Equipment Grounding Conductor Size Calculator
Enter the minimum equipment grounding conductor size from your code's table, the circuit conductor size before the increase and the size actually installed.
Equipment grounding conductor after the increase
8 AWG
How the size was set
Increased in proportion to the circuit conductor area
Increase in circuit conductor area
1.5901 ×
Proportional EGC area
16,510 cmil
Area of the size shown
16,510 cmil
How it works
An equipment grounding conductor (EGC) has a minimum size set by the rating of the circuit's overcurrent device, which you look up in your code's table (NEC Table 250.122 in the United States). This page does not reproduce that table: you enter the size you found.
When the circuit's ungrounded conductors are made larger than the minimum size that would otherwise be used — most often to reduce voltage drop on a long run — NEC section 250.122(B) is commonly described as requiring a wire-type EGC to be increased in proportion to the increase in their circular-mil area. The exact wording, conditions and any exceptions are in the code edition your jurisdiction adopts. This page applies that proportion: it divides the installed conductor area by the area before the increase and multiplies the EGC's area by that ratio.
The result is rounded up to the next size in this calculator's list at or above the proportional area. It is not shown larger than the installed circuit conductors, a limit also commonly described in the NEC's equipment grounding conductor sizing rules; whether that limit applies to an installation is for the adopted code.
Formula
ratio = cmil(installed) ÷ cmil(before the increase) proportional EGC area = cmil(table EGC) × ratio EGC = smallest listed size with cmil ≥ proportional area, but not larger than the installed conductor cmil (AWG n) = 25 × 92^(2(36 − n)/39); kcmil sizes: kcmil × 1,000
Example
A circuit whose table EGC is 10 AWG has its conductors increased from 8 AWG (16,510 cmil) to 6 AWG (26,251 cmil) for voltage drop. The ratio is 26,251 ÷ 16,510 = 1.5900, so the EGC needs 10,383 × 1.5900 = 16,510 cmil, which is 8 AWG.
If the same circuit's conductors were increased from 3 AWG to 1/0, the ratio is 105,535 ÷ 52,635 = 2.0050 and the EGC needs 20,818 cmil; the next listed size at or above that is 6 AWG (26,251 cmil).
Assumptions and limitations
- The minimum EGC size and the conductor size before the increase are values you take from the code and your own ampacity determination; this page does not supply or check them, and it does not reproduce NEC tables.
- Areas are the nominal circular-mil areas from the American Wire Gage definition (NBS Handbook 100, 1966) and the kcmil sizes. The listed sizes are 14 to 1 AWG (without 5, 7, 9, 11 and 13), 1/0 to 4/0 and 250 to 1000 kcmil (without 550, 650, 850 and 950); rounding goes to the next of these.
- One set of conductors per circuit. Parallel runs, multiple circuits sharing an EGC, raceways or cable armor used as the EGC, and other special cases in the code are not covered.
- The proportional rule depends only on the circuit conductors' areas, so the EGC material matters only through the table size you enter.
- This is an arithmetic aid, not an electrical code determination. The adopted code and the authority having jurisdiction govern; a licensed electrician determines the conductor sizes for an installation.
Frequently asked questions
Why is the grounding conductor made larger when the circuit wires are larger?
Circuit conductors are usually enlarged because the run is long. A common explanation is that during a ground fault the fault current returns on the EGC, and a long EGC of minimum size has a high resistance that limits that current. Enlarging the EGC in the same proportion as the circuit conductors keeps the fault path's resistance in step with theirs, so that enough current flows to operate the overcurrent device.
Why is the result sometimes more than one size larger than the proportional area?
Conductor sizes come in steps. The proportional area is rarely an exact size, so it is rounded up to the next listed size; between 1 AWG and 4/0 the steps are about 26 % apart in area, and this list skips the odd sizes 5 to 13 AWG, which are about 59 % apart in pairs.
More in Electrical calculators.