What maximum resistance in ohms is permitted across dual grounding rods when bonding a residential standby generator?

Under National Electrical Code standards for residential standby generators, no maximum ohm threshold is enforced across dual grounding rods spaced at least six feet apart, as installing the supplemental rod satisfies the requirement without resistance verification.

Inspectors and contractors frequently address whether earth-resistance testing is required during residential generator installations. While a single grounding rod must achieve a measured resistance of 25 ohms or less under National Electrical Code (NEC) 250.53(A)(2), driving a second bonded rod satisfies compliance automatically without requiring a fall-of-potential test.

If you only do one thing: Drive two 5/8-inch-diameter by 8-foot-long grounding rods at least 6 feet apart—ideally 16 feet apart—and connect them using a continuous 6 AWG copper grounding electrode conductor without conducting earth testing.

• NEC 250.53(A)(2) 25-Ohm Exception: A single ground rod measuring greater than 25 ohms to earth requires a supplemental electrode, but installing a second rod upfront waives all testing and ohm-measurement requirements.
• Electrode Spacing Standard: NEC 250.53(A)(3) mandates a minimum separation of 6 feet between rods, though separating them by twice the rod length (16 feet for 8-foot rods) achieves superior electrical dissipation.
• Rod Dimensions and Materials: Grounding rods must measure at least 8 feet in length and 5/8 inch in diameter for standard galvanized steel, or 1/2 inch for listed copper-clad steel, driven flush or below finished grade.
• Grounding Electrode Conductor Sizing: Connecting two ground rods solely to an electrical service or generator requires a minimum 6 AWG bare or insulated copper wire per NEC 250.66.
• System Classification: Under NEC 250.35, most residential standby setups use a 2-pole transfer switch with a solid neutral, classifying the generator as a non-separately derived system that bonds directly to the main service panel grounding system.
• Auxiliary Electrode Bonding: If an auxiliary grounding electrode is driven at the generator pad under NEC 250.54, it requires no resistance testing, but it must connect directly to the equipment grounding conductor run from the home.

• Watch out for: Placing grounding rods closer than 6 feet apart, which violates code standards and causes overlapping electrical resistance spheres in surrounding soil.
• Watch out for: Creating an illegal neutral-to-ground bond inside a non-separately derived generator enclosure when the neutral is bonded at the main service panel, violating NEC 250.24(A)(5).
• Watch out for: Using grounding pipe clamps not stamped with the "DB" direct burial rating when connecting the 6 AWG copper conductor to submerged rods.

Confirm whether your transfer switch switches the neutral conductor to determine grounding derivation, then drive two 8-foot rods at least 6 feet apart. For complex load calculations or local electrical permitting requirements, consult a licensed electrical professional.