Equipment grounding conductor sizing starts with NEC Table 250.122: look up the ampere rating of the fuse or breaker protecting the circuit, then read across for the minimum copper or aluminum size. From there, two adjustments can push the number up: if the phase conductors were enlarged for voltage drop, the grounding conductor scales with them, and parallel circuits need a full-size grounding conductor in each raceway rather than a shared or divided one. The conductor never has to be larger than the phase conductors it runs with.
Table 250.122: The Baseline Sizes
Table 250.122 reflects a straightforward principle. Bigger breakers let more fault current flow before tripping, so the grounding conductor needs more cross-sectional area to carry that current without overheating. The copper minimums for the most common breaker ratings:
- 15-amp breaker: 14 AWG copper (12 AWG aluminum)
- 20-amp breaker: 12 AWG copper (10 AWG aluminum)
- 60-amp breaker: 10 AWG copper (8 AWG aluminum)
- 100-amp breaker: 8 AWG copper (6 AWG aluminum)
- 200-amp breaker: 6 AWG copper (4 AWG aluminum)
- 400-amp breaker: 3 AWG copper (1 AWG aluminum)
- 800-amp breaker: 1/0 AWG copper (3/0 AWG aluminum)
Aluminum runs a step larger at every tier because it has higher resistivity than copper and needs more material to carry the same fault current safely. If you’re pulling aluminum, read the aluminum column directly rather than assuming a one-size bump covers it.
One cap applies at every ampere rating. The equipment grounding conductor never needs to be larger than the circuit’s phase conductors. If the table technically calls for a grounding conductor bigger than the ungrounded conductors, match the phase conductor size instead. This comes up on circuits where the overcurrent device is large relative to the wire actually being run.
Upsizing for Voltage Drop
When phase conductors get enlarged past the ampacity table, usually to compensate for voltage drop on a long feeder or branch circuit, NEC 250.122(B) requires the equipment grounding conductor to grow proportionately. The math uses circular mil area rather than AWG numbers because AWG steps aren’t linear.
The process: divide the circular mil area of the upsized phase conductor by the circular mil area of the phase conductor originally required. That ratio is what you apply to the grounding conductor. Multiply the circular mil area of the Table 250.122 grounding conductor by the same ratio, look up the result in NEC Chapter 9, Table 8, and round up to the next standard size. Skipping this step is a common plan-review rejection. If the phase conductors got bigger for voltage drop, the grounding conductor follows.
Parallel Circuit Runs
Circuits with conductors run in parallel across multiple raceways need an equipment grounding conductor in each raceway. Not one shared conductor threaded through a single path. Each grounding conductor is sized from Table 250.122 using the full rating of the circuit’s overcurrent device, not the per-raceway share of the current.
People miss this. If a 400-amp feeder splits across four raceways, each raceway still needs a 3 AWG copper grounding conductor. Not a smaller wire scaled to one-quarter of the load.
Two variations soften the rule in specific installations. When parallel circuit conductors run together in a single nonmetallic raceway or cable tray, one equipment grounding conductor sized per Table 250.122 is acceptable. And when parallel multiconductor cables are used, each cable must contain a grounding conductor sized per Table 250.122, with all of those grounding conductors bonded together at each end.
Feeders to Detached Buildings
A feeder to a detached garage, workshop, or barn brings NEC 250.32 into play. The equipment grounding conductor with the feeder is sized per Table 250.122 based on the feeder’s overcurrent protection, using the same table you’d use for any other circuit. The remote building also needs its own grounding electrode system, and the grounding electrode conductor there is sized separately from Table 250.66 based on the largest ungrounded feeder conductor.
A narrow exception exists for buildings supplied by a single branch circuit that includes an equipment grounding conductor. In that case, the separate grounding electrode system at the second building can be skipped. Once you install a feeder or multiple branch circuits, the full requirement applies regardless of whether an equipment grounding conductor is present.
At the detached building’s panel, do not bond neutral to ground. The grounded conductor must remain isolated from the equipment grounding conductor and from the grounding electrode at the remote building. Bonding them there creates a parallel path for normal neutral current through the earth.
Don’t Confuse the EGC With the Grounding Electrode Conductor
These two conductors follow different sizing tables, and mixing them up is one of the more common causes of undersized installations and failed inspections. The equipment grounding conductor (EGC) connects metal equipment enclosures back to the service panel so fault current can trip the breaker; it comes from Table 250.122 based on the overcurrent device. The grounding electrode conductor (GEC) connects the electrical system to earth through a rod, plate, or metal water pipe; it comes from Table 250.66 based on the largest ungrounded service-entrance conductor.
For example, a service with 2 AWG copper entrance conductors requires an 8 AWG copper grounding electrode conductor under Table 250.66, while the equipment grounding conductor for a 200-amp panel would be 6 AWG copper under Table 250.122. Same building, same panel, two different tables, two different sizes.
What Actually Counts as the Grounding Conductor
Sizing assumes you’re pulling a wire-type equipment grounding conductor, but NEC 250.118 lists over a dozen materials that qualify. Rigid metal conduit, intermediate metal conduit, and electrical metallic tubing all qualify as grounding paths on their own, provided every joint and fitting stays tight enough to maintain electrical continuity. Type AC cable armor and certain Type MC cable sheaths also count. The common requirement is a reliable low-impedance path from the equipment back to the panel.
Flexible metal conduit gets special treatment. It can serve as the sole grounding path only when the total ground-fault return path through the flex does not exceed six feet and the circuit is protected at a level appropriate for the connector fittings used. Past six feet, pull a separate wire-type equipment grounding conductor through the flex. Measure before assuming the flex armor is enough on a longer equipment whip.
Whatever conductor you use also has to travel in the same raceway, cable, or cord as the circuit conductors it protects. When the ungrounded and grounding conductors share the same physical space, their magnetic fields largely cancel during a fault, keeping the impedance of the ground-fault loop low. Separate them, and impedance climbs, which slows breaker response.
Identification
NEC 250.119 governs how the grounding conductor is marked so no one confuses it with a hot or neutral. Insulated equipment grounding conductors must have a continuous green outer finish, or green with one or more yellow stripes. Bare conductors need no color marking. For conductors 4 AWG and larger, you have flexibility: if the insulation isn’t already green, you can re-identify each end and every accessible point by stripping the insulation, coloring it green, or wrapping it with green tape so the marking encircles the conductor. Inside a multiconductor cable, the same re-identification options apply at each accessible point.
Terminations That Preserve the Rated Size
A correctly sized grounding conductor still fails if the connections at each end aren’t right. Green grounding screws threaded into tapped holes in metal boxes are the standard attachment. Where a box lacks a tapped hole, grounding clips that grip the edge of the box work. Every connection has to use listed pressure connectors or devices approved for grounding.
Pigtailing is the preferred technique when multiple cables enter a single box. Splice all the incoming grounding conductors together with a wire connector and extend a single short lead to the green terminal on the receptacle or switch. If you loop the grounding conductor through the device terminal instead, pulling that device out of the box breaks the ground path for everything downstream on the same circuit. Inspectors look for this.
Torque
NEC 110.14(D) requires terminations to be tightened to the manufacturer’s specified torque value using a calibrated torque tool whenever a numerical value is provided, and grounding terminations are covered by the same rule. Under-torqued connections loosen over time from thermal cycling and vibration. Over-torqued connections damage the conductor or terminal, creating a high-resistance point that heats up under fault current. A basic beam-type or click-type torque screwdriver handles most of the residential and commercial range. Shear bolts and breakaway-style connectors with visual indicators are also acceptable where the hardware is designed that way.
Aluminum
Modern AA-8000 series aluminum alloy conductors, standard for aluminum building wire for decades, do not require wire brushing or antioxidant compounds at terminations. UL testing under UL 486A-486B has confirmed that AA-8000 alloy performs reliably without oxide inhibitors. Older AA-1350 alloy wire manufactured before the early 1970s did require antioxidant treatment. If you run into old aluminum wiring during a retrofit, treat those terminations differently.
Whatever the alloy, any terminal receiving an aluminum conductor has to be rated for aluminum, typically marked “AL” or “CU-AL” on the device. Copper-only terminals with aluminum conductors lead to galvanic corrosion and eventual failure. Torque values for aluminum terminations come from the connector manufacturer and get applied with the same calibrated tools required for any other termination.