Ground Wire Size Calculator
The equipment grounding conductor (EGC) -- commonly called the 'ground wire' -- is the safety conductor that provides a low-impedance fault current path back to the source,...
Formula
Source: NEC Article 250, NFPA 70 | Last reviewed: July 26, 2026
Examples
100 Amps
= 10 AWG
- material = 1
100 A OCPD -> #8 AWG Cu EGC per NEC 250.122
400 Amps
= 40 AWG
- material = 1
400 A OCPD -> #3 AWG Cu EGC
1200 Amps
= 120 AWG
- material = 1
1200 A OCPD -> #3/0 AWG Cu EGC
Quick Reference Table
| OCPD Rating (A) | Cu EGC (AWG/kcmil) | Al EGC (AWG/kcmil) |
|---|---|---|
| 15 | #14 | #12 |
| 20 | #12 | #10 |
| 30 | #10 | #8 |
| 40 | #10 | #8 |
| 60 | #10 | #8 |
| 100 | #8 | #6 |
| 200 | #6 | #4 |
| 300 | #4 | #2 |
| 400 | #3 | #1 |
| 500 | #2 | #1/0 |
| 600 | #1 | #2/0 |
| 800 | #1/0 | #3/0 |
| 1000 | #2/0 | #4/0 |
| 1200 | #3/0 | 250 kcmil |
| 1600 | #4/0 | 350 kcmil |
| 2000 | 250 kcmil | 400 kcmil |
| 2500 | 350 kcmil | 600 kcmil |
| 3000 | 400 kcmil | 600 kcmil |
| 4000 | 500 kcmil | 800 kcmil |
| 5000 | 700 kcmil | 1200 kcmil |
| 6000 | 800 kcmil | 1200 kcmil |
Where is this used?
(1) Branch circuits in commercial buildings -- NM-B (Romex) cable includes a bare copper EGC (#14 for 15 A, #12 for 20 A circuits).
In EMT conduit installations, the conduit itself may serve as the EGC per NEC 250.118, but a wire-type EGC is always required in flexible metal conduit runs over 6 ft or where flexibility is a design requirement.
(2) Motor circuits -- the EGC is sized per NEC 250.122(D) based on the motor branch-circuit short-circuit and ground-fault protective device rating, not the motor FLA or conductor ampacity.
A 75 HP 460V motor on a 200 A thermal-magnetic breaker requires a #6 AWG Cu EGC, even though the phase conductors (sized at 125% of 96 A = #1 AWG) are larger.
The EGC size is pegged to the OCPD that must clear the fault, not the load current.
(3) Feeder circuits in industrial plants -- 600 A feeder breaker serving a motor control center requires a #1 AWG Cu EGC per Table 250.122.
If a 4-inch conduit carries three 500 kcmil phase conductors plus the #1 EGC, the EGC is significantly smaller than the phase conductors -- this is normal and intentional.
(4) Parallel conductors -- when circuit conductors are paralleled (multiple conductors per phase in separate conduits), NEC 250.122(F) requires a full-size EGC in each parallel raceway -- the EGC is never divided or paralleled.
A 3000 A feeder with six conduits, each carrying one 500 kcmil conductor per phase, requires a 400 kcmil Cu EGC in every one of the six conduits.
This ensures any single conduit containing a fault has the full EGC capacity to clear it.
(5) Flexible cords and portable equipment -- cord-connected tools and appliances have an EGC as the green conductor in the cord.
The EGC in flexible cords is sized per NEC Table 400.5(A)(1), which considers the finer stranding and higher temperature rating of cord insulation.
(6) Separately derived systems -- a transformer secondary EGC from the transformer to the first overcurrent device is sized per 250.122 based on the secondary OCPD rating.
However, the system bonding jumper (connecting X0 to the case and to the grounding electrode conductor) is sized per Table 250.102(C)(1), which is based on the secondary conductor size, not the OCPD -- a critical distinction that when confused results in undersized bonding jumpers.
(7) Data centers and IT equipment -- the EGC for rack PDUs must handle high-frequency fault current components from UPS and switch-mode power supplies.
While Table 250.122 provides the minimum, some data center specifications require upsized EGCs on branch circuits serving IT equipment to provide a lower impedance path for common-mode noise.
(8) PV systems -- NEC 690.45 specifies EGC sizing for photovoltaic source and output circuits, referencing Table 250.122 but with the maximum OCPD rating from the PV system's Isc (short-circuit current), not the inverter continuous current.
Real-World Usage Scenarios
Voltage drop upsizing triggers EGC increase on a remote pump station
A water utility installs a 50 HP 460V submersible well pump 750 ft from the control panel. The phase conductors require #4/0 AWG copper (350 kcmil ampacity) for voltage drop control, far above the #4 AWG minimum for the 65 A motor load. The original EGC per Table 250.122 for the 150 A motor OCPD is #6 AWG Cu (26,240 CM). NEC 250.122(B) requires proportional upsizing: the phase conductor increase ratio is (#4/0 at 211,600 CM) / (#4 at 41,740 CM) = 5.07. Required EGC = 26,240 x 5.07 = 133,000 CM. The standard EGC is therefore #2/0 AWG (133,100 CM). The electrician who originally installed #6 EGC on this long run must be corrected -- a fault at 750 ft on the undersized #6 EGC may not clear fast enough to prevent conductor melting and loss of the fault path, leaving the well casing energized.
Parallel feeder EGC application in a data center generator bus
A 2500 kW standby generator feeds a 4000 A bus via four parallel conduits, each with three 600 kcmil copper phase conductors. The electrical engineer calculates the EGC: per NEC Table 250.122, a 4000 A OCPD requires a 400 kcmil copper EGC. Per NEC 250.122(F)(1), each of the four parallel conduits must contain a full 400 kcmil EGC -- the EGC cannot be divided. The contractor proposes running two conduits with #4/0 EGC in each, arguing that two #4/0 in parallel equal 423,200 CM against the required 400,000 CM. The engineer correctly rejects this: paralleling EGCs is prohibited because if one conduit is damaged, the remaining EGC would be undersized. All four conduits get 400 kcmil EGCs, adding approximately $7,500 to the installation cost compared to the contractor's proposed shortcut.
Existing installation EGC verification during industrial facility audit
A food processing plant undergoes an arc flash hazard assessment, requiring verification of all equipment grounding paths. A 300 HP 460V mixer motor fed from a 600 A breaker has been operational for 15 years. The inspector opens the motor junction box and finds a #4 AWG copper EGC. Per Table 250.122, a 600 A OCPD requires a #1 AWG copper EGC -- the existing #4 is three sizes too small. Further investigation reveals the phase conductors were upsized from #4/0 to 350 kcmil for a 280 ft run, but the EGC was never proportionally increased. The facility manager faces a choice: pull a new, code-compliant EGC through the existing conduit (challenging with existing conductors), run a separate EGC outside the conduit (prohibited by 250.134(B)), or install new conduit and conductors. The production line shutdown cost for the replacement is estimated at $32,000 -- versus the potential liability of an uncleared ground fault causing an arc flash incident or electrocution.
Common Mistakes to Avoid
Confusing EGC (Table 250.122) with GEC (Table 250.66)
The EGC is the 'green wire' in every circuit, sized by the OCPD rating. The GEC connects the electrical system neutral/ground bus to the grounding electrode system (ground rods, concrete-encased electrode) and is sized by the largest service entrance conductor per Table 250.66. A common error on residential services: a 200 A service with #2/0 copper service entrance conductors requires a #4 AWG copper GEC per Table 250.66. Meanwhile, the 200 A main breaker requires only a #6 AWG copper EGC per Table 250.122. Using Table 250.122 for the ground rod wire (needing only #6) instead of Table 250.66 (needing #4) results in an undersized GEC that may fuse during a lightning strike or primary-to-secondary fault on the utility transformer. The GEC carries high-energy events; the EGC carries branch-circuit-level faults. Different requirements for different purposes.
Running the EGC outside the conduit or cable
NEC 250.134(B) and 300.3(B) require all conductors of a circuit, including the EGC, to be contained within the same raceway, cable, or trench. Running a separate EGC outside the conduit -- for example, attaching a #6 bare copper wire to the outside of an EMT run because the electrician forgot to pull it through -- creates a choke coil during ground faults. The fault current flowing in the phase conductor inside the steel conduit produces a magnetic field that induces a counter-voltage in the external EGC, increasing its impedance by 10-50 times. A fault that would have cleared in 0.02 seconds on a properly contained EGC may take 0.5 seconds or longer on an external EGC -- during which time the conduit is energized at line voltage. This has caused documented electrocution fatalities where workers touched a 'grounded' conduit that was actually floating at 277V.
Using conduit as the EGC without verifying continuity in modified installations
NEC 250.118 recognizes EMT, IMC, RMC, and other metallic raceways as equipment grounding conductors -- but only when properly installed with tight, wrench-tight fittings. In older facilities, corrosion at coupling joints, paint overspray on threads, loose locknuts after vibration, and dielectric breaks from PVC expansion joints or flexible metal conduit sections can interrupt the conduit's grounding path. Before relying on conduit as the sole EGC, test continuity with a low-resistance ohmmeter (ideally under 1 ohm end-to-end). Many industrial facilities now specify a separate wire-type EGC in all conduits regardless of conduit type, even though the NEC permits reliance on the conduit alone. This 'belt and suspenders' approach costs approximately 5% more in conductor material but provides a redundant ground-fault path and simplifies ground-fault coordination studies.
Industry Standards Referenced
Frequently Asked Questions
What size ground wire for a 100 amp service?
Per NEC Table 250.122, a 100 A overcurrent device requires a #8 AWG copper equipment grounding conductor (EGC) or #6 AWG aluminum. This is for the EGC inside the conduit or cable -- not the grounding electrode conductor (ground rod wire), which is sized per NEC Table 250.66 based on the service entrance conductor size (typically #8 Cu for 100 A residential service). For a 200 A service, the EGC is #6 Cu and the GEC is #4 Cu for #2/0 service entrance conductors -- the two values diverge significantly at higher capacities.
Do I need to upsize the ground wire if I upsize for voltage drop?
Yes. NEC 250.122(B) requires that when ungrounded (phase) conductors are increased in size, the EGC must be proportionally increased. The ratio = (new phase conductor CM) / (minimum required phase conductor CM). Multiply the original EGC CM by this ratio to get the required upsized EGC, then select the next standard AWG. This ensures the EGC impedance remains low enough to clear a fault at the end of a long run within the breaker's instantaneous trip time. This rule is not optional -- it is mandatory and is a common item in electrical plan review checklists. Failure to upsize creates a scenario where the longer circuit reduces fault current (higher impedance), which increases the OCPD trip time, which increases the thermal energy the undersized EGC must dissipate -- a spiral that can result in the EGC melting before the breaker opens.
What is the difference between EGC, GEC, and bonding conductor?
EGC (Equipment Grounding Conductor): the 'green wire' that connects equipment enclosures and receptacle grounds back to the panel ground bus. Sized per NEC 250.122 based on OCPD rating. GEC (Grounding Electrode Conductor): connects the electrical system neutral/ground bus to the grounding electrode (ground rods, Ufer ground, water pipe electrode). Sized per NEC 250.66 based on the largest service entrance conductor size. Bonding conductor: connects metal parts together to ensure they're at the same potential (e.g., bonding water pipes, gas pipes, structural steel to the grounding system). Sized per NEC 250.102(C)(1) based on the phase conductor size. Different conductors, different sizing tables, different purposes -- don't confuse them.
Can I use the metal conduit as the equipment ground?
Yes, NEC 250.118 recognizes EMT, IMC, RMC, and galvanized rigid steel conduit as equipment grounding conductors when properly installed with tight fittings. However, there are important caveats: (1) flexible metal conduit (FMC, 'Greenfield') and liquid-tight flexible metal conduit (LFMC) have limitations -- FMC over 6 ft and LFMC over 6 ft for 20 A circuits or less require an internal wire-type EGC. (2) Aluminum conduit cannot be used as an EGC when embedded in concrete or in direct contact with earth (corrosion risk). (3) Any painted, corroded, or loose fittings can break the ground path -- testing end-to-end continuity with a low-resistance meter is essential before relying on conduit as the sole ground. Many industrial specs now require a wire-type EGC in all conduits as a redundant safety measure, regardless of NEC permission to use the conduit alone. For PVC conduit, a wire-type EGC is always mandatory -- PVC is non-conductive.
How is the EGC sized for parallel conductor installations?
Per NEC 250.122(F), when phase conductors are run in parallel in two or more raceways or cables, each parallel raceway or cable must contain a full-size EGC sized per Table 250.122 based on the total OCPD rating. You do NOT divide the EGC among the parallel runs. For example, a 3000 A feeder run in five parallel 4-inch conduits, each with one 600 kcmil Cu per phase: Table 250.122 requires a 400 kcmil Cu EGC for the 3000 A OCPD, and ALL FIVE conduits must each contain a full 400 kcmil Cu EGC. The reason: a ground fault in any one conduit involves only that conduit's phase conductors, and it must be cleared by the EGC in that same conduit. Paralleling smaller EGCs across conduits would not provide the required fault-clearing capacity in the faulted conduit.
Does the EGC have to be green or can I use a bare conductor?
NEC 250.119 permits both bare and green-insulated copper conductors as EGCs in most installations. Bare copper is standard in NM-B (Romex) cable and in many industrial conduit installations where cost is prioritized. Green-insulated copper (or green with yellow stripe for isolated grounds) is required in: (1) flexible cords and cables where more than one EGC may be present (e.g., isolated ground circuits for sensitive electronics per NEC 250.146(D)), (2) where the EGC is larger than #6 AWG and in a raceway where bare could be confused with a neutral or other conductor, and (3) in healthcare facilities per NEC 517.13 where redundant grounding is required. The green color is reserved exclusively for the EGC -- using green for any other purpose (e.g., phase conductor, switch leg) is a code violation that can cause a fatal misidentification during maintenance.
Reviewed for accuracy
Reviewed against NEC 2023 Article 250 (Grounding and Bonding), NEC Table 250.122, and NFPA 70 · Last reviewed: July 26, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.