Grounding Conductor Size Calculator

The grounding electrode conductor runs from the service equipment to the grounding electrode — the driven rods, the metal underground water pipe, the rebar in the footing. Its purpose is establishing the system's reference to earth: limiting the voltage imposed by lightning and by contact with higher-voltage lines, and stabilising voltage to ground during normal operation. It is not a fault-current path, and earth is a poor conductor for that purpose anyway.

Its size comes from the largest ungrounded service-entrance conductor, on the reasoning that a bigger service implies a bigger system to reference. Table 250.66 is a band table — each row covers a range of service conductor sizes, and the ranges differ between copper and aluminium service conductors, which is why the material of the service conductor is an input even though it is not the material of the conductor being sized.

The equipment grounding conductor does something else entirely. It runs with the circuit conductors, bonds the metal enclosures along the way, and provides a low-impedance path back to the SOURCE — not to earth — so that a fault between a hot conductor and an enclosure draws a large current and opens the overcurrent device quickly. Under normal conditions it carries nothing.

That purpose determines its index. Table 250.122 asks for therating of the overcurrent device, because the conductor has to pass enough current, for long enough, to trip that specific device. Not the load current, not the conductor ampacity — the device.

Two examples, because the page has two halves and doing one would leave the other looking like a footnote.

Grounding electrode conductor. A service run in400 kcmil copper. Table 250.66 is entered with the largest ungrounded service conductor, and 400 kcmil falls in the over 350 through 600 kcmil band of the copper column. That row gives 1/0 AWG copper, or3/0 AWG if the electrode conductor itself is aluminium.

Then check the caps before pulling it. If this conductor's only job is reaching a driven ground rod, 250.66(A) says it need not be larger than 6 AWG copper — a long way below the 1/0 AWG the table gives. On a concrete-encased electrode the cap is 4 AWG. The full table size is for a run to a water pipe or a ground ring.

Equipment grounding conductor. A branch circuit protected by a100 A breaker. Table 250.122 is entered with the device rating, 100 A is a printed row, and it gives8 AWG copper or6 AWG aluminium.

Note what was not asked. Not what the circuit draws, not what size the circuit conductors are, not how long the run is. Only the breaker — because the only thing this conductor ever has to do is make that breaker trip.

Now put the two side by side. A 100 A service would run roughly 3 AWG copper conductors, and Table 250.66 gives those an 8 AWG electrode conductor — the same 8 AWG the 100 A breaker gets from 250.122. At that amperage the two tables happen to agree, which is exactly why the confusion survives. Go up to 400 A and the electrode conductor is 1/0 while the equipment conductor is 3 AWG: twice the area, from the same nominal amperage. The agreement at the bottom is a coincidence, not a relationship.

Both tables in full, and worth keeping side by side for the same reason they share a page. Notice how differently they are shaped: the first is seven bands covering every service from a small dwelling to a large industrial one, and the second is a row per device rating running from a 15 A lighting circuit to 6,000 A switchgear.

Use them when you already know which conductor you are sizing and just want the row. If you are not certain which one applies, use the calculator instead — it says which question it answered next to every number it gives, which a printed table cannot.