Grounding and Bonding Practice

Two sizing tables indexed by different things, the electrode requirements, and why the earth is not a fault-current path.

Practise Grounding

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What it is, and why electricians need it

Grounding connects to earth; bonding joins metal parts together. Bonding is what clears faults, and grounding does not — and getting that distinction straight is most of what this topic asks of you.

Here is why it matters. When an ungrounded conductor faults to a metal enclosure, something has to carry enough current back to the source to operate the overcurrent device, fast. That path is the equipment grounding conductor and the bonding connections — a deliberate low-impedance metallic route. The connection to earth plays no part in it. Earth resistance runs to ohms or tens of ohms, which would limit fault current to a few amperes: not remotely enough to trip a 20-ampere breaker, leaving the enclosure energized and lethal. That is precisely why the Code states the earth shall not serve as the effective ground-fault current path.

What grounding does do is stabilise voltage to ground, limit the voltage imposed by lightning and line surges, and give the system a reference. Both functions are needed; they are just not the same function.

The calculation side comes down to two tables that are indexed by different things. The equipment grounding conductor table is keyed to the rating of the overcurrent device — not to the circuit conductor size. The grounding electrode conductor table is keyed to the size of the largest ungrounded service-entrance conductor. Candidates mix these up routinely, so before you open the book, decide which one the question wants.

Formulas and principles

  • EGC by device rating (copper): 15 A → 14, 20 A → 12, 60 A → 10, 100 A → 8, 200 A → 6, 300 A → 4

    Minimum equipment grounding conductor for the overcurrent device ahead of the equipment.

    The left column reads "not exceeding", so a 30 A or 50 A device uses the 60 A row.

  • GEC by service conductor (copper): 2 or smaller → 8, 1 or 1/0 → 6, 2/0 or 3/0 → 4, over 3/0 through 350 → 2

    Minimum grounding electrode conductor, keyed to the largest ungrounded service-entrance conductor.

    Continues: over 350 through 600 → 1/0, over 600 through 1100 → 2/0, over 1100 → 3/0.

  • GEC caps: 6 AWG Cu to rod/pipe/plate, 4 AWG Cu to concrete-encased

    Where the conductor does not continue on to an electrode type requiring a larger size.

    To a ground ring, it need not exceed the ring conductor itself.

  • Rod electrode: 8 ft minimum length, 8 ft in soil contact, 25 ohms or supplement

    A single rod needs a supplemental electrode unless its resistance to earth is 25 ohms or less.

    Where rock is struck: up to 45 degrees from vertical, or a trench at least 30 in. deep.

  • Upsized conductors → proportionally upsized EGC

    Increase the EGC in proportion to the circular-mil increase of the ungrounded conductors.

The two tables side by side

Presented together deliberately, because the point to internalise is that they are keyed to different inputs.

Equipment grounding conductor

Indexed by overcurrent device rating.

Equipment grounding conductor size by device rating
DeviceCuAl
15 A1412
20 A1210
60 A108
100 A86
200 A64
300 A42
400 A31
500 A21/0
600 A12/0
800 A1/03/0
1000 A2/04/0
1200 A3/0250

Grounding electrode conductor

Indexed by largest service-entrance conductor.

Grounding electrode conductor size by service conductor size
Service CuGEC CuGEC Al
2 or smaller86
1 or 1/064
2/0 or 3/042
Over 3/0–35021/0
Over 350–6001/03/0
Over 600–11002/04/0
Over 11003/0250

Source: NEC 2017 Table 250.122 and Table 250.66, from Public.Resource.Org's publication of incorporated standards. Values are given as facts for working problems; no NEC table is reproduced.

Now try it on 18 questions

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Worked examples

Scenario: A feeder is protected at 200 amperes and run in 4/0 copper. What size copper equipment grounding conductor and what size copper grounding electrode conductor are required?

  1. The EGC is keyed to the device rating. A 200-ampere device gives 6 AWG copper.
  2. The GEC is keyed to the service-entrance conductor size. 4/0 copper falls in the "over 3/0 through 350" row, giving 2 AWG copper.
  3. Note the two answers differ, and neither is derived from the other.
  4. If the GEC ran only to a driven rod and no further, the 6 AWG cap would apply and 6 AWG would suffice.

Answer: 6 AWG copper EGC; 2 AWG copper GEC, or 6 AWG if it serves only a rod

Scenario: A circuit is protected by a 50-ampere breaker. The table has rows for 20 and 60 amperes but none for 50. What copper EGC applies?

  1. The left column reads "rating or setting of automatic overcurrent device not exceeding".
  2. 50 amperes does not exceed 60, so the 60-ampere row applies.
  3. That row gives 10 AWG copper.
  4. The same reasoning covers 25, 30 and 40-ampere devices — all take the 60-ampere row.

Answer: 10 AWG copper, from the 60-ampere row

Common mistakes

  • Using the wrong table for the question asked. The equipment grounding conductor is sized from the device rating; the grounding electrode conductor from the service-entrance conductor size. Decide which the question wants before looking anything up.
  • Sizing the EGC from the circuit conductor size. It is keyed to the overcurrent device, not the conductor. An upsized conductor does not by itself change the EGC — though upsizing for voltage drop does require a proportional increase.
  • Looking for an exact device rating in the EGC table. There are no rows between 20 and 60 amperes. Use the next larger listed rating.
  • Assuming a ground rod clears faults. It does not. The rod is for voltage stabilisation and surge dissipation. Fault clearing is done by the equipment grounding conductor and the bonding connections back to the source.
  • Bonding the grounded conductor to the equipment grounding system downstream. The two are joined at exactly one point, the main bonding jumper at the service disconnect. Downstream they must stay separate, which is why a subpanel needs an isolated neutral bar.
  • Applying the 6 AWG cap to a GEC that serves other electrodes too. The cap only applies where the conductor does not extend on to an electrode type requiring a larger conductor. If it continues to a water pipe electrode, the full table size applies.

Related practice topics

Topics that come up alongside this one on the exam.

Questions about this topic

What is the difference between grounding and bonding?

Grounding connects a system or equipment to earth, which stabilises voltage to ground and helps dissipate lightning and surges. Bonding joins metal parts to one another so they sit at the same potential and so a fault has a low-impedance path back to the source. Bonding is what actually clears faults; grounding does not. Confusing the two is the single most common conceptual error in this subject.

Why can the earth not serve as the fault-current path?

Because its impedance is far too high. Earth resistance is measured in ohms or tens of ohms, which would limit fault current to a few amperes — nowhere near enough to trip a breaker. The faulted equipment would stay energized indefinitely. That is why a metallic equipment grounding conductor is required, and why the Code states the earth shall not be used as the effective ground-fault current path.

How are the two sizing tables different?

They are indexed by completely different things, and candidates mix them up constantly. The equipment grounding conductor table is keyed to the rating of the overcurrent device ahead of the equipment. The grounding electrode conductor table is keyed to the size of the largest ungrounded service-entrance conductor. Read which one the question is asking for before you open the book.

The EGC table has no row for 30 or 50 amperes. What do I use?

The next larger listed rating. The left column reads "rating or setting of automatic overcurrent device not exceeding", so a 30-ampere or 50-ampere device falls in the 60-ampere row, giving 10 AWG copper. The table jumps straight from 20 to 60 amperes, which catches candidates who look for an exact match and conclude the question is unanswerable.

When does a single ground rod need a second one?

Always, unless it can be shown to have a resistance to earth of 25 ohms or less. Because measuring that resistance requires specialised test equipment, the normal practice is simply to drive a second rod — cheaper and faster than proving the first one qualifies.

Why are there caps on grounding electrode conductor size?

Because the electrode's own resistance to earth limits how much current can flow into the ground, so a larger conductor achieves nothing. A grounding electrode conductor serving only rod, pipe or plate electrodes need not exceed 6 AWG copper; for a concrete-encased electrode the cap is 4 AWG copper, one size larger because that electrode makes much better contact with earth; and for a ground ring it need not exceed the ring conductor itself.

If I upsize circuit conductors, does the grounding conductor change?

Yes. Where ungrounded conductors are increased in size for any reason, including voltage drop, the equipment grounding conductor must be increased proportionally according to circular-mil area. The purpose is to keep the fault-path impedance in proportion to the circuit's, so the device still clears a fault at the far end quickly.

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