The first time you pull a 200-amp feeder, something in the pipe looks like a mistake. The ungrounded conductors are 3/0 copper — thick, heavy, stubborn on every bend. And lying beside them, almost apologetic, is the equipment grounding conductor: a 6 AWG. Every apprentice eventually asks the question out loud. If that little wire has to carry a fault big enough to trip a 200-amp breaker, why isn't it as big as the wires that fed the fault?

It's a good question, and the answer is one of the more elegant pieces of reasoning buried in the Code. The hots and the ground aren't doing the same job at different scales. They're doing entirely different jobs, on entirely different timescales — and once you see that, Table 250.122 stops being a lookup chart and starts being physics.

Two Wires, Two Different Jobs

The ungrounded conductors are sized by ampacity: the current they can carry continuously without their insulation exceeding its temperature rating. Ampacity is a steady-state idea. It assumes the wire will run warm for hours, shedding heat into the raceway and the air around it exactly as fast as the current generates it. That equilibrium — heat in equals heat out — is what Table 310.16 describes.

The equipment grounding conductor spends its entire working life carrying nothing. It isn't a return path in normal operation; that's the neutral's job. The EGC exists for one moment: the fraction of a second between a hot conductor touching a metal enclosure and the breaker slamming open. It will never reach thermal equilibrium, because if it's doing its job, the event is over before equilibrium is even a meaningful idea.

One wire is built to run a marathon. The other is built to survive a single sprint. You don't size them the same way, because heat doesn't treat them the same way.

Heat Is Current Squared Times Time

During a bolted fault, the heating in a conductor is effectively adiabatic — the event is so brief that almost none of the heat escapes into the insulation or the air. All of it stays in the copper. The energy deposited is proportional to the square of the current multiplied by the time it flows, the quantity engineers write as I²t.

That little t is the whole trick. A breaker's magnetic element — the instantaneous trip — clears a solid fault in one to two cycles, which at 60 hertz means somewhere around 17 to 33 milliseconds. Copper melts at 1,084 °C, and the mass of metal in even a 6 AWG conductor can absorb thousands of amps for a couple of cycles and come out merely warm. The conductor's ampacity is irrelevant to this event; ampacity is a statement about hours, and the fault lasts hundredths of a second.

This short-time capability isn't hand-waving — it's the same withstand calculation, in the family of the Onderdonk fusing equation, that engineers use to check whether a conductor survives its own protective device. The NEC did the work once and baked the conclusion into a table.

Table 250.122 Keys to the Breaker, Not the Wire

Notice what the table indexes on: not the size of the circuit conductors, but the rating of the overcurrent device ahead of them. A 20-amp breaker gets a 12 AWG copper ground. A 60-amp breaker gets a 10. A 100 gets an 8, and a 200 gets that suspicious-looking 6.

The logic follows directly from I²t. The breaker's rating and trip curve bound how much energy can pass through the circuit before the fault is cleared. A bigger breaker lets more energy through before it opens, so it demands a ground wire with more thermal mass to soak it up. The size of the hots never enters into it — they're sized for a different problem.

Two footnotes worth knowing. First, the EGC never has to be larger than the circuit conductors themselves. Second, under 250.118 a metallic raceway — EMT, IMC, or rigid — qualifies as the equipment grounding conductor in its own right, and the cross-sectional steel of a conduit dwarfs any wire you'd pull through it. The green wire many electricians add inside metal conduit is often redundancy, not requirement.

The Other Half of the Job: Impedance

Surviving the fault is only half of what the EGC does. Section 250.4(A)(5) names the other half: the effective ground-fault current path. The ground wire's job is to make the fault loop — out on the hot, back on the EGC — so low in impedance that the fault current is enormous, far above the breaker's magnetic threshold, so the breaker trips instantly.

This is where the whole scheme becomes self-referential in a way worth sitting with. The EGC is sized small because the fault clears in a cycle or two. But the fault only clears in a cycle or two because the EGC keeps the loop impedance low. If that path is compromised — a loose locknut, a corroded coupling, a ground wire that's genuinely undersized for the run — the fault current sags down into the breaker's thermal region. Now the breaker takes seconds or minutes instead of milliseconds, the sprint becomes the marathon the wire was never sized for, and the whole time, every metal surface on that circuit sits energized. The small ground wire is safe only inside a system where every connection holds up its end.

When You Upsize the Hots, Upsize the Ground

That impedance logic explains the rule that catches people on long runs: 250.122(B). If you increase the size of the ungrounded conductors — almost always to fight voltage drop — you must increase the equipment grounding conductor proportionally, by the ratio of the circular-mil areas.

Take a 20-amp circuit run 250 feet to a gate operator. Normally the hots are 12 AWG and so is the ground. Voltage drop pushes you to upsize the hots to 8 AWG — a jump from 6,530 circular mils to 16,510, a ratio of about 2.5. The ground has to grow by the same ratio: 6,530 times 2.5 lands you right at 8 AWG as well. On a long upsized run, the ground often ends up the same size as the hots.

The reason isn't ceremony. That long run adds impedance to the fault loop, and impedance is exactly what starves a fault of the current it needs to trip the breaker fast. Less current means more time; more time means more I²t; more I²t means the table-minimum ground wire is no longer protected by the assumptions that made it legal. Growing the EGC pulls the loop impedance back down and restores the sprint.

The Table Is a Conclusion, Not a Rule of Thumb

Once you've seen the reasoning, the 6 AWG in the 200-amp pipe stops looking like a corner cut. It looks like what it is: a conductor sized precisely for a job that lasts two cycles, inside a system engineered so the job never lasts longer than that. The hots answer to hours; the ground answers to milliseconds. Same pipe, different physics.

What this asks of you in the field is bookkeeping — the 250.122 lookup, the circular-mil ratio when a run gets upsized, the voltage-drop calculation that triggers the whole cascade. That's the kind of math that's easy at a desk and error-prone on a ladder, which is why Voltly keeps all of it in your pocket: voltage drop, ampacity, conduit fill, and the NEC tables, all working offline in the crawlspaces and basements where signal goes to die. Run the numbers before you pull, and the ground wire that looks too small will be exactly the right size — provably. Try it at https://voltly.lumenlabs.works.