Grounding & Bonding Conductor Calculator
Size the equipment grounding conductor (NEC Table 250.122) and grounding electrode conductor (NEC Table 250.66) in copper or aluminum. Free, no account required.
Equipment Grounding Conductor (EGC)
NEC Table 250.122 — sized by the rating of the overcurrent device protecting the circuit.
EGC: 6 AWG copper · 4 AWG aluminum
Based on OCPD not exceeding 200 A.
Grounding Electrode Conductor (GEC)
NEC Table 250.66 — sized by the largest ungrounded service-entrance conductor.
GEC: 4 AWG copper · 2 AWG aluminum
For a 3/0 copper service conductor.
NEC 250 Reference
- Table 250.122: EGC sized by OCPD rating. Upsize proportionally if ungrounded conductors are increased for voltage drop (250.122(B)).
- Table 250.66: GEC sized by the largest ungrounded service conductor (or equivalent area for parallel sets).
- 250.66(A): GEC to a ground rod need not be larger than 6 AWG copper.
- 250.66(B): GEC to a concrete-encased electrode need not be larger than 4 AWG copper.
- 250.66(C): GEC to a ground ring need not be larger than the ring conductor (min 2 AWG).
EGC vs GEC
The EGC bonds equipment back to the source and is sized by the breaker (Table 250.122). The GEC connects the system to the earth electrodes and is sized by the service conductors (Table 250.66). They are not interchangeable.
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This calculator uses NEC 2023 Tables 250.66 and 250.122. Always verify with your local AHJ and the NEC edition adopted in your jurisdiction. Not a substitute for an engineered design.
How the grounding and bonding conductor calculation works
Grounding questions are missed more often than any other topic, and almost always for the same reason: two different conductors, two different tables, two different inputs. Get the table right and the lookup is trivial. Get it wrong and every downstream number is wrong.
The equipment grounding conductor, sized from Table 250.122, is indexed by the rating of the overcurrent device ahead of the circuit. It is the fault-return path that lets a breaker trip, so it scales with the device that has to trip. A 100 A breaker means 8 AWG copper regardless of what the circuit conductors happen to be.
The grounding electrode conductor, sized from Table 250.66, is indexed by the size of the largest ungrounded service-entrance conductor. It connects the system to earth and scales with the service, not with any breaker. Table 250.66 also caps out: the GEC is never required to be larger than 3/0 copper, and connections to a rod, pipe, or plate electrode never need to exceed 6 AWG copper under 250.66(A), while a concrete-encased electrode connection never needs to exceed 4 AWG under 250.66(B).
Step by step
- 1
Decide which conductor you are sizing
EGC for the fault path within a circuit, GEC for the connection to the grounding electrode system.
- 2
For an EGC, find the device rating
Table 250.122, indexed by the overcurrent device ahead of the equipment, not by the circuit conductor size.
- 3
For a GEC, find the service conductor size
Table 250.66, indexed by the largest ungrounded service-entrance conductor or equivalent area for parallel sets.
- 4
Apply the electrode caps
250.66(A) caps rod, pipe, and plate connections at 6 AWG copper. 250.66(B) caps concrete-encased at 4 AWG copper.
- 5
Check for proportional increase
250.122(B): where circuit conductors are upsized, the EGC must be increased proportionally by circular-mil area.
- 6
Size bonding jumpers
Supply-side bonding jumpers follow 250.102(C), which uses Table 250.102(C)(1).
Worked example
A feeder protected at 200 A uses 3/0 copper conductors upsized to 250 kcmil for voltage drop. What size copper equipment grounding conductor is required?
- Table 250.122 at 200 A = 6 AWG copper, the base requirement.
- 250.122(B) applies because the conductors were increased in size.
- Circular mils: 3/0 copper is 167,800. 250 kcmil is 250,000.
- Ratio = 250,000 / 167,800 = 1.49.
- 6 AWG is 26,240 circular mils. 26,240 x 1.49 = 39,098 circular mils.
- Next size up meeting that: 4 AWG at 41,740 circular mils.
Answer: 4 AWG copper. The base Table 250.122 answer of 6 AWG is the trap. Whenever conductors are upsized for voltage drop, 250.122(B) requires the same proportional increase in the equipment grounding conductor.
How this is tested on the exam
Article 250 is consistently the lowest-scoring article on licensing exams. The fix is mechanical: before you open a table, say out loud whether you are sizing an EGC or a GEC. EGC means device rating and Table 250.122. GEC means service conductor size and Table 250.66. The 250.122(B) proportional increase is the most-missed single rule in the article.
Frequently asked questions
What is the difference between an EGC and a GEC?
The equipment grounding conductor bonds metal equipment back to the source so a fault has a low-impedance path and the breaker opens. It is sized from Table 250.122 by device rating. The grounding electrode conductor connects the system to the earth electrode. It is sized from Table 250.66 by service conductor size. They serve different purposes and are almost never the same size.
Do I ever need a grounding electrode conductor larger than 3/0?
No. Table 250.66 tops out at 3/0 copper for service conductors larger than 1,100 kcmil. Additionally 250.66(A) caps the portion running to a rod, pipe, or plate electrode at 6 AWG copper, and 250.66(B) caps the portion to a concrete-encased electrode at 4 AWG copper.
When does the equipment grounding conductor have to be increased?
250.122(B). When the ungrounded conductors are increased in size for any reason, most commonly voltage drop, the equipment grounding conductor must be increased proportionally based on circular-mil area. This is the most frequently missed rule in Article 250.
Can the equipment grounding conductor be aluminum?
Yes. Table 250.122 lists both copper and aluminum columns. Aluminum requires a larger size for the same device rating, and 250.120(B) restricts where bare or insulated aluminum equipment grounding conductors may be installed, including a prohibition on direct earth contact.