How to Calculate Earthing Cable Size for Home Panels and Subpanels

To calculate earthing cable size (technically called the Equipment Grounding Conductor, or EGC) for standard residential branch circuits and feeders, you match the wire gauge to the rating of the overcurrent protective device (breaker or fuse) using NEC Table 250.122. For a standard 20A branch circuit, use a 12 AWG copper EGC; for a 100A subpanel feeder, use an 8 AWG copper EGC. If you are in an IEC-governed region (UK, EU, AU) or dealing with extreme industrial fault currents, you must calculate the size using the adiabatic equation based on available short-circuit current.

The Hazard: What Happens When an Earthing Cable is Undersized?

The primary hazard of an undersized earthing cable is chassis energization and lethal shock. When a hot wire fails and contacts the metal casing of an appliance or subpanel, the EGC is supposed to carry that massive fault current back to the source, instantly tripping the breaker. If the EGC is too thin, its electrical resistance limits the fault current. The breaker sees the fault as a normal load and fails to trip. Worse, the undersized copper wire acts like a fuse, overheating and melting inside the wall while the metal chassis remains energized at 120V or 240V.

To understand why this happens, you must distinguish between three terms that DIYers frequently confuse:

• Neutral (Grounded Conductor): Carries the normal return current back to the transformer during standard operation. It is a current-carrying conductor.
• Ground (Equipment Grounding Conductor / EGC): Carries current only during a fault. It provides a low-impedance path back to the source to trip the breaker. It carries zero current during normal operation.
• Bonding: The physical act of connecting all non-current-carrying metal parts (panel boxes, conduit, appliance chassis) together and to the ground system. Bonding ensures that if a fault occurs, the metal parts don't float to a lethal voltage relative to the earth.

Without proper bonding and an adequately sized EGC, a ground fault simply turns your refrigerator or subpanel enclosure into a 120V shock hazard. According to OSHA electrical safety guidelines, improper grounding and bonding are among the most frequently cited causes of fatal workplace and residential electrocutions.

How to Calculate Earthing Cable Size: The OCPD Method

In the United States, the National Electrical Code (NEC) simplifies EGC sizing by tying it directly to the Overcurrent Protective Device (OCPD) rating. The logic is straightforward: a larger breaker allows more current to flow before tripping, so the ground wire must be thick enough to handle that maximum potential fault current without melting.

NEC Table 250.122: Minimum Size Equipment Grounding Conductors

Step-by-Step: Sizing a 60A Subpanel Feeder

1. Identify the OCPD: You are feeding a detached garage subpanel with a 60A double-pole breaker in the main panel.
2. Consult the Table: Look at the 60A row. The minimum copper EGC is 10 AWG.
3. Check for Voltage Drop Upsizing (Crucial Edge Case): If the garage is 150 feet away, you must upsize your hot and neutral conductors from 6 AWG to 4 AWG to prevent voltage drop. Under NEC 250.122(B), if you upsize the ungrounded (hot) conductors for voltage drop, you must proportionally upsize the EGC. Since 4 AWG is two sizes larger than 6 AWG, your 10 AWG ground must also increase by two sizes, making the required EGC 6 AWG copper.
4. Pull the Wire: Run two 4 AWG hots, one 4 AWG neutral, and one 6 AWG copper EGC in your conduit.

The Adiabatic Equation: Sizing for High Fault Currents (IEC/Industrial)

If you are designing a system under IEC standards (like BS 7671 in the UK), or if you are working on a US industrial service with massive available fault currents (e.g., 40,000 Amps at the main switchgear), table lookups are insufficient. The magnetic forces and thermal stress of a high-magnitude short circuit can vaporize a standard table-sized EGC in milliseconds, before the breaker's mechanical parts can even begin to open.

Engineers use the adiabatic equation to calculate the exact cross-sectional area required to survive the thermal stress of a fault. The Institution of Engineering and Technology (IET) outlines this calculation extensively for compliance with modern wiring regulations.

The formula is: S = √(I² × t) / k

• S = Minimum cross-sectional area of the earthing conductor (in mm²)
• I = Prospective fault current (in Amps)
• t = Disconnection time of the protective device (in seconds)
• k = A constant based on the conductor material and insulation type (typically 115 for copper conductors with PVC insulation, or 143 for bare copper).

Worked Numeric Example

Imagine a commercial 3-phase panel where the available fault current (I) is measured at 8,000A. The upstream molded case circuit breaker has a let-through trip time (t) of 0.05 seconds at that current. You are using copper wire with THHN/PVC insulation (k = 115).

1. Square the current: 8,000² = 64,000,000
2. Multiply by time: 64,000,000 × 0.05 = 3,200,000
3. Take the square root: √3,200,000 ≈ 1,788.8
4. Divide by k: 1,788.8 / 115 = 15.55 mm²

The calculated minimum size is 15.55 mm². Since wire is manufactured in standard metric sizes, you must round up to the next available standard size, which is 16 mm² (roughly equivalent to 5 AWG in the US system). If you had blindly used a standard table meant for a 60A breaker, you might have installed a 10 mm² wire, which would violently fail during a fault.

Verification, Testing, and When to Call a Pro

Calculating the size is only half the battle; verifying the physical installation is what actually keeps you safe. A perfectly sized 6 AWG ground wire is useless if the lug is loose or corroded.

How to Verify the Ground Path

Do not rely on a $10 neon receptacle tester to verify a subpanel feeder or heavy appliance ground. You need quantitative data.

• Continuity Test (De-energized): With the breaker OFF and locked out, use a digital multimeter on the lowest ohms setting. Place one probe on the appliance chassis or subpanel ground bar, and the other on the main panel's ground bar. A healthy, properly sized, and well-bonded EGC will read less than 1.0 ohm (typically 0.1 to 0.3 ohms). If it reads OL (open) or greater than 5 ohms, you have a broken wire or a loose, corroded mechanical connection.
• Ground Impedance Test (Energized): For advanced verification, use a dedicated ground impedance tester. This device injects a small current through the hot and ground wires to measure the actual impedance of the fault loop without tripping the breaker. A high impedance reading indicates a poor bond somewhere in the chain.

When a Licensed Electrician is Required

While running a ground wire to a new outlet or shed subpanel is a common DIY task, you must hire a licensed electrician for the following scenarios:

• Service Entrance Work: Sizing and installing the main Grounding Electrode Conductor (GEC) that connects your main panel to the physical earth (ground rods or ufer ground). This involves the utility side of the meter and carries extreme arc-flash risks.
• Main Bonding Jumper Installation: The main bonding jumper connects the neutral bar to the ground bar in the main service panel only. If this is missing, undersized, or improperly installed in a subpanel (where neutrals and grounds must remain strictly separated), it creates a parallel neutral path that can energize your plumbing and gas lines.
• Upgrading Service Capacity: Moving from a 100A to a 200A main service requires recalculating the main GEC size, upgrading the meter base, and coordinating with the local utility provider.