The size of the ground wire for a 200-amp electrical service is a specific requirement governed by the National Electrical Code (NEC) to ensure the safety and proper function of the electrical system. This conductor, known as the Grounding Electrode Conductor (GEC), provides a path to the earth for high-voltage events like lightning strikes or line surges. The correct sizing is determined by the size of the main power conductors supplying the service, not the 200-amp rating alone. Understanding this relationship and the context of the code is necessary for a compliant and safe installation.
Grounding vs. Bonding: Clarifying the Concepts
The terms grounding and bonding are often used interchangeably, but they serve two distinct purposes in an electrical system. Grounding is the connection of the electrical system to the earth, typically through a grounding electrode system. This connection stabilizes the system’s voltage relative to the earth during normal operation. It also provides a path to limit the voltage imposed by external events, such as lightning or accidental contact with higher-voltage lines.
Bonding is the permanent joining of all non-current-carrying metallic parts to establish electrical continuity. This includes metal enclosures, raceways, and piping systems. The primary function of bonding is to ensure that all these metal components are at the same electrical potential, preventing dangerous voltage differences from developing. This creates a low-impedance path, necessary to quickly trip a circuit breaker or fuse during a fault condition.
The Specific Answer: Grounding Electrode Conductor Size
The size of the Grounding Electrode Conductor (GEC) for a 200-amp service is determined by the size of the largest ungrounded Service Entrance Conductors (SECs). The NEC provides Table 250.66 to correlate the size of the SECs with the required GEC size. A typical 200-amp residential service uses either 2/0 AWG copper or 4/0 AWG aluminum for the SECs.
For a service utilizing 2/0 AWG copper SECs, the GEC must be at least #4 AWG copper. If the service uses 4/0 AWG aluminum SECs, the GEC must be at least #2 AWG aluminum.
The GEC is not intended to carry continuous current, but rather to handle transient events like surges. The code specifies that the GEC is never required to be larger than the SECs themselves. This sizing ensures the conductor can safely handle the energy associated with high-voltage events.
Grounding Electrode Types and Size Exceptions
The required size of the Grounding Electrode Conductor can be affected by the type of grounding electrode it connects to. The NEC recognizes several types of electrodes, including ground rods, metal underground water pipes, and concrete-encased electrodes. The full size determined by the SECs must be used when connecting to a metal underground water pipe or building steel.
Rod, Pipe, or Plate Electrodes
If the GEC connects only to a rod, pipe, or plate electrode, the conductor is not required to be larger than #6 AWG copper or #4 AWG aluminum. This exception is common in residential installations where ground rods are the primary electrode.
Concrete-Encased Electrodes
An exception applies to concrete-encased electrodes, which are highly effective due to their large surface area. The portion of the GEC connecting solely to a concrete-encased electrode is not required to be larger than #4 AWG copper.
If the GEC connects to multiple types of electrodes, such as a ground rod and a metal water pipe, the conductor size must be determined by the electrode that requires the largest size. This often means reverting to the size dictated by the service entrance conductors.
Equipment Grounding and Professional Installation
Separate from the Grounding Electrode Conductor (GEC) is the Equipment Grounding Conductor (EGC), which is the bare or green wire found within branch circuits. The EGC runs with the circuit conductors to outlets and appliances, providing the low-impedance path necessary for fault current to return to the source and trip the circuit breaker.
The size of the EGC is determined by the rating of the circuit’s overcurrent protection device, using NEC Table 250.122. For example, a 20-amp circuit requires a 12 AWG copper EGC, while a 200-amp circuit breaker requires a 6 AWG copper EGC.
Working on a main electrical service is complex and carries significant risk of severe injury or property damage. All work involving the main service panel, including the installation of the grounding system, requires permits and must be performed or inspected by a licensed electrician. A licensed professional ensures the installation is compliant with local codes.