The thickness of the ground wire (equipment grounding conductor) in a secondary distribution box is determined by the rating of the overcurrent protection device, typically following NEC Table 250.122.

Equipment Grounding Conductor (EGC) Sizing

The equipment grounding conductor (EGC) is the wire that runs with your circuit conductors to provide a low-impedance path for fault current, ensuring that breakers trip quickly during a fault. Its size is based on the rating of the upstream breaker or fuse protecting the circuit, not the continuous load current . For copper conductors, typical minimum sizes according to NEC Table 250.122 are:

• 15–20 A breaker: 14–12 AWG
• 30–60 A breaker: 10 AWG
• 100 A breaker: 8 AWG
• 200 A breaker: 6 AWG
• 300 A breaker: 4 AWG
• 400 A breaker: 3 AWG
• 500 A breaker: 2 AWG
• 600 A breaker: 1 AWG
• 800 A breaker: 1/0 AWG
• 1000 A breaker: 2/0 AWG For aluminum conductors, the size is generally two gauge sizes larger than copper for the same breaker rating .

Special Considerations for Transformer Secondary

If the ground wire runs between a transformer secondary and the panel, it may be considered a supply side bonding jumper (SSBJ) rather than a standard EGC. In this case, NEC Table 250.102(C)(1) applies, and the size may differ from the EGC table. For example, a 400 A main breaker on a transformer secondary may require a #3 AWG copper EGC to the panel, while the SSBJ could be sized at 1/0 AWG depending on the transformer and system configuration . It is important to distinguish between:

• EGC: Runs with circuit conductors to the panel, sized by breaker rating (Table 250.122).
• SSBJ: Connects transformer secondary neutral to ground at the first panel, sized by Table 250.102(C)(1).
• Grounding Electrode Conductor (GEC): Runs from the panel to the earth, sized by Table 250.66 based on the secondary conductor size .

Summary

• For a standard secondary distribution box, the ground wire thickness is determined by the breaker rating using NEC Table 250.122.
• For transformer secondary connections, check if the wire is a supply side bonding jumper; if so, use Table 250.102(C)(1).
• Always verify whether the conductor is copper or aluminum, as aluminum requires a larger gauge.
• Proper sizing ensures safety, code compliance, and reliable fault clearing. By following these NEC tables and guidelines, you can select the correct ground wire thickness for your secondary distribution box installation.