole purpose is to provide a low-impedance path for fault current. If a hot wire shorts to a metal tool chassis or a subpanel enclosure, the ground wire must carry enough current to instantly trip the breaker's magnetic mechanism.

A standard 100-amp thermal-magnetic breaker requires roughly 500 to 1000 amps of instantaneous fault current to trip the magnetic latch in under 0.1 seconds. If you undersize the ground wire (for example, mistakenly pulling 12 AWG or 14 AWG), the wire's resistance limits the fault current. Instead of 500 amps, only 40 amps might flow. This is enough to melt the 12 AWG ground wire into a glowing fuse, but not enough to trip the 100A breaker. The metal enclosure remains energized at 120V or 240V, creating a lethal shock hazard for the next person who touches it while grounded.

Safety Warning: Never rely on the earth (ground rods) to clear a line-to-ground fault. The earth has too much resistance. The fault current must travel back to the source (the transformer) via the equipment grounding conductor to trip the breaker.

Ground vs. Neutral vs. Bond: Clearing the Terminology

Misunderstanding these three terms is the root cause of 90% of residential wiring failures. Here is the exact functional distinction:

  • Neutral (Grounded Conductor): The white or gray wire. It is a current-carrying conductor that provides the normal return path for 120V circuits back to the transformer.
  • Ground (Equipment Grounding Conductor / EGC): The bare or green wire. It is a non-current-carrying safety path that only sees current during a fault. It connects all metal enclosures, tool chassis, and device straps together.
  • Bond (Main Bonding Jumper): The physical connection (usually a green screw or a metal strap) that ties the Neutral bus bar to the Ground bus bar. This connection is made at exactly one location: the main service disconnect. In a subpanel, the neutral and ground buses must remain strictly isolated.

If you bond neutral and ground in a subpanel, normal neutral return current will split and travel back on the bare ground wire. This energizes all metal enclosures downstream and can cause neutral currents to flow through plumbing or gas lines, creating a severe fire and shock risk.

The Voltage Drop Trap: Proportional Upsizing (NEC 250.122)

This is where even experienced DIYers fail inspections. If your 100-amp subpanel is located 150 feet away, standard 3 AWG copper hot wires will suffer excessive voltage drop. To fix this, you upsize the hot wires to 1 AWG or 1/0 AWG copper.

Under NEC Article 250.122(B), if you increase the size of the ungrounded (hot) conductors to compensate for voltage drop, you must increase the equipment grounding conductor proportionally.

Worked Example: Upsizing the Ground

  1. Base Hot Wire: 3 AWG Copper (Circular Mil Area = 52,620)
  2. Base Ground Wire: 8 AWG Copper (Circular Mil Area = 16,510)
  3. Upsized Hot Wire: 1 AWG Copper (Circular Mil Area = 83,690)
  4. Ratio of Increase: 83,690 / 52,620 = 1.59x
  5. Required Ground Area: 16,510 x 1.59 = 26,250 Circular Mils
  6. New Ground Wire Size: 6 AWG Copper (41,740 CM) or 4 AWG Aluminum (41,740 CM)
Shortcut Rule: If you upsize your hot wires by two AWG steps (e.g., 3 AWG to 1 AWG), upsize your ground wire by two AWG steps (e.g., 8 AWG to 6 AWG). Always verify with the exact circular mil math for compliance.

How to Verify Your Grounding Path with a Tester

Once the panel is wired, you must verify the integrity of the grounding path before energizing the system. Relying on a simple outlet tester at a receptacle is insufficient for verifying the main service or feeder ground.

Step 1: De-Energized Continuity Test (Main Bonding Jumper)

With the main breaker OFF and the utility feed verified dead, set your multimeter (like a Fluke 117) to the lowest Ohms setting. Place one probe on the neutral bus bar and the other on the ground bus bar in the main panel. You should read less than 1.0 ohm (ideally < 0.5 ohms), confirming the main bonding jumper is intact.

Step 2: De-Energized Continuity Test (EGC Path)

At the subpanel (with the feeder breaker OFF), measure resistance between the subpanel's isolated ground bus bar and the main panel's ground bus bar. A solid 8 AWG copper run over 100 feet should read roughly 0.06 ohms. If you read 'OL' (Open Line), your ground wire is broken or disconnected.

Step 3: Energized Ground Impedance Testing (Professional)

For advanced verification of the actual Grounding Electrode system (the rods in the earth), professionals use a ground impedance tester like the Fluke 1625-2 GEO Earth Ground Tester. This device uses the fall-of-potential method or stakeless clamp method to measure the actual resistance of the earth connection, ensuring it meets the NEC requirement of 25 ohms or less for a single rod (or the use of two rods if it exceeds 25 ohms).

Code Authority and When to Call a Licensed Electrician

The sizing rules outlined above reflect standard NEC-style guidance (specifically NFPA 70, Articles 250.66 and 250.122). However, the NEC is a model code. Your local Authority Having Jurisdiction (AHJ)—usually the city or county electrical inspector—has the final legal authority and may have local amendments regarding aluminum wire restrictions, conduit fill limits, or specific grounding electrode requirements.

You must hire a licensed electrician and pull a permit when:

  • You are installing or modifying the Service Entrance Conductors (the wires coming from the utility meter to the main panel).
  • You are upgrading the main service disconnect or replacing the meter base.
  • You are installing a new Grounding Electrode System (driving new rods, tapping into the municipal water pipe, or installing a Ufer ground in a new concrete footing).

Running a subpanel feeder (EGC included) from an existing main panel is a common DIY task, provided you understand the neutral/ground isolation rules and properly torque all lugs to the manufacturer's specifications using a calibrated inch-pound torque screwdriver. If your local AHJ requires a licensed contractor for any work inside the main panel dead-front, defer to their rules to avoid failed inspections and voided insurance coverage.