Why is this calculator's result not a wire gauge?

Because NEC Table 250.122 sizes equipment grounding conductors at specific breakpoints, not by a continuous formula, and a proportional calculation like this one cannot produce an actual wire gauge. The breakpoints run, for example, 60A→10 AWG copper, 100A→8 AWG copper, 200A→6 AWG copper and 400A→3 AWG copper; this calculation only illustrates how sizing roughly scales with OCPD rating.

Can I use this result to select my actual grounding conductor?

No. Always look up the required size directly from NEC Table 250.122 (or your local code) for your actual OCPD rating — never use a proportional or interpolated calculation for the real conductor size.

What determines the equipment grounding conductor size in the real code table?

It's set by the rating of the automatic overcurrent device (fuse or breaker) ahead of the circuit — larger OCPD ratings require larger grounding conductors, stepping up at the specific breakpoints defined in NEC Table 250.122.

Does this account for conductors upsized for voltage drop, or for a feeder run as parallel sets?

No to both. The page multiplies two numbers and has no field for conductor size, raceway count or conductor material, so none of the rules that sit around the table can be applied here — and two of them change the answer regularly. Where the ungrounded conductors have been increased beyond the size their own ampacity would need, most often to hold voltage drop on a long run, NEC 250.122(B) requires a wire-type equipment grounding conductor to be increased in proportion to the circular-mil increase, so the table row is a floor rather than the finished size. Where a feeder runs as parallel sets in separate raceways, NEC 250.122(F) puts an equipment grounding conductor in each raceway, each one sized on the same device rating; the requirement is not shared out between them. In the opposite direction, 250.122(A) does not require the grounding conductor to be larger than the circuit conductors supplying the equipment. Past that, the point where this stops being a table lookup at all is when the ground-fault path has to be shown to carry the available fault current and open the device fast enough — feeder and service work, or anything with a fault-current study behind it. The table row still sets the minimum there; what a study can do is call for more, and that judgement belongs to the licensed electrician or engineer of record rather than to this page.

Where does the 0.0225 sizing factor come from, and what belongs in the OCPD field?

The 0.0225 is not a code value and carries no unit — it is a scaling constant chosen so the output moves with device rating, and the field accepts anything from 0.015 to 0.03. On a 100 A breaker that range returns 1.5 at one end and 3.0 at the other: a two-to-one spread produced by a field with no physical meaning, against a code table that gives one required size at 100 A for each conductor material. Read the output as a relative indicator and nothing more. In the other field, enter the rating of the fuse or breaker ahead of the circuit — not the load current, not the conductor ampacity, not the equipment nameplate. A 60 A feeder serving a 40 A load is a 60 A entry, because the table is indexed on the protective device. Two limits are worth knowing before you trust the shape of the curve: the field steps in 5 A and will accept numbers that are not standard overcurrent device ratings, and it stops at 800 A while the code table continues well beyond that. There is also no copper or aluminum choice on this page, and the aluminum entries in the real table are larger than the copper ones.

The breaker was upsized after the wire was pulled — does the grounding conductor have to change?

It has to be re-checked every time, and often the answer is yes — this is one of the quietest ways an existing installation stops complying. Because the table is indexed on the protective device, swapping a 100 A breaker for a 200 A one during a panel change or an equipment upgrade moves the required grounding conductor up with it. A breaker can only be increased in the first place where the ungrounded conductors already have the ampacity for the larger device, and where they do — because they were pulled oversized — the grounding conductor sharing that raceway is the part that gets forgotten. The reverse mistake turns up on retrofit work, where a small green conductor gets pulled alongside a much larger circuit on the reasoning that it never carries current. It carries all of it for the fraction of a second that matters — the fault current, for as long as the device takes to open — and undersizing it raises the impedance of that path, which can slow the clearing of the one fault it exists for. When ordering, treat it as a full-length conductor in every raceway, parallel sets included; it counts toward conduit fill and box fill, and it is commonly worth pulling a wire-type conductor rather than leaning on the raceway itself where the run includes flex, expansion fittings or painted enclosures, which is where a metal fault path tends to turn high-impedance.

Materials & Quantities

Equipment Grounding Conductor Sizing Reference Calculator

Estimate a proportional equipment grounding conductor size from the circuit's overcurrent device rating.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Overcurrent Protective Device Rating (A)

The rating of the fuse or circuit breaker protecting the circuit ahead of the equipment grounding conductor.

The device ahead of the conductor, not the load it serves and not the conductor's own ampacity. Grounding conductors are sized from the protective device because their job is to carry fault current long enough to open it. One consequence gets missed often: where phase conductors are upsized for voltage drop, the grounding conductor has to be upsized in proportion — the fault path has to keep up with the run it protects.

Sizing Factor (Illustrative Scaling Constant)

An illustrative scaling constant used only to produce a rough proportional estimate — not a code value.

This is a proportionality knob for a rough figure, and the result it produces is NOT a code-compliant size. Grounding conductors are sized from a published table against the protective device rating, not from a formula, and that table is not linear. Use this to see roughly how the size moves with the device, then take the actual size from the table that applies where the work is.

Approximate grounding conductor sizing factor

2.3 (illustrative scaling units)

Low confidence

This proportional calculation is illustrative ONLY — NEC Table 250.122 defines equipment grounding conductor size using specific breakpoints (e.g. 60A→10AWG copper, 100A→8AWG copper, 200A→6AWG copper, 400A→3AWG copper), NOT a continuous formula. Always look up the required size directly from NEC Table 250.122 (or your local code) for the actual OCPD rating — never use a proportional/interpolated calculation for the real conductor size.

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

NEC Table 250.122 sets equipment grounding conductor size based on the rating of the automatic overcurrent device ahead of the circuit — sizes step up at defined breakpoints rather than scaling continuously. This calculator applies a simplified proportional approximation (OCPD rating × a scaling factor) as a rough estimate only; the actual required size MUST be taken directly from NEC Table 250.122, not calculated proportionally.

Inputs used

Overcurrent Protective Device Rating (A): 100
Sizing Factor (Illustrative Scaling Constant): 0.02

Final result2.25 (illustrative scaling units)

Confidence note: This proportional calculation is illustrative ONLY — NEC Table 250.122 defines equipment grounding conductor size using specific breakpoints (e.g. 60A→10AWG copper, 100A→8AWG copper, 200A→6AWG copper, 400A→3AWG copper), NOT a continuous formula. Always look up the required size directly from NEC Table 250.122 (or your local code) for the actual OCPD rating — never use a proportional/interpolated calculation for the real conductor size.

What this calculation does not cover

One conductor of several that get confused with it. Table 250.122 sizes the equipment grounding conductor — the one running with the circuit back to the panel. The grounding electrode conductor out to the rod or the concrete-encased electrode comes from 250.66 off the service-entrance conductors, the main and system bonding jumpers from 250.28(D), and a supply-side bonding jumper from 250.102(C). Every one of those keys off conductor size rather than device rating, so entering a breaker rating for any of them returns an answer from the wrong table entirely.
The overcurrent device rating is the index in the NEC and almost nowhere else. BS 7671 and the IEC-derived codes size the protective conductor from the LINE conductor — a table keyed to the phase cross-section, or the adiabatic equation run against the prospective fault current and the device's let-through energy — so the same circuit is sized on a different quantity in most of the world. A reader outside the United States will not find the breaker rating in the method at all.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

NEC Table 250.122 sets equipment grounding conductor size based on the rating of the automatic overcurrent device ahead of the circuit — sizes step up at defined breakpoints rather than scaling continuously. This calculator applies a simplified proportional approximation (OCPD rating × a scaling factor) as a rough estimate only; the actual required size MUST be taken directly from NEC Table 250.122, not calculated proportionally.

Which documents these citations point at

National Electrical Code (NFPA 70) — Table 250.122 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Earthing and Bonding a Serviceuses this calculator
Earthing judged the only way it can be — by the ohms you measure, and by which bonds still conduct when a meter is put across them.
Installing Cable Tray and Containmentuses this calculator
Cable tray comes down to two numbers — how full the section is and how far apart it is held — and every field decision trades one against the other.
Fitting a Consumer Unit

Called something else where you work? Consumer unit / breaker panel · Earthing and grounding — the term in each market, how close the equivalence really is, and the standard that governs it.

Worked example: Electric vehicle charger circuit — step 8 of 9

The data behind it: Copper conductor sizes and ampacity

How to calculate equipment grounding conductor sizing reference in 3 steps

Overcurrent Protective Device Rating (A)The rating of the fuse or circuit breaker protecting the circuit ahead of the equipment grounding conductor.
Sizing Factor (Illustrative Scaling Constant)An illustrative scaling constant used only to produce a rough proportional estimate — not a code value.
Approximate grounding conductor sizing factorThe tool computes the approximate grounding conductor sizing factor from those figures and shows the formula, its sources, and a confidence rating alongside it.

Approximate grounding conductor sizing factor by overcurrent protective device rating (A)

Page defaults, not your figures above.

Overcurrent Protective Device Rating (A)	Approximate grounding conductor sizing factor ((illustrative scaling units))
20	0.45
50	1.13
100	2.25
200	4.5
500	11.3

Frequently asked questions

Why is this calculator's result not a wire gauge?: Because NEC Table 250.122 sizes equipment grounding conductors at specific breakpoints, not by a continuous formula, and a proportional calculation like this one cannot produce an actual wire gauge. The breakpoints run, for example, 60A→10 AWG copper, 100A→8 AWG copper, 200A→6 AWG copper and 400A→3 AWG copper; this calculation only illustrates how sizing roughly scales with OCPD rating.
Can I use this result to select my actual grounding conductor?: No. Always look up the required size directly from NEC Table 250.122 (or your local code) for your actual OCPD rating — never use a proportional or interpolated calculation for the real conductor size.
What determines the equipment grounding conductor size in the real code table?: It's set by the rating of the automatic overcurrent device (fuse or breaker) ahead of the circuit — larger OCPD ratings require larger grounding conductors, stepping up at the specific breakpoints defined in NEC Table 250.122.
Does this account for conductors upsized for voltage drop, or for a feeder run as parallel sets?: No to both. The page multiplies two numbers and has no field for conductor size, raceway count or conductor material, so none of the rules that sit around the table can be applied here — and two of them change the answer regularly. Where the ungrounded conductors have been increased beyond the size their own ampacity would need, most often to hold voltage drop on a long run, NEC 250.122(B) requires a wire-type equipment grounding conductor to be increased in proportion to the circular-mil increase, so the table row is a floor rather than the finished size. Where a feeder runs as parallel sets in separate raceways, NEC 250.122(F) puts an equipment grounding conductor in each raceway, each one sized on the same device rating; the requirement is not shared out between them. In the opposite direction, 250.122(A) does not require the grounding conductor to be larger than the circuit conductors supplying the equipment. Past that, the point where this stops being a table lookup at all is when the ground-fault path has to be shown to carry the available fault current and open the device fast enough — feeder and service work, or anything with a fault-current study behind it. The table row still sets the minimum there; what a study can do is call for more, and that judgement belongs to the licensed electrician or engineer of record rather than to this page.
Where does the 0.0225 sizing factor come from, and what belongs in the OCPD field?: The 0.0225 is not a code value and carries no unit — it is a scaling constant chosen so the output moves with device rating, and the field accepts anything from 0.015 to 0.03. On a 100 A breaker that range returns 1.5 at one end and 3.0 at the other: a two-to-one spread produced by a field with no physical meaning, against a code table that gives one required size at 100 A for each conductor material. Read the output as a relative indicator and nothing more. In the other field, enter the rating of the fuse or breaker ahead of the circuit — not the load current, not the conductor ampacity, not the equipment nameplate. A 60 A feeder serving a 40 A load is a 60 A entry, because the table is indexed on the protective device. Two limits are worth knowing before you trust the shape of the curve: the field steps in 5 A and will accept numbers that are not standard overcurrent device ratings, and it stops at 800 A while the code table continues well beyond that. There is also no copper or aluminum choice on this page, and the aluminum entries in the real table are larger than the copper ones.
The breaker was upsized after the wire was pulled — does the grounding conductor have to change?: It has to be re-checked every time, and often the answer is yes — this is one of the quietest ways an existing installation stops complying. Because the table is indexed on the protective device, swapping a 100 A breaker for a 200 A one during a panel change or an equipment upgrade moves the required grounding conductor up with it. A breaker can only be increased in the first place where the ungrounded conductors already have the ampacity for the larger device, and where they do — because they were pulled oversized — the grounding conductor sharing that raceway is the part that gets forgotten. The reverse mistake turns up on retrofit work, where a small green conductor gets pulled alongside a much larger circuit on the reasoning that it never carries current. It carries all of it for the fraction of a second that matters — the fault current, for as long as the device takes to open — and undersizing it raises the impedance of that path, which can slow the clearing of the one fault it exists for. When ordering, treat it as a full-length conductor in every raceway, parallel sets included; it counts toward conduit fill and box fill, and it is commonly worth pulling a wire-type conductor rather than leaning on the raceway itself where the run includes flex, expansion fittings or painted enclosures, which is where a metal fault path tends to turn high-impedance.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.