Minimum equipment grounding conductor size

Table of sizes according to overcurrent device rating

The equipment grounding conductor size is based on the rating of the overcurrent protective device that protects the circuit upstream, not on the load current. The following table presents the minimum standard sizes for copper and aluminum conductors.

Overcurrent device rating	Copper conductor (AWG/kcmil)	Copper conductor (mm²)	Aluminum conductor (AWG/kcmil)	Aluminum conductor (mm²)
15 A	14 AWG	2,08 mm²	12 AWG	3,31 mm²
20 A	12 AWG	3,31 mm²	10 AWG	5,26 mm²
30 A	10 AWG	5,26 mm²	8 AWG	8,37 mm²
40 A	10 AWG	5,26 mm²	8 AWG	8,37 mm²
60 A	10 AWG	5,26 mm²	8 AWG	8,37 mm²
100 A	8 AWG	8,37 mm²	6 AWG	13,3 mm²
200 A	6 AWG	13,3 mm²	4 AWG	21,2 mm²
300 A	4 AWG	21,2 mm²	2 AWG	33,6 mm²
400 A	3 AWG	26,7 mm²	1 AWG	42,4 mm²
500 A	2 AWG	33,6 mm²	1/0 AWG	53,5 mm²
600 A	1 AWG	42,4 mm²	2/0 AWG	67,4 mm²
800 A	1/0 AWG	53,5 mm²	3/0 AWG	85,0 mm²
1000 A	2/0 AWG	67,4 mm²	4/0 AWG	107 mm²
1200 A	3/0 AWG	85,0 mm²	250 kcmil	127 mm²
1600 A	4/0 AWG	107 mm²	350 kcmil	177 mm²
2000 A	250 kcmil	127 mm²	400 kcmil	203 mm²
2500 A	350 kcmil	177 mm²	600 kcmil	304 mm²
3000 A	400 kcmil	203 mm²	600 kcmil	304 mm²
4000 A	500 kcmil	253 mm²	800 kcmil	405 mm²
5000 A	700 kcmil	355 mm²	1200 kcmil	608 mm²
6000 A	800 kcmil	405 mm²	1200 kcmil	608 mm²

Adjustment for increase in phase conductor size

When the ungrounded conductors of a circuit are increased in cross section — typically to compensate for voltage drop in long runs — the equipment grounding conductor must be increased proportionally in copper area. The rule applies even if the protective device does not change. The general formula is:

S_ground_final = S_ground_initial × (S_phase_increased / S_phase_initial)

Variable	Description	Unit
S_ground_initial	Area of the grounding conductor per base table for the protective device	mm² or kcmil
S_phase_increased	Area of the phase conductor after increase for voltage drop	mm² or kcmil
S_phase_initial	Minimum area of the phase conductor for the load current without distance adjustment	mm² or kcmil
S_ground_final	Minimum required area for the adjusted equipment grounding conductor	mm² or kcmil

Short-circuit withstand capacity limit

Every equipment grounding conductor must have a cross section sufficient to withstand the maximum ground-fault current that the protective device allows to flow during its clearing time. The minimum size according to the table guarantees this requirement for faults up to 5 cycles (0,083 s) in standard circuit breakers, provided the conductor is copper and installed in the same raceway conditions as the phase conductors. In systems with short-circuit currents exceeding 25 kA / 25 000 A symmetrical, it must be verified that the Joule integral of the conductor (I²t) exceeds the specific pass-through energy of the protective device.

I²t_conductor ≥ I² × t_clearing

Variable	Description	Unit
I²t_conductor	Maximum allowable Joule integral for the grounding conductor	A²·s
I	Symmetrical rms short-circuit current available at the fault point	A
t_clearing	Total fault clearing time of the upstream protective device	s

Relationship with overcurrent protection

The sizing of the equipment grounding conductor depends exclusively on the rating of the overcurrent protective device that feeds the branch circuit, not on the size of the phase conductors. This rule is reversed only in the case of adjustment for increased phase size, where the grounding conductor must be scaled to maintain the same fault impedance ratio. In installations with multiple circuits in a single conduit, a single grounding conductor must be selected sized for the largest protective device present. Systems with fuse protection may, in certain cases, allow the use of slightly smaller grounding conductors than those required for circuit breakers of the same amperage when the melting time is less than 0,01 s.

Comparison with IEC 60364-5-54

The international standard IEC 60364-5-54 uses a calculation approach rather than a fixed table per device. The protective conductor (PE) is sized using the formula based on short-circuit energy and clearing time. The simplified table of the IEC yields values very close to the NEC table for circuit breakers up to 63 A / 63 amperes; above that threshold, the IEC tends to be slightly more conservative, requiring on average 8 % more cross section in copper for ratings of 400 A / 400 amperes. In aluminum the difference reaches up to 15 % for sizes larger than 500 kcmil / 253 mm², due to the lower conductivity and higher resistivity of the material, which the IEC penalizes with a larger correction factor.

Frequently Asked Questions (FAQ)

1. What is the minimum grounding conductor size for a 20-ampere breaker in copper?
   The minimum size is 12 AWG (3,31 mm²) for a 20 A / 20 ampere protective device, provided the phase conductors have not been increased for voltage drop.

2. Can aluminum conductor be used as equipment grounding conductor?
   Yes, aluminum conductor is permitted for equipment grounding. For a 200 A / 200 ampere device, a minimum of 4 AWG aluminum (21,2 mm²) is required, compared to 6 AWG copper (13,3 mm²).

3. What grounding conductor size corresponds to a 1000-ampere main breaker?
   For a 1000 A / 1000 ampere protective device, the minimum size in copper is 2/0 AWG (67,4 mm²) and in aluminum is 4/0 AWG (107 mm²).

4. When must the grounding conductor size be increased?
   When the phase conductors are increased in size to compensate for voltage drop, the grounding conductor must be increased proportionally in cross-sectional area. If a 20 A circuit changes from 12 AWG to 8 AWG in phase, the ground from 12 AWG (3,31 mm²) increases to 8 AWG (8,37 mm²).

5. Can a grounding conductor be smaller than the neutral?
   Yes, the grounding conductor is often of smaller size than the neutral. In a 100 A circuit with a 4 AWG neutral, the ground can be 8 AWG copper (8,37 mm²) depending on the protective device, since it only conducts current during faults.

6. What happens if a grounding conductor is installed undersized?
   An undersized grounding conductor can melt before the protective device clears a ground fault, leaving metal parts energized and creating a severe risk of electrocution. The table ensures the conductor withstands the fault current for at least 0,083 s / 83 ms.