About Grounding Conductor Size Calculator (NEC 250.66 & 250.122)
The grounding conductor size calculator answers the two sizing questions the NEC keeps in separate tables. In GEC mode, pick the largest ungrounded service-entrance conductor — metal and size, with a conductors-per-phase count for parallel sets — and the tool reads NEC 2023 Table 250.66 to give the grounding electrode conductor in both copper and aluminum. In EGC mode, enter the rating of the circuit's overcurrent protective device and the tool reads Table 250.122, rounding up to the next table row when your rating falls between rows, again in both metals.
EGC mode also handles the rule most charts skip: NEC 250.122(B). When the ungrounded conductors are increased in size for a reason other than ampacity — long-run voltage drop is the classic case — the equipment grounding conductor must be increased proportionately by circular-mil area. The calculator takes the size the load required and the size actually installed, scales the table EGC by that Chapter 9 Table 8 area ratio, and rounds up to the next standard conductor size.
This tool covers the common cases. Table 250.66's reliefs in 250.66(A)–(C) — a GEC that is the sole connection to a rod, pipe, or plate electrode never needs to be larger than 6 AWG copper, a concrete-encased electrode caps at 4 AWG copper, and a ground ring follows the ring conductor — plus installation conditions and local amendments can change the answer, so the result is a code-table starting point, not a substitute for the full Article 250 check.
How It Works
- Pick the mode. GEC (grounding electrode conductor) sizes the wire from the service to the electrode system per Table 250.66; EGC (equipment grounding conductor) sizes the fault-return wire run with the circuit conductors per Table 250.122.
- For a GEC, select the service-conductor metal and the largest ungrounded service-entrance conductor. For parallel sets, set conductors per phase — the table note works on the equivalent circular-mil area of the set, so two 3/0 copper conductors count as 335,600 cmil.
- For an EGC, enter the OCPD rating in amperes (15–3000 A). Ratings between table rows round UP to the next row, exactly as 250.122(A) directs — a 45 A breaker uses the 60 A row.
- If the ungrounded conductors were upsized for voltage drop, toggle the 250.122(B) upsize and pick the size the load required and the size installed. The tool multiplies the table EGC's circular-mil area by the installed-to-required area ratio and rounds up to the next standard size from Chapter 9 Table 8.
- Read both answers: every result gives the copper and the aluminum/copper-clad-aluminum conductor, matching the two columns of the printed tables.
Worked Example
A 40 A branch circuit needs 8 AWG copper for the load, but the run is long, so 6 AWG copper is installed for voltage drop. Table 250.122 gives a 10 AWG copper EGC for a 40 A OCPD. Applying 250.122(B): the area ratio is 26,240 ÷ 16,510 = 1.589, so the EGC must have at least 10,380 × 1.589 = 16,498 cmil — the next standard size up is 8 AWG copper (16,510 cmil). The aluminum answer scales the same way: the table's 8 AWG aluminum (16,510 cmil) becomes 26,240 cmil, exactly 6 AWG. On the service side, a 350 kcmil copper service falls in Table 250.66's "over 3/0 through 350 kcmil" row, so the grounding electrode conductor is 2 AWG copper or 1/0 aluminum.
Formulas
- Equivalent area of a parallel service set (Table 250.66 note)
A_equiv = cmil(size) × conductors per phase- 250.122(B) proportional EGC upsize
A_EGC,new ≥ A_EGC,table × (A_installed / A_required), then next standard size up- OCPD row selection (250.122(A))
row = smallest table rating ≥ OCPD rating
Standards & References
- NEC 2023 (NFPA 70) Table 250.66 — grounding electrode conductor for AC systems, keyed to the largest ungrounded service-entrance conductor or equivalent parallel area; reliefs in 250.66(A)–(C) for rod/pipe/plate (max 6 AWG Cu), concrete-encased (max 4 AWG Cu), and ground-ring electrodes
- NEC 2023 (NFPA 70) Table 250.122 — minimum equipment grounding conductors by OCPD rating, with 250.122(B) proportional upsizing by circular-mil area
- NEC 2023 (NFPA 70) Chapter 9 Table 8 — conductor properties (circular-mil areas) used for the parallel equivalent area and the 250.122(B) ratio
Frequently Asked Questions
What size grounding electrode conductor do I need for a 200 A service?
A typical 200 A residential service uses 2/0 or 3/0 copper service-entrance conductors, which is Table 250.66's third row: a 4 AWG copper or 2 AWG aluminum grounding electrode conductor. If the GEC runs only to a ground rod, 250.66(A) caps it at 6 AWG copper regardless — that is why 6 AWG bare copper to the rods is so common.
What is the difference between a GEC and an EGC?
The grounding electrode conductor (GEC) connects the service's grounded conductor to the earth electrode system and is sized from the service-entrance conductors per Table 250.66. The equipment grounding conductor (EGC) is the fault-clearing path run with every circuit and is sized from the overcurrent device per Table 250.122. They answer different physics — the GEC handles lightning and surge gradients, the EGC must carry fault current long enough to trip the breaker.
When do I have to upsize the equipment grounding conductor?
Whenever the ungrounded conductors are made larger than ampacity requires — usually for voltage drop on a long run — 250.122(B) requires the EGC to grow by the same circular-mil ratio. Example: a 40 A circuit upsized from 8 AWG to 6 AWG copper (ratio 1.589) pushes the 10 AWG table EGC to 8 AWG. Without this rule the fault-loop impedance of a long run could keep the breaker from tripping fast.
My breaker rating is not a row in Table 250.122 — which row applies?
Use the next row whose rating is not less than the device. A 45 A or 50 A breaker uses the 60 A row (10 AWG copper); a 175 A device uses the 200 A row (6 AWG copper). The calculator does this rounding automatically and shows which row it matched.
How do parallel service conductors change the GEC size?
Table 250.66 works on the equivalent area of the set: add the circular-mil areas of the conductors on one phase and read the row that range lands in. Two 4/0 copper conductors per phase are 423,200 cmil — the "over 350 through 600 kcmil" row — so the GEC is 1/0 copper even though each individual conductor is only 4/0.
Why does the calculator stop at a 3000 A overcurrent device?
Published reproductions of Table 250.122 disagree on the aluminum cells of the 4000–6000 A rows, and services that size are engineered systems anyway. Rather than encode a cell we could not verify against the code text, the tool caps OCPD entry at 3000 A — above that, the design belongs to the electrical engineer of record.