The 10 Most Common Grounding and Bonding Mistakes

Why Grounding and Bonding Matter

Grounding and bonding are not the same thing, and confusing the two is the first mistake many electricians make. Grounding provides a path to the earth to limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines. Bonding connects all metallic parts of the electrical system together to create a low-impedance fault current path that allows overcurrent devices to operate quickly and clear faults.

When grounding or bonding is done incorrectly, the consequences range from nuisance tripping and equipment damage to electrocution and fire. Article 250 of the NEC is one of the longest and most complex articles in the code, spanning over 40 pages. It is also one of the most frequently cited articles on inspection failure reports.

Mistake 1: Using the Wrong Size Grounding Electrode Conductor

The grounding electrode conductor (GEC) must be sized according to Table 250.66 based on the size of the largest ungrounded service-entrance conductor. A common error is using a GEC that is too small, or using the same size GEC regardless of the service size.

Mistake 2: Improper Bonding of Water Piping

Metal water piping systems must be bonded to the electrical system per Section 250.104(A). Common errors include:

• Failing to bond the water piping system at all, especially when the main water pipe is metallic but enters the building in an area away from the electrical panel
• Attaching the bonding jumper to a section of pipe that is downstream of a dielectric fitting or water meter, which breaks the bonding path
• Using an improper clamp or connection method that does not maintain good electrical contact

Mistake 3: Not Bonding CSST Gas Piping

Corrugated stainless steel tubing (CSST) used for gas piping must be bonded per the manufacturer's instructions and local code requirements. Lightning-induced surges can arc through CSST, creating a fire hazard. Most manufacturers require a 6 AWG copper bonding conductor connected to the CSST system and run back to the grounding electrode system.

Mistake 4: Misusing the Neutral as a Ground

In a subpanel (any panel that is not the service disconnecting means), the neutral (grounded conductor) and the equipment grounding conductors must be kept separate. The neutral bus must be isolated from the panel enclosure, and a separate equipment grounding bus must be installed and bonded to the enclosure.

Bonding the neutral to the enclosure in a subpanel creates parallel paths for neutral return current, which can put current on metal parts that should not be energized and can cause interference with GFCI and AFCI devices.

Mistake 5: Wrong Size Equipment Grounding Conductor

The equipment grounding conductor (EGC) must be sized according to Table 250.122 based on the rating of the overcurrent device protecting the circuit. This is different from the GEC sizing in Table 250.66. Common mistakes:

• Using the same size ground wire for all circuits regardless of the breaker size
• Confusing Table 250.66 with Table 250.122 — they serve completely different purposes
• Not increasing the EGC size when conductors are increased in size for voltage drop

Mistake 6: Inadequate Grounding Electrode System

Section 250.50 requires that all grounding electrodes present at a building be bonded together to form the grounding electrode system. This includes:

• Metal underground water pipe (first 3 m / 10 ft of entry)
• Metal frame of the building (if effectively grounded)
• Concrete-encased electrode (Ufer ground)
• Ground ring
• Rod, pipe, or plate electrodes

A common mistake is installing two ground rods and calling it done, without bonding to the metal water pipe or concrete-encased electrode that may also be present.

Mistake 7: Ground Rod Resistance Not Verified

Section 250.53(A)(2) requires that a single rod, pipe, or plate electrode must have a resistance to ground of 25 ohms or less. If it does not, a supplemental electrode must be installed. Most electricians install two ground rods by default (spaced at least 1.8 m / 6 ft apart) to avoid having to test the resistance. This is acceptable, but if only one rod is installed, resistance testing is required.

Mistake 8: Improper Intersystem Bonding

Section 250.94 requires an intersystem bonding termination at the service equipment or metering equipment. This provides a connection point for communications systems (cable TV, telephone, internet) to bond to the building's grounding electrode system. Many electricians forget to install this device, resulting in inspection failures.

Mistake 9: Splicing the GEC Without Proper Fittings

The grounding electrode conductor must be installed in one continuous length without a splice, except where spliced by irreversible compression connectors or by the exothermic welding process (Section 250.64(C)). Wire nuts and standard split-bolt connectors are not acceptable for GEC splices.

Mistake 10: Missing Bonding Bushings on Service Raceways

At service equipment, standard locknuts are not sufficient to ensure a good bonding connection through concentric and eccentric knockouts. Section 250.92(B) requires one of the following methods:

• Bonding bushings with bonding jumpers
• Threadless connectors made tight
• Other listed devices such as bonding-type locknuts