Grounding and bonding for the master exam
Article 250 questions on the master exam lean on three tables: Table 250.66 for grounding electrode conductors, Table 250.102(C)(1) for supply-side bonding jumpers, and Table 250.122 for equipment grounding conductors. Knowing which table a question needs is half the answer.
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free grounding and bonding drillWhich table to use
| Conductor | Sized from | Table |
|---|---|---|
| Grounding electrode conductor | Largest service conductor (or equivalent area of parallel sets) | Table 250.66 |
| Main and system bonding jumpers, supply-side bonding jumpers | Largest ungrounded supply conductor | Table 250.102(C)(1) |
| Equipment grounding conductor | Rating of the overcurrent device ahead of the circuit | Table 250.122 |
Table 250.66 for copper conductors
| Largest service conductor (copper) | GEC (copper) |
|---|---|
| 2 AWG or smaller | 8 AWG |
| 1 or 1/0 AWG | 6 AWG |
| 2/0 or 3/0 AWG | 4 AWG |
| Over 3/0 through 350 kcmil | 2 AWG |
| Over 350 through 600 kcmil | 1/0 AWG |
| Over 600 through 1100 kcmil | 2/0 AWG |
| Over 1100 kcmil | 3/0 AWG |
The conductor to a rod, pipe or plate electrode never needs to be larger than 6 AWG copper, and to a concrete-encased electrode never larger than 4 AWG copper (250.66(A) and (B)).
Worked example: parallel service
Three parallel sets of 500 kcmil copper per phase
- Equivalent area: 3 × 500 = 1,500 kcmil.
- Grounding electrode conductor to building steel: over 1,100 kcmil → 3/0 copper (Table 250.66).
- One supply-side bonding jumper for all three sets: over 1,100 kcmil, so 12.5% × 1,500 kcmil = 187.5 kcmil → 4/0 copper (Table 250.102(C)(1) Note 1).
- A 400 A feeder from this service needs at least a 3 AWG copper equipment grounding conductor (Table 250.122).
Practice grounding and bonding questions
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free grounding and bonding drillRules that come up often
- When ungrounded conductors are increased in size for voltage drop, the equipment grounding conductor increases in proportion to their circular-mil area (250.122(B)).
- A single rod, pipe or plate electrode needs a supplemental electrode unless it has a resistance to earth of 25 ohms or less; the two electrodes must be at least 6 ft apart (250.53(A)(2) and (A)(3)).
- A separately derived system needs a system bonding jumper at one point between the source and the first disconnecting means (250.30(A)(1)).
- A building supplied by a feeder needs its own grounding electrode system, and the grounded conductor is not bonded to the equipment at the second building (250.32).
Common questions
How do I size the grounding electrode conductor for parallel service conductors?
Add the areas of the parallel conductors in one phase to get the equivalent area, then use Table 250.66.
When is a supply-side bonding jumper sized at 12.5%?
When the ungrounded supply conductors are larger than 1,100 kcmil copper (1,750 kcmil aluminum), the jumper must have at least 12.5% of their area (Table 250.102(C)(1), Note 1).
Is the equipment grounding conductor sized from the wire or the breaker?
From the rating of the overcurrent device ahead of the circuit, using Table 250.122.
Practice grounding and bonding questions
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free grounding and bonding drillMore master exam guides
Section numbers follow the 2023 National Electrical Code (NFPA 70); rules are paraphrased, not quoted. Independent preparation, not affiliated with NFPA, ICC, PSI, Pearson VUE or any state board.