What the equipment grounding conductor has to do
The equipment grounding conductor has one job: carry fault current back to the source fast enough and in enough quantity to open the overcurrent device. It is not there to carry load current, and it is not the same thing as the grounding electrode conductor that connects the system to the earth. When a hot conductor touches a metal enclosure, the EGC is the low-impedance path that turns that contact into a short circuit the breaker can see, rather than an energised enclosure waiting for someone to touch it.
That job explains why the code sizes it from the overcurrent device rating rather than from the load. The device rating is what sets the current the fault path has to deliver and the time it will be delivered for. A 200 A breaker on a lightly loaded feeder still needs a grounding conductor able to carry a 200 A-class fault long enough for the breaker to clear, so Table 250.122 is indexed by device rating alone.
The table is also a minimum, not a design. NEC 250.4(A)(5) requires the fault path to have impedance low enough to facilitate operation of the overcurrent device, and on very long runs the table size may not achieve that. Where the run is long, the conductor may have to be larger than the table for the same reason a feeder gets upsized for voltage drop — impedance. Table 250.122 does not model length at all.
Finally, the EGC is only one of the permitted equipment grounding means. NEC 250.118 lists the others: rigid and intermediate metal conduit, EMT, listed flexible metal conduit within limits, cable armour and metal raceways all qualify under stated conditions. The wire-type conductor this calculator sizes is required when none of those is used, and is commonly installed anyway.
The proportional upsizing rule that catches everyone
NEC 250.122(B) is the rule most often missed on a plan review. It says that where the ungrounded conductors are increased in size from the minimum that has sufficient ampacity, a wire-type equipment grounding conductor, where installed, must be increased in size proportionately to the increase in circular mil area of the ungrounded conductors.
Read that carefully, because two details matter. First, the comparison is against the minimum size with sufficient ampacity, not against the size in Table 310.16 for that breaker, and not against the last job's size. You must know what the smallest compliant phase conductor would have been. Second, the ratio is in circular mils, which is area, not in AWG steps. Going up one AWG size is a factor of 1.2610 in area; going up two is 1.5900; three is 2.0050.
The most common trigger is voltage drop. A 200 A feeder might need 3/0 copper for ampacity, but 400 feet of run pushes you to 4/0. The moment you make that change, the required grounding conductor becomes 26,240 × (211,600/167,800) = 33,089 circular mils. That is more than 6 AWG's 26,240 and less than 4 AWG's 41,740, and since the requirement is a minimum you must take the larger: 4 AWG. The rule pushed the ground up a full trade size for what looked like a modest change to the phases.
Two limits keep this bounded. NEC 250.122(A) provides that the grounding conductor is not required to be larger than the circuit conductors themselves, which caps runaway results. And the rule applies to wire-type grounding conductors; where a metal raceway serves as the equipment grounding conductor under 250.118 there is no wire to enlarge. Parallel installations have their own treatment in 250.122(F), where a full-size EGC is generally required in each raceway.
Worked example: a 200 A feeder upsized for voltage drop
A 200 A feeder to a detached workshop runs 380 feet. The ampacity calculation permits 3/0 copper; the voltage-drop calculation pushes it to 4/0. What size copper equipment grounding conductor is required?
- Base size from the table. Table 250.122 for a 200 A device gives 6 AWG copper. From Chapter 9 Table 8, 6 AWG is 26,240 circular mils.
- Circular mils of each phase option. 3/0 is 167,800 cmil; 4/0 is 211,600 cmil.
- Upsizing ratio. 211,600 ÷ 167,800 = 1.261025. The phases grew by 26.1% in area.
- Increase the EGC in the same proportion. 26,240 × 1.261025 = 33,089 circular mils.
- Pick a standard size. 4 AWG is 41,740 cmil, which exceeds 33,089. The next size down, 6 AWG at 26,240, does not. So the answer is 4 AWG copper.
- Check the 250.122(A) cap. 41,740 cmil is far smaller than the 211,600 cmil phase conductors, so the cap does not bind.
Notice how much the answer moved. A single AWG step on the phase conductors forced a single AWG step on the ground, from 6 AWG to 4 AWG, because 26,240 × 1.261 lands above 26,240 and there is nothing between 6 AWG and 4 AWG in the standard series. The ratio only had to exceed 1.0000 for the size to change, since 6 AWG was already exactly at the requirement.
Contrast a case where the base and the required phase size coincide. A 20 A circuit takes 12 AWG copper phases and, from the table, a 12 AWG copper ground — both 6,530 cmil. Upsize the phases to 8 AWG (16,510 cmil) for a long run and the ratio is 16,510/6,530 = 2.528330. Multiply the ground: 6,530 × 2.528330 = 16,510 cmil, which is exactly 8 AWG. Whenever the table's base EGC happens to equal the minimum phase conductor, the proportional rule always lands precisely on the installed phase size — the algebra is base × (installed/required) with base equal to required, which cancels to installed.
Reading the result and the checks it does not perform
Take the named size, not the circular-mil figure, to the wholesaler. The calculator reports both because the code arithmetic is done in circular mils while conductors are sold by AWG and kcmil, and rounding in the wrong direction between them is a real source of error. Always round up to the next standard size; there is no allowance for rounding down to the nearest, however close.
Check that the size you land on can actually be terminated. A 4 AWG grounding conductor in a device box needs a lug or a listed connector rated for it, and it takes real space — the box fill calculator handles the 5.00 in³ allowance a 6 AWG conductor claims and the corresponding effect on the enclosure. In raceway, the enlarged grounding conductor counts towards conduit fill even though it never counts as a current-carrying conductor for ampacity derating.
Understand what the table does not consider. It does not consider run length, so a very long circuit may need a larger conductor to satisfy the effective-ground-fault-current-path requirement of 250.4(A)(5). It does not consider available fault current or the clearing time of the specific device, which is what an engineered study would evaluate. And it does not apply to motor circuits protected by an instantaneous-trip breaker in the way you might expect: NEC 250.122(D) sizes those from the rating of the branch-circuit short-circuit and ground-fault protective device with reference to the motor overload protection, so check that section rather than assuming the table row for the breaker.
Finally, remember that the grounding conductor is not a neutral. Load current must never be carried on the EGC in normal operation, and the separation of the grounded conductor from the grounding conductor downstream of the service is what makes that true. Bonding them anywhere but at the service or at a separately derived system puts normal load current on metal parts.
NEC Table 250.122 minimum equipment grounding conductor sizes
| Device rating (A) | Copper | Copper (cmil) | Aluminium | Aluminium (cmil) |
|---|---|---|---|---|
| 15 | 14 AWG | 4,110 | 12 AWG | 6,530 |
| 20 | 12 AWG | 6,530 | 10 AWG | 10,380 |
| 60 | 10 AWG | 10,380 | 8 AWG | 16,510 |
| 100 | 8 AWG | 16,510 | 6 AWG | 26,240 |
| 200 | 6 AWG | 26,240 | 4 AWG | 41,740 |
| 300 | 4 AWG | 41,740 | 2 AWG | 66,360 |
| 400 | 3 AWG | 52,620 | 1 AWG | 83,690 |
| 500 | 2 AWG | 66,360 | 1/0 AWG | 105,600 |
| 600 | 1 AWG | 83,690 | 2/0 AWG | 133,100 |
| 800 | 1/0 AWG | 105,600 | 3/0 AWG | 167,800 |
| 1000 | 2/0 AWG | 133,100 | 4/0 AWG | 211,600 |
| 1200 | 3/0 AWG | 167,800 | 250 kcmil | 250,000 |
| 1600 | 4/0 AWG | 211,600 | 350 kcmil | 350,000 |
| 2000 | 250 kcmil | 250,000 | 400 kcmil | 400,000 |
| 2500 | 350 kcmil | 350,000 | 600 kcmil | 600,000 |
| 3000 | 400 kcmil | 400,000 | 600 kcmil | 600,000 |
| 4000 | 500 kcmil | 500,000 | 800 kcmil | 800,000 |
A device rating between two rows takes the next row up: a 30 A breaker uses the 60 A row and a 150 A breaker uses the 200 A row. The rating is that of the overcurrent device, not the calculated load.
Where grounding conductor sizing goes wrong
- Sizing from the load instead of the device. Table 250.122 is indexed by the rating of the overcurrent device. A 120 A load behind a 200 A breaker takes the 200 A row.
- Missing the proportional increase. Any time the phase conductors are larger than the ampacity minimum — for voltage drop, for future capacity, for standardisation across a job — 250.122(B) applies to a wire-type grounding conductor.
- Applying the ratio in AWG steps instead of circular mils. One AWG step is a factor of 1.2610 in area, not 1.0. Working in gauge numbers gives the wrong ratio every time.
- Rounding to the nearest standard size. The calculated area is a minimum. 33,089 cmil requires 4 AWG, not 6 AWG, even though 6 AWG is closer in AWG terms.
- Forgetting the 250.122(A) cap. The grounding conductor is not required to exceed the circuit conductors, which matters on heavily upsized small circuits.
- Assuming the raceway does the job. Metal raceways qualify as equipment grounding conductors only under the conditions listed in 250.118, including length and size limits for flexible types. Where they do not qualify, a wire-type conductor is required.
- Treating a motor circuit like a general circuit. NEC 250.122(D) has its own rule for motor circuits protected by instantaneous-trip breakers, and the branch-circuit protection there is sized for inrush rather than for the conductor — see the motor overload and breaker sizing calculator.
- Ignoring run length. The table has no length term. NEC 250.4(A)(5) still requires a fault path of low enough impedance to operate the device, which on very long runs can demand more than the table.
Key terms
- Equipment grounding conductor (EGC)
- The conductive path installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor or the grounding electrode conductor, providing the return path for fault current.
- Grounding electrode conductor
- A different conductor entirely, connecting the system or equipment to the grounding electrode. It is sized under NEC 250.66, not 250.122.
- Circular mil
- The area of a circle one thousandth of an inch in diameter, equal to 5.067 × 10⁻⁴ mm². The unit the NEC uses for proportional conductor comparisons.
- Effective ground-fault current path
- The intentionally constructed low-impedance path designed to carry fault current from the point of a fault back to the source, required by NEC 250.4(A)(5) to facilitate operation of the overcurrent device.
How this fits with the rest of Article 250
Article 250 contains several conductors that people conflate. The equipment grounding conductor, sized here from 250.122, bonds equipment enclosures and carries fault current. The grounding electrode conductor, sized from 250.66, connects the system to the earth and does not carry fault current in normal operation. The main bonding jumper at the service, sized from 250.28(D), joins the grounded conductor to the enclosure and is the reason fault current has anywhere to go. The supply-side bonding jumper for a separately derived system follows 250.30. Using the wrong table gives a conductor that is legal for a different purpose.
Two supporting calculations usually run alongside this one. The conductor ampacity itself comes from Table 310.16 with the correction and adjustment factors of 310.15 — that is the ampacity derating calculator, and the “minimum required” size in the upsizing ratio is exactly its output. Voltage drop, which is what usually triggers the upsizing in the first place, is the voltage drop calculator. Between them they tell you the two circular-mil figures the ratio needs.
For paralleled sets, NEC 250.122(F) generally requires a full-size wire-type equipment grounding conductor in each raceway or cable, sized from the device rating rather than divided among the sets. That is a common and expensive surprise on large services, and it is deliberate: each raceway must be able to clear a fault on its own.
Finally, keep in mind that this is a code minimum for a listed installation. Where the equipment is sensitive, where the available fault current is very high, or where the run is unusually long, an engineered ground-fault path study can call for more. The AWG to mm² converter is the tool for translating any of these sizes into a metric specification for imported equipment.