Safety & Protection calculator
Grounding Calculator
This page is an NEC Article 250 conductor-sizing screen. It helps you sort the difference between the main bonding jumper, the equipment grounding conductor, and the grounding electrode conductor, then size the selected conductor family on the correct basis. It is not a rod-resistance calculator, a grounding-grid design engine, or a substitute for project-specific grounding layout review.
Updated July 10, 2026
A service with 4/0 copper ungrounded conductors screens at a 2 AWG copper main bonding jumper under Table 250.102(C)(1), while a 200A feeder screens at a 6 AWG copper equipment grounding conductor under Table 250.122.
Grounding and bonding screen: MBJ uses the largest ungrounded conductor | EGC uses OCPD rating | GEC uses Table 250.66 plus the applicable electrode rule.
Choose main bonding jumper equipment grounding conductor or grounding electrode conductor below to size the correct Article 250 conductor family
Example Calculations
More examples. Open to review 1 additional calculation example.
How to Use
How to use the grounding calculator
- Choose the exact conductor family you are checking:
- Main Bonding Jumper for service, system, or supply-side bonding jumper review based on Table 250.102(C)(1)
- Equipment Grounding Conductor for feeder or branch-circuit EGC review based on Table 250.122
- Grounding Electrode Conductor for service or separately derived grounding-electrode-conductor review based on Table 250.66 and the applicable electrode rule
- Enter the correct basis. For bonding jumpers and GECs, use the largest ungrounded conductor size or equivalent area. For EGCs, use the overcurrent-device rating.
- Read the returned size together with the installation note. Some grounding electrode connections have practical size caps, and aluminum conductors still carry installation restrictions.
- If the real question is rod resistance, fall-of-potential testing, or grounding-electrode performance in soil, move to the Grounding Resistance Calculator instead.
What each conductor does
| Conductor family | Sizing basis | Typical job | What it is not |
|---|---|---|---|
| Main bonding jumper or similar bonding means | Largest ungrounded conductor or equivalent area under Table 250.102(C)(1) | Bonding the grounded conductor to the equipment-grounding and enclosure path at the permitted bonding point | A generic green grounding wire for every circuit |
| Equipment grounding conductor | Overcurrent-device rating under Table 250.122 | Providing a low-impedance fault-return path for feeders and branch circuits | The conductor that bonds the service to the grounding electrode system |
| Grounding electrode conductor | Largest service or derived conductor size under Table 250.66, then the applicable electrode rule | Connecting the service or source to the grounding electrode system | The metallic fault-return path that normally clears a branch-circuit fault |
Scope notes that matter
- The main bonding jumper, equipment grounding conductor, and grounding electrode conductor are not interchangeable.
- This page helps size conductors, but it does not design the whole grounding and bonding layout for the project.
- If multiple services or separately derived systems serve the same building or structure, review the common grounding electrode system requirement instead of creating isolated electrode systems.
- A ground rod does not replace the bonded metallic fault-return path that actually clears most feeder and branch-circuit faults.
Example 1: a service with 4/0 copper ungrounded conductors screens at a 2 AWG copper main bonding jumper under Table 250.102(C)(1).
Example 2: a feeder protected at 200 A screens at a 6 AWG copper equipment grounding conductor under Table 250.122.
Example 3: a service with 500 kcmil aluminum conductors and a rod-electrode connection can screen the grounding electrode conductor through Table 250.66, while still checking the practical rod-electrode size limit under the applicable rule.
Use the Grounding Resistance Calculator when the question is rod resistance to earth. Use the GEC Sizing Calculator when you want a dedicated grounding-electrode-conductor page. Use the grounding and bonding guide when the real issue is where the neutral-ground bond belongs or how the fault-return path works.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
What is the difference between the main bonding jumper and the equipment grounding conductor?
The main bonding jumper makes the intentional bond between the grounded conductor and the equipment-grounding or enclosure path at the permitted bonding point. The equipment grounding conductor runs with feeders or branch circuits and carries fault current back to that bonding point.
Why is this page not a grounding-resistance calculator?
Because conductor sizing and rod resistance are different problems. This page sizes Article 250 conductor families. Rod resistance depends on soil resistivity, rod geometry, and field conditions, so it belongs on the grounding-resistance page.
Does a ground rod clear most branch-circuit faults?
No. In ordinary feeder and branch-circuit faults, the overcurrent device clears through the bonded metallic fault-return path and the equipment grounding conductor, not through the earth alone.
What does NEC 250.58 mean in practice?
When one building or structure has multiple services or separately derived systems, they normally use the same grounding electrode system rather than isolated electrode systems scattered around the building. This keeps grounding and bonding on a common reference.
Can I use one conductor size rule for every grounding and bonding conductor on the job?
No. The main bonding jumper, the equipment grounding conductor, and the grounding electrode conductor each have their own sizing basis. That is why this page starts by asking which conductor family you are checking.
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