Ground Wire Size Calculator and Chart (NEC 250.122)
A 20 A overcurrent device takes a #12 AWG copper equipment grounding conductor, or #10 AWG aluminum. A 100 A circuit takes #8 AWG copper, and 200 A takes #6 AWG.
Calculator
Equipment Grounding Conductor
#12 AWGCU
Base EGC (NEC Table 250.122)
#12 AWG
Per NEC 2023 Table 250.122
Ground wire size chart
The smallest wire-type equipment grounding conductor for each breaker or fuse rating, per NEC 2023 Table 250.122, before any increase for larger circuit conductors.
area of the smallest conductor the circuit required, for its overcurrent device and load, after any correction for heat or bundling
Ainstalled
area of the conductor actually run
Aground
the area the grounding conductor must reach; the next size up the ladder that meets it is the answer
A rating between two rows of the table reads the next row up.
The increase applies only when the installed conductors are larger than the circuit needed; it is never a reduction.
The grounding conductor is not required to be larger than the circuit conductors.
The table stops at 6000 A.
Assumptions
A wire-type equipment grounding conductor. Metal raceways and cable armors that are themselves recognized as grounding conductors are not sized here.
The rating entered is the device ahead of the circuit. For a circuit with several devices it is the one protecting these conductors.
Copper and aluminum are read in their own columns; copper-clad aluminum is not offered.
The ratio is compared exactly, in circular mils, so a conductor that lands on a size boundary is not nudged by rounding.
Conductors made larger only for the ampacity corrections for heat or bundling do not enlarge the ground; enter the corrected size as the size the circuit needs.
A qualified person may size the grounding conductor another way where an effective ground-fault current path is shown; this page does not.
The size the ground has to reach is the area calculation; the conductor chosen is the next standard size at or above it.
Worked example
A 100 A feeder with conductors increased for voltage drop
The load needs #3 AWG copper, but the run is long and 1/0 AWG went in instead.
The table's size for a 100 A device:#8 AWG copper= 16,510 cmil
How much bigger the circuit conductors became, by area:105,600 ÷ 52,620= 2.007
The ground grows by the same ratio:16,510 × 2.007= 33,133 cmil
The smallest conductor with at least that area:#4 AWG = 41,740 cmil= the first size at or above 33,133 cmil
Answer: #4 AWG copper, up from the #8 AWG the table alone would call for.
Sizing from the load or the wire instead of the device
The table is read with the rating of the overcurrent device. A tool that asks for the load current, or for the phase conductor size, will land a row out whenever the device is not the obvious one for that conductor.
Check: what the other tool asked for.
Missing the increase
Conductors are often upsized for voltage drop or for future load. A table-only answer is then too small, sometimes by two sizes.
Check: whether the other tool asked what the circuit actually needed.
Increasing by diameter or by gauge steps
The increase is by circular-mil area, not by AWG steps and not by diameter. Counting sizes up the ladder gives a different, usually larger, answer.
Check: whether the other tool worked in circular mils.
Confusing it with the grounding electrode conductor
The conductor to the rods, the water pipe or the footing steel is sized from the service conductors, by a different table. The two answers are rarely the same size.
Check: which conductor the answer was for.
Splitting a parallel set's ground
Where wire grounds are used, each raceway of a parallel run carries a full-size one. Dividing one conductor between the raceways, or sizing each for its share of the load, is the common mistake.
Check: how the other tool handled parallel raceways.
Questions
What size ground wire for a 200 amp service?
Two different conductors answer that. The equipment grounding conductor run with a 200 A feeder is #6 AWG copper or #4 AWG aluminum, from this table. The conductor to the ground rods or the footing steel is the grounding electrode conductor, which is sized from the service conductors instead — the grounding electrode conductor calculator does that one.
What size ground for a 100 amp sub panel?
#8 AWG copper or #6 AWG aluminum, on a 100 A breaker. The rating of the device ahead of the feeder is what the table reads, not the panel's own rating and not the load.
Does the ground get bigger when the conductors do?
Yes, where they were made larger than the circuit needs for a reason other than the ampacity corrections for heat and for bundled conductors — voltage drop is the usual one. The grounding conductor goes up by the same ratio of circular-mil area, and it never has to be larger than the circuit conductors themselves.
Parallel runs in separate raceways?
Where wire-type grounding conductors are used, each raceway gets one, and each is full size for the overcurrent device ahead of the whole parallel set — not a share of it. A metal raceway that qualifies as a grounding conductor can serve instead, and parallel sets that share one raceway need only one.
Do I need a green wire in metal conduit?
Not always. Several metal raceways and cable armors are themselves recognized as the equipment grounding conductor, and a wire run with the circuit is one option among them. This calculator sizes that wire; whether the raceway can do the job instead is a different question.
Is this the same as the neutral?
No. The grounded (neutral) conductor carries current in normal use and is sized for it; the equipment grounding conductor carries fault current only, and is sized from the overcurrent device.
Written and maintained by Yusuf Can Samiloglu: MEP/HVAC design background, mechanical-engineering degree, not a licensed electrician.
How it’s checked: the calculation is the method of the Electric Master app, ported to the web and tested case by case against the app’s source code. Every table value was checked against the NEC 2023 values in at least two independent published sources.
The App Store version of Electric Master (2.2.0) predates the NEC corrections this page follows; until its next update, its result can differ from this page for some inputs. Version 2.2.0 applies the ratio even when the installed conductors are smaller than the size the circuit needed, which gives a ground below the table size; it rounds the area down before choosing a size, and it does not hold the result at the circuit conductor size. Set “Actual Conductor Size” no smaller than “Minimum Required Size” there.
: First version, NEC 2023. Table 250.122 by overcurrent-device rating for copper and aluminum, the 250.122(B) increase found by an exact comparison of circular mils, the 250.122(A) limit at the circuit conductors, and a reminder for parallel sets in separate raceways.