Let's break down NEC 2017 Article 250.122(B) and see if the #8 AWG ground is undersized.NEC 2017 Article 250.122(B) states:"Increase in Size of Ungrounded Conductors. Where ungrounded conductors are increased in size, as permitted in 310.15(B)(3)(a), for voltage drop, the equipment grounding conductor shall be increased in size proportionately according to the circular mil area of the ungrounded conductors. Where a feeder or branch-circuit is derived from a separately derived system, the grounded conductor shall be increased in size proportionately according to the increase in circular mil area of the ungrounded conductors."Analysis


Article 250.122(B) applies ONLY if the ungrounded conductors (phase and neutral) were increased in size due to voltage drop. The question states the conductors are #4 AWG, and the breaker is 70A. A #4 AWG conductor is appropriate for a 70A circuit (NEC Table 310.15(B)(16) as limited by 240.4(D) for #14-#10 AWG). Therefore, if the engineer sized the conductor appropriately for 70 amps, then 250.122(B) does NOT apply.
2. If the ungrounded conductors were not increased due to voltage drop, then we consult Table 250.122.3. NEC Table 250.122 - Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment. This table dictates the minimum equipment grounding conductor (EGC) size based on the rating of the overcurrent protective device (OCPD).4. For a 70A OCPD, Table 250.122 requires a minimum #8 AWG copper EGC.Conclusion

Based on the information provided, a #8 AWG equipment grounding conductor is

NOT undersized unless the #4 AWG phase and neutral conductors were increased in size beyond what is required for the 70A load for voltage drop. If that is the case, then the EGC would need to be increased proportionately based on the circular mil increase in the ungrounded conductors.

Important Considerations



Material

The above analysis assumes the conductors and grounding conductor are copper. Different materials have different ampacities and require different grounding conductor sizes.

Type of Grounding Conductor

This analysis assumes the ground is copper.

Local Codes

Always check local amendments to the NEC. Some jurisdictions may have stricter requirements.

Installation Method

Certain installation methods may affect ampacity and require adjustments.*

Professional Judgment: Consult with a qualified electrician or engineer to ensure the installation meets all applicable codes and safety standards. They can verify the design assumptions and make the final determination.In summary, the #8 AWG ground appears to be sufficient if the #4 AWG conductors are sized based on the ampacity requirements of the 70A breaker. However, double-check if the #4 AWG conductors were selected for voltage drop, as this would necessitate a larger EGC. Flag for review