Weekly digest #110: grounding gotchas

Grounding vs bonding: stop conflating them

Grounding connects the electrical system to earth. Bonding connects metallic parts together so they stay at the same potential. They serve different purposes and the NEC treats them differently. Mix them up on paper and you will mix them up in the field.

NEC 250.4(A)(1) covers the performance requirements for grounded systems: the earth connection limits voltage from lightning, surges, and line contact with higher-voltage systems. NEC 250.4(A)(3) covers bonding of electrically conductive materials to establish an effective ground-fault current path. The earth is not that path. A ground rod by itself will not clear a fault.

If your only fault return path is through dirt, the breaker will not trip. Load-side current goes back to the source through the equipment grounding conductor, not through the earth.

The main bonding jumper: one place, one time

The neutral and the equipment grounding conductor get bonded together exactly once, at the service disconnect. NEC 250.24(A)(5) prohibits a grounded conductor connection to the grounding electrode or grounding electrode conductor on the load side of the service disconnecting means. Downstream panels get a separate neutral bar and ground bar, with the bonding screw removed or the green screw left out.

This rule saves lives and blows up jobs when ignored. A downstream neutral-ground bond creates parallel paths for neutral current on metal raceways and equipment grounds. You will see current on conduit, nuisance GFCI trips, and strange readings on your clamp meter.

If you clamp a feeder EGC and read more than a trace of current with normal load, you have a neutral-ground bond somewhere downstream. Walk it until you find it.

Separately derived systems trip people up

A generator, transformer, or UPS output can be a separately derived system (SDS), and if it is, it needs its own system bonding jumper and grounding electrode conductor per NEC 250.30. The catch: whether it qualifies as separately derived depends on whether the neutral is switched.

If your transfer switch is 3-pole and the neutral runs solid through, the generator is not separately derived. Do not bond neutral-to-ground at the genset. If the transfer switch is 4-pole and switches the neutral, it is separately derived, and you do bond at the genset. Getting this backwards either leaves you with no effective fault path or creates objectionable current on the grounding system.

• 3-pole transfer, solid neutral: bond only at service, not at genset
• 4-pole transfer, switched neutral: bond at service AND at genset (separately derived)
• Portable generator feeding a cord-and-plug load: bonded neutral at the frame is fine
• Portable generator feeding a building via transfer switch: follow the transfer switch rules above

Ground rod resistance: the 25-ohm myth

NEC 250.53(A)(2) is the source of endless field arguments. A single made electrode (ground rod, pipe, plate) that does not achieve 25 ohms or less to ground must be supplemented by one additional electrode. That is it. You drive a second rod, and you are done. You do not keep driving rods chasing 25 ohms.

Two rods in parallel will not necessarily get you under 25 ohms either, and the code does not require it once you have installed the supplementary electrode. The 25-ohm number applies to a single made electrode, not to the overall system. If your building steel, concrete-encased electrode, or metal water pipe qualifies under 250.52(A), the 25-ohm rule does not apply at all.

Inspectors and engineers sometimes demand fall-of-potential testing and sub-25-ohm readings on every job. That is a project spec, not a code requirement. Know the difference when you are pricing work.

CEE (Ufer) is mandatory when present

NEC 250.50 requires that all electrodes present at a building be bonded together to form the grounding electrode system. If a concrete-encased electrode exists, it has to be used. You cannot skip it and drive rods instead because rods are easier.

On new construction, that means coordinating with the concrete crew before the pour. NEC 250.52(A)(3) defines the CEE: at least 20 feet of bare or zinc-galvanized or electrically conductive coated steel reinforcing bars not less than 1/2 inch in diameter, or at least 20 feet of bare copper not smaller than 4 AWG, encased in at least 2 inches of concrete.

If you show up after the pour and there is no stub-up, you are stuck bonding to exposed rebar by approved means or adding electrodes. Either way, document what you found and get it in writing before you close the wall.

GEC sizing: do not oversize blindly

Grounding electrode conductor sizing comes from NEC Table 250.66, based on the largest ungrounded service conductor. But 250.66(A), (B), and (C) cap the required size when the connection is only to specific electrodes.

1. Connection to rod, pipe, or plate electrodes: GEC does not need to be larger than 6 AWG copper
2. Connection to concrete-encased electrode: GEC does not need to be larger than 4 AWG copper
3. Connection to ground ring: GEC does not need to be larger than the ring conductor, minimum 2 AWG

These caps apply to the portion of the GEC that is the sole connection to that electrode. If one conductor serves multiple electrodes, the larger Table 250.66 size governs the common portion. Read the exceptions carefully before you pull 3/0 to a ground rod.