Understanding the Ins and Outs of Rarely Used Codes in NEC Article 250


Article 250 of the National Electrical Code (NEC) covers grounding and bonding requirements for electrical systems. While most electricians are familiar with the common rules in Article 250, there are some rarely used and lesser known codes that are still important to understand. In this article, I will provide an in-depth look at these rarely used NEC 250 codes, why they exist, and how they apply.

Bonding of Piping Systems and Exposed Structural Steel

Bonding of Metal Water Piping (250.104)

NEC 250.104 requires metal water piping systems to be bonded together to create an equipotential grounding system. This prevents voltage potential between different parts of the system which could shock anyone touching different sections of pipe.

While plastic piping is common today, many older buildings still use metal water pipes. If any part of the piping is metal, all sections must be bonded. The main bonding jumper should connect to the metal water pipe within 5 feet of its entrance to the building.

Bonding Interior Metal Gas Piping (250.104(B))

Similar to water piping, interior metal gas piping must also be bonded to the grounding system per NEC 250.104(B). This applies to natural gas piping and liquefied petroleum gas piping systems. The main bonding jumper should connect to the metal gas pipe within 5 feet of its entrance to the building.

Bonding Exposed Structural Steel (250.104(C))

In commercial and industrial buildings, exposed structural steel such as beams and columns must be bonded to the grounding system as per 250.104(C). This prevents the steel framework from becoming energized if a fault occurs. The connections must be accessible.

Grounding Electrode System Inspections and Resistance Testing

Inspecting Grounding Electrode Connections (250.119)

Per NEC 250.119, all grounding electrode system connections must be inspected to ensure they are accessible and meet code requirements. This includes connections for ground rods, ground rings, concrete-encased electrodes, and grounding electrodes present at separate buildings/structures. The inspector needs to verify that the connections are tight and properly bonded.

Testing Resistance of Grounding Electrode Systems (250.56)

NEC 250.56 allows an optional resistance test of the grounding electrode system using an earth ground resistance tester. As per the code, the measured resistance cannot exceed 25 ohms.

This test helps ensure the grounding system can sufficiently discharge voltage to earth in a fault condition. High resistance indicates poor connections or inadequate electrodes.

Grounding of Systems on Alternating Current (AC) Supply Systems Over 1kV

Grounding Metal Structures and Equipment (250.170)

For AC systems over 1,000 volts, NEC 250.170 requires that metal structures and equipment enclosures be grounded. This applies to indoor and outdoor equipment and structures that could become energized. Multiple connections spaced at least 6 feet apart are required.

The earth grounding prevents shock hazards and protects equipment from damage in a fault condition.

Stepping Down Voltages (250.182)

Per 250.182, AC systems over 1 kV supplying secondary circuits or equipment must have a grounded conductor. This grounded (neutral) conductor must have an ampacity of at least 1/2 the largest ungrounded (hot) conductor.

The grounded conductor steps down the voltage to the utilization level. This code prevents an unnecessarily high voltage from being supplied to end-use equipment.

Grounding Separately Derived AC Systems (250.30)

Typical Separately Derived Systems

Separately derived AC systems are sources that are not in direct connection to a service point or other power source. Some examples are:

Grounding Requirement (250.30)

NEC 250.30 mandates that separately derived AC systems have both a grounded conductor and a grounding electrode conductor. The grounded conductor ties the derived system back to earth, while the grounding electrode conductor connects to nearby grounding electrodes.

This critical grounding prevents shock hazards and allows fault currents to dissipate properly.

Grounding Systems Located in Different Structures (250.32)

Bonding Separate Buildings (250.32(A))

Where an electrical system spans multiple structures, NEC 250.32(A) requires the grounding electrode system in each structure to be bonded together. This equalizes potential and creates a common ground.

The bonding conductor can be underground, or overhead where conditions allow. Insulation is required for conductors spanning overhead.

Separate Buildings Supplied by Feeder or Branch Circuits (250.32(B))

For separate buildings supplied by feeders or branch circuits originating in another building, 250.32(B) mandates that each building have its own grounding electrode system. The wiring method provides an equipment ground, so separate grounding electrodes are needed in each building for fault protection.

This prevents one building's grounding system from having to handle surge currents from a different structure.

Ground Fault Circuit Interrupter (GFCI) Protection for Personnel (250.121)

General Requirements for GFCI Protection

NEC 250.121 covers where GFCI protection is mandated to prevent electric shock hazards to personnel. GFCIs monitor current flow and trip when an abnormal imbalance is detected, rapidly shutting off power in as little as 0.025 seconds.

Locations Requiring GFCIs

Some locations requiring GFCI protection for personnel safety include:

GFCIs are critical for preventing electrocutions in areas with elevated risk of electric shock.

Ground Fault Protection of Equipment (250.36)

Purpose of Ground Fault Protection

NEC 250.36 covers ground fault protection systems designed to protect electrical equipment itself from damage. If a ground fault current exceeds a preset value for a specified time, the system will trip and open the circuit.

Where GFCI Protection is Required

Locations requiring equipment ground fault protection include:

This protects vital equipment from damage in ground fault conditions. The trip level and time delay provides selectivity.

Surge Protective Devices (SPDs) for Critical Systems (250.184)

Why SPDs Are Used

Surge protective devices (SPDs) absorb voltage spikes caused by lightning strikes or other power events. NEC 250.184 covers special SPD requirements for critical systems.

Where SPDs are Required

SPDs are required for the following critical systems:

This provides extra protection from surges that could damage vital building systems and inhibit their operation during emergencies.


While lesser-used, these important NEC 250 codes address specialized grounding and bonding requirements for systems often found in commercial and industrial buildings. Understanding proper application of these codes keeps electrical systems safe and reliable. Implementation does take careful coordination - consultants can advise to ensure grounding integrity and code compliance. With expertise and diligence, even rarely used NEC 250 rules can be effectively applied.