Sizing Grounding and Bonding Conductors with NEC Table 250.122

Your Answer-First Guide to the Equipment Grounding Conductor Chart

Properly sizing an equipment grounding conductor (EGC) is a critical safety task governed by NEC Table 250.122, the definitive equipment grounding conductor chart. The key principle is that the minimum size equipment grounding conductor is determined by the rating or setting of the circuit’s overcurrent protective device rating (OCPD), not the ampacity of the circuit conductors. To perform the EGC sizing calculation, you locate the amperage of the fuse or circuit breaker in the left column of the table and find the corresponding minimum copper or aluminum EGC size in the right column. This ensures the EGC can carry enough fault current to trip the OCPD, creating an effective ground-fault current path. This process is fundamentally different from using a standard wire ampacity chart, which is based on a conductor’s heat rating under normal load.

The Fundamental Role of the Equipment Grounding Conductor (EGC)

In NEC Article 250 grounding, the Equipment Grounding Conductor (EGC) is the unsung hero of electrical safety. Its job isn’t to carry current under normal conditions but to stand ready for a ground fault. When an energized conductor accidentally touches a conductive surface like a metal raceway or equipment chassis, the EGC provides a low-impedance path back to the source. This creates a massive surge of current that trips the circuit breaker or blows the fuse, de-energizing the circuit almost instantly. Without a properly sized EGC, this effective ground-fault current path would be compromised, leaving equipment energized and creating a lethal shock hazard. Achieving electrical code compliance grounding is non-negotiable, and understanding the EGC is central to that goal.

A Deep Dive into the NEC Equipment Grounding Conductor Chart: Table 250.122

NEC Table 250.122 is the primary tool for wire-type EGC sizing. Electricians frequently refer to it as the nec ground wire size chart or simply the ground wire size chart. Mastering this table is essential and a common topic in journeyman electrician exam grounding questions. The table simplifies the wire size computation for EGCs into a straightforward lookup process.

Step-by-Step EGC Sizing Calculation

1. Identify the OCPD Rating: Determine the amperage rating of the circuit breaker or fuse protecting the circuit. This is the single most important value for this calculation.
2. Locate the Rating in Table 250.122: Find this amperage in the left-hand column, “Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes).”
3. Find the Minimum EGC Size: Read directly across to the right-hand column to identify the minimum required cable gauge awg for the EGC. The table provides sizes for both copper and aluminum (or copper-clad aluminum) conductors.
4. Verify Material: Ensure you are using the correct column for your chosen conductor material. For example, a 60 amp breaker wire size for the EGC is #10 AWG copper but #8 AWG aluminum.
5. Check for Special Conditions: Before finalizing, review NEC sections for adjustments, such as those for voltage drop or parallel conductors, discussed below. This final step is crucial for complex installations.

Practical Examples: From 30 Amp Wire Size to 200 Amp Service Wire Size

Let’s apply this. Unlike determining how many outlets on a 20 amp circuit or how many receptacles on a 20 amp circuit, EGC sizing is not a load calculation. It’s a safety calculation.

• 20-Amp Circuit: For a standard 20A circuit using 12awg wire for the ungrounded conductors, Table 250.122 requires a #12 AWG copper or #10 AWG aluminum EGC. The rules are different for questions like how many watts on a 15 amp circuit or whether a 20 amp outlet on a 15 amp circuit is compliant.
• 50-Amp Circuit: If you’re determining what size wire for 50 amp breaker, you might choose #6 AWG THHN for the circuit conductors. The 50 amp wire size for the EGC, however, is found on the table: #10 AWG copper or #8 AWG aluminum. This is the correct EGC size regardless if it’s for an EV charger or another appliance. Many ask, “what size wire for 50 amps?” and it’s critical to differentiate between the EGC and the current-carrying conductors. The same logic applies to a 40 amp wire size or a 30 amp wire size, where you might be considering factors like dryer wattage. For more on modern requirements, see our guide on how the 2023 NEC simplifies grounding for ranges and dryers.
• 100-Amp Subpanel: For a feeder protected by a 100A breaker, the correct 100 amp sub panel wire size for the EGC is #8 AWG copper or #6 AWG aluminum. This is independent of the feeder conductor size (e.g., #3 copper or 1/0 al wire), which is based on load calculations. The question of what size wire for 100 amp service or a wire size for 100 amp sub panel has two parts: the service conductors and the EGC. This might also apply to a mobile home feeder wire 100 amp service.
• Large Services: For a 200 amp wire size EGC, the table dictates #6 AWG copper or #4 AWG aluminum. For a 400 amp wire size EGC, it’s #3 AWG copper or #1 AWG aluminum or copper-clad aluminum. You might use large conductors like 4/0 aluminum wire (or 4 0 aluminum wire) for the service itself, but the EGC follows the table. The 4/0 aluminum wire ampacity (180A in the 75°C column) is for load, not grounding.

Special Conditions & Adjustments for Wire-Type EGC Sizing

The NEC accounts for situations that go beyond a simple table lookup. These are common sources of confusion and are essential for advanced electrical code compliance grounding.

• Voltage Drop (250.122(B)): If you increase the size of the ungrounded conductors to compensate for voltage drop, the EGC must also be increased proportionately. You cannot use a simple size electrical wire calculator for this; you must do the math based on the circular mil area. Mentioning a wire length calculator is relevant here as longer runs often necessitate this adjustment.
• Parallel Conductors (250.122(F)): The parallel conductor grounding rules require a full-sized EGC, based on the circuit’s OCPD rating, to be run in each parallel raceway. You cannot simply add the circular mils of smaller EGCs together. This is a significant change from past practices and is a crucial part of the latest code.
• Grounding for Motor Circuits (250.122(D)): For motor circuits, the EGC must be sized based on the rating of the motor branch-circuit short-circuit and ground-fault protective device.
• Sizing Bonding Jumpers: On the load side of the final OCPD, sizing bonding jumpers typically follows the rules of 250.122. This is essential for grounding raceways and equipment effectively.

The NEC is constantly evolving. For licensed professionals, staying current on changes is paramount. Our courses cover critical updates, including how EGC sizing and splicing rules changed in the 2023 NEC and broader topics like changes to grounding electrode conductor sizing.

Ground Wire Sizing Chart vs. Ampacity Chart NEC

A frequent point of confusion is the difference between a ground wire sizing chart (Table 250.122) and a wire ampacity chart (like Table 310.16). They serve entirely different purposes. An ampacity chart nec, also called a nec wire ampacity chart or ampacity table, tells you the maximum current a wire can carry continuously without heat damage. It answers questions about the ampacity of a conductor, such as the 8 gauge wire amp rating, 4 awg wire ampacity, or 3/0 copper wire ampacity. For instance, the amp capacity of 8 gauge wire for 8 thhn ampacity is 50A at 75°C, while its 8awg wire ampacity might vary by insulation type. These tables—whether called a nec ampacity chart, electrical wire size chart, or ampacity of cable table—are for sizing current-carrying conductors only.

In contrast, the equipment grounding conductor chart is based solely on the OCPD. A circuit with a 20A breaker needs a #12 copper EGC, even if you used oversized #10 conductors for voltage drop (unless you must then upsize the EGC per 250.122(B)). Knowing how to know what gauge wire to use means knowing which table to consult: the nec wiring size chart for ampacity or the ground wire size chart NEC for grounding.

Key Takeaways for Sizing EGCs

• The EGC is sized based on the circuit’s OCPD rating, not the conductor ampacity or the connected load.
• NEC Table 250.122 is the definitive equipment grounding conductor chart for this task.
• Always check for special conditions requiring an upsized EGC, such as voltage drop compensation or parallel conductor installations.
• Sizing bonding jumpers on the load side of an OCPD typically follows the same rules as sizing EGCs.
• Do not use a standard ampacity chart, wire amperage chart, or nec amp chart for EGC sizing. These are for normal operating currents.

Master the critical art of grounding and bonding with our in-depth courses. ExpertCE provides the NEC knowledge you need to stay licensed, compliant, and safe on the job.

Primary Sources

• NFPA 70, National Electrical Code (NEC), 2023 Edition. For direct access to the code, visit the National Fire Protection Association (NFPA) website.

Frequently Asked Questions (FAQ)

What is the fastest way to use the equipment grounding conductor chart?

The fastest way is to first identify the amperage of the overcurrent protective device (breaker or fuse) for the circuit. Then, go directly to NEC Table 250.122, the primary equipment grounding conductor chart, find that amperage in the left column, and read across to find the minimum required EGC size for either copper or aluminum.

Do I use a wire ampacity chart or a ground wire size chart for the EGC?

You must use the ground wire size chart, which is NEC Table 250.122. A wire ampacity chart (like Table 310.16) is used for sizing the normal current-carrying conductors (the “hots” and neutral) based on load and heat. The EGC is sized for fault current, which is a completely different purpose tied to the OCPD rating.

What is the correct ground wire size for a 60 amp breaker?

According to the nec ground wire size chart (Table 250.122), for a circuit protected by a 60-amp breaker, the minimum equipment grounding conductor size is #10 AWG Copper or #8 AWG Aluminum/Copper-Clad Aluminum. The size of wire for 60 amp phase conductors will be larger, but this is the correct EGC size.

How does EGC sizing calculation work for parallel runs?

Per NEC 250.122(F), when conductors are run in parallel in separate raceways, a full-sized EGC must be installed in each raceway. The EGC sizing calculation is based on the rating of the circuit’s overcurrent protective device. For example, if a 400A circuit uses two parallel conduits, each conduit must contain an EGC sized for 400A (#3 AWG copper or #1 AWG aluminum or copper-clad aluminum), not an EGC sized for the ampacity of the conductors within that single conduit.

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