Ground Wire Size Calculator — Free Online Calculator
Find the correct equipment grounding conductor size per NEC Table 250.122 based on overcurrent device rating.
How to Use This Calculator
Select the breaker or fuse size and conductor material.
The Formula Explained
NEC Table 250.122 specifies the minimum equipment grounding conductor size based on the rating of the overcurrent device protecting the circuit.
Grounding: The Invisible Safety System
Grounding and bonding is the part of the electrical system that does nothing 99.99% of the time. When you need it, you need it desperately — during a ground fault, lightning strike, or equipment insulation failure. The grounding system provides a low-impedance path back to the source, causing overcurrent devices to trip quickly, while bonding ensures that exposed metal parts never develop dangerous voltage differentials during a fault.
NEC Article 250 is one of the longest and most frequently updated articles in the code because grounding is both critical and poorly understood. Miss a bonding jumper at a water meter and a small fault can energize the entire plumbing system. Undersize an equipment grounding conductor and a fault on a 100A feeder will not trip the breaker fast enough to prevent insulation damage. Install a single ground rod with 50 ohms to earth and lightning can raise the entire electrical system to dangerous potentials during a storm.
Worked Example: Sizing the Ground for a 200A Service
Residential 200A service, copper conductors. NEC Table 250.66 governs the grounding electrode conductor (GEC) — the wire from the service neutral to the grounding electrodes. For service conductors 2/0 through 3/0 copper, the GEC is 4 AWG copper. For 4/0 copper service conductors, the GEC bumps up to 2 AWG copper.
The equipment grounding conductor (EGC) that runs with the service is a different calculation. Since this is the grounded (neutral) service conductor, there is no separate EGC at the service — bonding is provided by the main bonding jumper between neutral bar and enclosure. For subpanels downstream, NEC Table 250.122 applies: for a 200A feeder breaker, the EGC is 6 AWG copper or 4 AWG aluminum. This is smaller than the phase conductors because it only needs to carry fault current for a brief time, not continuously.
Worked Example: Detached Garage Feeder Grounding
A 100A feeder to a detached garage 150 feet from the main house. Per NEC 250.32, detached buildings fed by a feeder require their own grounding electrode system, connected to the equipment grounding conductor of the feeder (not the neutral — that would create objectionable current on the EGC per 250.6).
Ground rod requirements: at least one ground rod driven 8 feet deep, unless a single rod shows 25 ohms or less to earth. If over 25 ohms, a second rod at least 6 feet away is required per 250.53(A)(2). Size of the GEC at the detached building: per Table 250.66, for 100A feeder with 1 AWG copper conductors, the GEC is 8 AWG copper minimum. The equipment grounding conductor in the feeder itself is 8 AWG copper per Table 250.122 for the 100A breaker.
Important: the neutral and ground must be kept separate at the detached building panel (do NOT install a main bonding jumper there). The neutral-to-ground bond only exists at the service disconnect back at the main house. Miss this and you create parallel paths for neutral current through the equipment grounding conductor and even through the earth.
Five Grounding Mistakes That Cause Real Harm
1. Using an undersized EGC because it is only the ground. The EGC must carry the full fault current until the breaker trips. Undersize it and the EGC itself fails during a fault, removing the safety path entirely. Table 250.122 is not optional.
2. Bonding neutral to ground at a subpanel. This creates parallel paths for neutral return current, energizing every grounded metal surface in the building with small voltages that can become dangerous during faults. The main bonding jumper goes at the SERVICE disconnect only.
3. Relying on a single ground rod without testing. Code requires either two rods 6 feet apart OR a single rod with verified 25 ohm or lower resistance. Testing costs a few dollars with a clamp-on ground resistance meter; assuming a single rod works costs lives.
4. Forgetting to bond metal water and gas pipes. NEC 250.104 requires bonding of interior metal water and gas piping to the grounding electrode system. Miss this and a fault on the range can energize the copper water lines, creating a shock hazard at every tap.
5. Using aluminum for GECs in corrosive environments. Aluminum is permitted for GECs but degrades rapidly in damp or chemically aggressive locations. Copper GECs are worth the extra cost for service entrances, pools, and any outdoor installation.
Grounding Best Practices Beyond Code Minimums
Use Ufer grounds whenever building new. A Ufer ground (concrete-encased electrode) is 20 feet of bare 4 AWG copper or 1/2-inch rebar embedded in the foundation footer. It provides extraordinary low-impedance earth contact because concrete stays moist and provides huge surface area. Required by code for new construction with concrete footings per 250.52(A)(3).
Install two ground rods as a matter of practice. Even when your single rod tests below 25 ohms, adding a second rod provides redundancy and future-proofing as soil conditions change with drought cycles.
Upsize the GEC one size beyond code minimum. The incremental cost is tiny and the safety margin is meaningful. A 6 AWG GEC instead of 8 AWG on a 200A service costs very little more in copper.
Test ground resistance periodically on critical installations. Industrial sites, data centers, and medical facilities should test grounding system impedance annually. A ground that was 10 ohms at install can drift to 100+ ohms over years as soil chemistry changes.
NEC Article 250 Key Sections
250.50 through 250.70 — grounding electrode system requirements (what constitutes a grounding electrode, how to connect them). Table 250.66 — grounding electrode conductor sizing based on service conductor size. Table 250.122 — equipment grounding conductor sizing based on overcurrent device rating.
250.90 through 250.104 — bonding requirements for service equipment, metal raceways, and other normally non-current-carrying metal. 250.32 — two or more buildings supplied by feeders (the detached garage rules). 250.53 — grounding electrode installation including the 25-ohm single-rod rule and rod spacing requirements. 250.6 — objectionable current prevention, the reason neutrals and grounds separate downstream of the service.
Equipment grounding conductor size: NEC 250.122 and why it matters
The equipment grounding conductor (EGC) is the wire that creates the fault-current path back to the source. Its job is to carry fault current long enough for the overcurrent device (breaker or fuse) to trip. NEC 250.122 sizes it based on the rating of the overcurrent device protecting the circuit, not on the size of the circuit conductors.
This is different from the grounding electrode conductor (GEC), which connects the panel ground bar to the earth ground rod/water pipe/Ufer. The GEC is sized by NEC 250.66 based on service-entrance conductor size.
The formula and what it does
15-20 A breaker: 14 AWG copper EGC. 30-60 A: 10 AWG. 100 A: 8 AWG. 200 A: 6 AWG. 400 A: 3 AWG. 1200 A: 250 kcmil. Aluminum sizes are typically two AWG larger. Note: the EGC is sized to the breaker, not the wire. A 12 AWG circuit on a 20 A breaker uses a 12 AWG EGC because that is what Table 250.122 calls for at 20 A.
Worked example
Scenario: 100 A subpanel feeder using 2 AWG copper THHN on a 100 A breaker in the main panel.
Per NEC 250.122 Table at 100 A: 8 AWG copper EGC. The feeder needs four conductors total: two 2 AWG hots, one 2 AWG neutral (if used), and one 8 AWG ground. The EGC is sized at 100 A regardless of the larger phase conductor size.
Variation: long feeder where you upsized the phase conductors to 1/0 AWG for voltage drop. NEC 250.122(B) says if you upsize phase conductors for voltage drop, you must proportionally upsize the EGC. 1/0 is two sizes up from 2, so the EGC also goes up two sizes: from 8 AWG to 4 AWG.
Code references and standards
NEC 250.122(A) base sizing table. EGC sized to OCPD rating, not conductor size.
NEC 250.122(B) if you upsize phase conductors for voltage drop, upsize the EGC proportionally. This catches a lot of long-feeder installs.
NEC 250.122(F) on parallel feeders, run a full-size EGC in each raceway. Do not split the EGC across raceways.
NEC 250.66 is the grounding electrode conductor (panel to earth), not the EGC. Sized by service entrance conductor: 200 A service usually gets a 4 AWG copper GEC.
Common mistakes to avoid
Sizing the EGC to the phase conductor instead of the breaker. A 12 AWG circuit on a 20 A breaker needs a 12 AWG EGC, even if it is part of a feeder with larger phase wires.
Forgetting to upsize when phases are upsized. NEC 250.122(B) catches a lot of long feeders. If you went from 2 AWG to 1/0 AWG for voltage drop, the EGC goes from 8 AWG to 4 AWG.
Using metal conduit as the only ground path. Allowed by 250.118 but unreliable. Modern practice runs a separate green EGC in every raceway as a redundant fault path.
Frequently asked questions
Why is the EGC based on breaker size, not load?
Because the EGC has to carry fault current long enough for the breaker to trip. The breaker is what defines the maximum fault current that conductor will see, so it sets the EGC requirement.
Can my EGC be the same size as my neutral?
On most feeders, no. The neutral is sized for unbalanced load current; the EGC is sized for fault current. They serve different purposes and have different size rules.
Do I need an EGC in a metal conduit?
NEC 250.118 allows metal conduit to serve as the EGC if all connections are properly made up. In practice, almost every modern install runs a green EGC anyway because corrosion, vibration, and improper joint makeup degrade the metal-path reliability over time.
How is the GEC different from the EGC?
The GEC connects the main panel ground bar to the grounding electrode (rod, plate, Ufer). Sized by NEC 250.66 based on service-entrance conductor size. The EGC runs from each branch panel back to the source, sized by 250.122 based on the breaker.
Does PVC conduit require a separate ground conductor?
Yes. PVC is non-conductive, so the only ground path is the separate EGC. NEC 352 and 250 require it in every PVC conduit.
What size ground rod conductor do I need?
NEC 250.66(A): for ground rods only, the GEC does not need to be larger than 6 AWG copper or 4 AWG aluminum, regardless of service size. For water pipe or Ufer, use the full 250.66 table.