Ground wire size calculator (NEC)

How to use the ground wire size calculator

Choose the NEC edition and what you are sizing. For an equipment grounding conductor (EGC), the wire that runs with a circuit back to the panel, enter the rating of the breaker or fuse ahead of the circuit, the EGC material, and the size of the phase conductors you are installing. If you made the phase conductors bigger than the ampacity required, for voltage drop or for future load, switch on the upsizing option and enter the size the ampacity alone called for. For parallel runs, enter the number of sets and whether each set has its own raceway or cable.

For a grounding electrode conductor (GEC), the conductor from the service to the ground rods, water pipe or footing electrode, enter the largest service-entrance conductor, the number of parallel sets, the materials, and the electrode it runs to. Each result shows the table row, any proportional increase, any upper limit that applies and the section numbers for your edition.

EGC, GEC and bonding jumpers

These conductors do different jobs, and the Code sizes each one differently. The equipment grounding conductor gives fault current a low-impedance path back to the source, so the breaker trips quickly when a hot conductor touches a metal enclosure. It is sized from the overcurrent device, because the device decides how much fault current flows before it opens. The grounding electrode conductor connects the system to the earth. It carries little fault current in normal service, so it is sized from the service conductors and capped when it runs only to rods or a footing. Bonding jumpers at the service, including the main bonding jumper, follow a third table, 250.102(C)(1); this calculator does not size those.

Sizing the EGC from the breaker

Table 250.122 lists the minimum EGC for each overcurrent device rating, in copper and in aluminum. A 20 A circuit needs 12 AWG copper, a 60 A circuit 10 AWG, a 100 A circuit 8 AWG and a 200 A circuit 6 AWG. The table values have not changed since 2020, when the aluminum entry for the 5000 A and 6000 A rows went from 1200 kcmil to 1250 kcmil so that it named a real Chapter 9 size. Two limits sit beside the table. The EGC never has to be larger than the circuit conductors (250.122(A)), which matters on motor circuits where the breaker is far larger than the wire. And a metal raceway or cable armor that qualifies as an EGC under 250.118 can do the job with no wire at all.

When the phase conductors are upsized

If you install phase conductors larger than the minimum that has the ampacity you need, the fault current still has to flow back through the EGC, and a long upsized run raises the impedance of that path. So the wire-type EGC must grow in the same proportion as the phase conductors’ circular-mil area:

A 200 A feeder that needs 3/0 AWG copper but is run in 4/0 AWG for voltage drop scales its 6 AWG EGC by 211,600 ÷ 167,800 to 33,089 cmil, which means 4 AWG. The rule is 250.122(B) through 2023 and 250.122(D) in 2026. Since 2020 it says plainly that an increase made only because of ambient correction or bundling adjustment does not count, and it lets a qualified person size the EGC to give an effective ground-fault path instead. The 2017 text did not spell out the exemption, so some inspectors applied it more broadly.

The borderline case, and our rounding policy

The proportional rule sets a minimum area, so the result has to round up to a standard size. Usually the answer is clear. Now and then the arithmetic lands a hair above a size. Take a 60 A circuit whose copper conductors go from 6 AWG to 4 AWG for voltage drop. The 10 AWG EGC scales to 10,380 × 41,740 ÷ 26,240 = 16,511 cmil, while 8 AWG is 16,510 cmil: 1.48 cmil short, or 0.009 %.

Read strictly, 8 AWG does not meet the minimum and the answer is 6 AWG. That is what this calculator gives. It then flags every case where the next smaller size misses by 0.5 % or less and explains the edge, because a shortfall that small is within the rounding of Table 8 itself and some inspectors will accept the smaller size. We never round down for you; if you want the smaller conductor, get the inspector’s agreement first.

Parallel runs

When a feeder runs in parallel sets, each in its own raceway or cable, every raceway gets its own EGC (250.122(F); 250.122(H) in 2026). Each one is sized from Table 250.122 on the full rating of the overcurrent device, not the rating divided by the number of sets, because a fault inside one raceway can send most of the fault current down that raceway’s EGC. Sets sharing one raceway share one EGC.

The 2026 edition adds a limit: the EGC in each raceway need not be larger than the largest ungrounded conductor in it. Eight sets of 1/0 copper on a 1200 A device need 8 × 3/0 AWG under NEC 2023 but 8 × 1/0 AWG under NEC 2026.

The grounding electrode conductor

Table 250.66 sizes the GEC from the largest ungrounded service-entrance conductor, or from the sum of the areas per phase when the service runs in parallel. Read the row in the column for the service conductors’ metal, then the GEC size for the GEC’s own metal. A 200 A service on 2/0 AWG copper needs a 4 AWG copper GEC. Where the GEC runs only to ground rods, pipes or plates, that part need not be larger than 6 AWG copper or 4 AWG aluminum (250.66(A)), so the same service can use 6 AWG copper to its rods; a GEC that runs only to a concrete-encased electrode stops at 4 AWG copper (250.66(B)), and one that runs only to a ground ring need not be larger than the ring conductor (250.66(C)).

Aluminum GECs may not touch masonry or the earth and may not run outdoors within 18 in. of the earth (250.64(A)). A GEC smaller than 6 AWG must be protected in a raceway or cable armor (250.64(B)).

Worked examples

Common mistakes

• Sizing the EGC from the conductor instead of the breaker. Table 250.122 is read by the overcurrent device rating.
• Dividing the rating across parallel sets. Each raceway’s EGC is sized on the full rating.
• Forgetting the proportional increase. Upsizing phase conductors for voltage drop without upsizing the EGC is a common inspection failure.
• Using Table 250.122 for the GEC. The GEC comes from Table 250.66 and the service conductors.
• Running a full-size GEC to rods. The part that serves only rods never needs more than 6 AWG copper.
• Rounding the proportional area down. The required area is a minimum; even 1 cmil over a size takes the next one.

Method and verification

The calculator reads Table 250.122 with its 2017 aluminum entries, applies the proportional rule with strict rounding, then the 250.122(A) and 2026 parallel limits. Table 250.66 rows are matched by circular-mil area so parallel sets add correctly. The tables were checked against at least two independent copies for each edition. Tests cover the borderline case above, the 200 A feeder example, parallel runs in 2023 and 2026, the motor-circuit cap, GEC sizes for copper and aluminum services, the electrode limits, and properties such as “a larger breaker never needs a smaller EGC”. To size the circuit conductors first, use the wire size calculator, which applies this rule automatically when it upsizes for voltage drop.

Updated 2026-10-14. Found a problem? Report it; every fix is logged publicly.