The short answer: bond neutral to the grounding system at the service disconnect, or at the defined bonding point for a separately derived system. After that point, keep the neutral isolated from equipment grounding conductors, panel enclosures, raceways, and building metal. A second downstream bond gives normal neutral current a parallel route over metal that should carry current only during a fault.
That rule is better stated as one intentional neutral-to-ground bond for each grounded electrical system, not “one bond for the whole property.” NFPA 70-2026 Sections 250.24 and 250.30 distinguish service-supplied systems from separately derived systems. A transformer secondary or a generator whose neutral is switched can create another separately derived system with its own required bonding point. The drawing has to show which system each bond belongs to.
Scope: this guide covers common premises-wiring topologies under the 2026 edition of NFPA 70 in the United States. The edition actually enforceable on a project depends on local adoption, amendments, permit date, and the authority having jurisdiction (AHJ). Utility grounding, impedance-grounded industrial systems, shore power, mobile generators, and non-U.S. IEC earthing arrangements need separate analysis. Last checked August 14, 2026.
Grounding and bonding are different jobs
The words are often collapsed into “ground,” which hides the current path the drawing needs to prove.
| Conductor or connection | Normal job | Current in normal operation? |
|---|---|---|
| Grounded conductor (neutral) | Returns intended load current to the source | Yes, when line-to-neutral loads operate |
| Equipment grounding conductor (EGC) | Provides a low-impedance fault-current path so the protective device can open | No |
| Grounding electrode conductor (GEC) | Connects the system to the grounding electrode system | Not as a normal load-current path |
| Main or system bonding jumper | Connects the grounded conductor to equipment grounding and the source enclosure at the designated point | It completes the fault path; it is not a downstream neutral conductor |
The earth connection helps reference the system to ground and serves lightning, surge, and other grounding functions. It is not a substitute for the metallic equipment grounding path back to the source. A ground rod alone is generally too high-impedance to clear a line-to-case fault promptly. The EGC and bonding path are what let fault current return to the source and operate the breaker or fuse.
The current-path test
Use this four-step decision procedure on a one-line or panel schedule:
- Find the source of the grounded system. It may be the utility service, a transformer secondary, or a generator.
- Find the first defined bonding point. For a service-supplied system, show the main bonding jumper at the service equipment. For a separately derived system, show the system bonding jumper at the permitted source or first disconnect location.
- Trace every downstream neutral. It must remain insulated from enclosures, EGC bars, raceways, and other bonded metal after that point.
- Trace the fault path back to the source. The EGC must return a fault to the same bonding point without depending on soil, incidental piping, or a normal neutral-current path.
The decisive review question is simple: if a line-to-neutral load is running normally, can any of its return current flow on an equipment enclosure or EGC? If yes, the topology contains an objectionable parallel path.
Worked example: outdoor service disconnect, indoor panel
Consider a 120/240-volt dwelling service with the service disconnect mounted outside and a panelboard inside.
At the outdoor service equipment, the drawing should show the grounded service conductor, the main bonding jumper to the enclosure and equipment grounding system, and the grounding electrode conductor. From that point to the indoor panel, the feeder carries two ungrounded conductors, an insulated neutral, and an EGC.
Inside the downstream panel:
- the neutral bar is insulated from the cabinet;
- the bonding screw or strap is not installed on that neutral bar;
- the EGC bar is bonded to the cabinet; and
- branch-circuit neutrals land only on the isolated neutral bar, while EGCs land on the bonded grounding bar.
Calling the indoor cabinet the “main panel” does not change its electrical role. The location of the service disconnect does. If the first service disconnect is outside, the indoor panel is feeder equipment and neutral isolation belongs on both the one-line and the panel detail.
The common failure is to leave the indoor panel’s green bonding screw installed because the enclosure is the largest panel in the building. That second connection lets neutral current split between the feeder neutral and the feeder EGC, with bonded raceways and metal potentially joining the return network.
Worked example: detached garage
A new feeder to a detached garage normally needs both an EGC with the feeder and a grounding electrode system at the garage. Those are not interchangeable.
The EGC connects the garage equipment and enclosure back to the source bonding point so a fault can operate the upstream protective device. The grounding electrode system connects the garage building to earth for the functions assigned to electrodes. At the garage disconnect or panel, keep the feeder neutral isolated from both the EGC and the grounding electrode conductor.
NFPA 70-2026 Section 250.32 contains limited allowances for some existing installations that lack an equipment grounding conductor and meet every listed condition. That exception is not a design pattern for new work. A review drawing should state whether the feeder is new or existing rather than silently treating a legacy three-wire arrangement as the default.
Transformers and generators: decide whether there is a new system
A transformer secondary is the straightforward case. It is a separately derived system, so the drawing needs a system bonding jumper, grounding electrode connection, and downstream neutral isolation arranged under Section 250.30. Do not copy the service bond symbol onto every secondary panel; place it at the permitted transformer or first-disconnect point and make the downstream boundary visible.
A generator depends on the transfer equipment:
| Transfer arrangement | Generator status | Drawing consequence |
|---|---|---|
| Neutral remains solidly connected through the transfer switch | Typically not separately derived | Do not add a second neutral bond at the generator; show the continuous neutral path back to the service bond |
| Transfer switch opens and switches the neutral | Typically separately derived while supplying the load | Show the generator-side system bond and grounding electrode connection, plus the switched-neutral pole |
ASCO’s August 2024 transfer-switch bulletin makes the same design dependency explicit: neutral configuration follows whether the alternate source uses a separately derived grounding arrangement. Real systems with multiple transfer switches, ground-fault protection, parallel sources, or mission-critical loads need a coordinated engineering review; “three-pole means this, four-pole means that” is a useful first test, not a complete specification.
What the drawing must show
A one-line does not need every conductor termination, but it must expose enough topology for a reviewer to answer the current-path question. Include:
- the service point and service disconnecting means;
- each main or system bonding jumper, labeled by system;
- the grounding electrode conductor and electrode system connection;
- feeder neutrals and EGCs as distinct conductors after the bond;
- neutral switching at transfer equipment;
- separately derived source boundaries; and
- a note identifying downstream neutral bars as isolated where the graphic symbol is not explicit.
Do not use a ground symbol as a catch-all. The earth-electrode symbol, chassis or equipment-bond symbol, neutral conductor, and bonding jumper make different claims. If the drawing legend maps all four to the same icon, the reviewer cannot determine whether normal current stays off exposed metal.
Common failure modes
- Bonding by panel name. “Main” and “subpanel” are informal labels; the service and source topology determine the bond location.
- Treating a ground rod as the fault path. The electrode does not replace the feeder EGC.
- Bonding at both generator and service with a solid neutral. That creates parallel neutral paths unless the system design provides the required isolation.
- Drawing a switched neutral as an ordinary three-pole ATS. The omitted pole changes whether the generator is separately derived.
- Showing an isolated neutral note without an EGC. Isolation works only when a deliberate low-impedance fault path remains.
- Applying a detached-building legacy exception to new work. Existing-condition allowances have prerequisites; they are not a shortcut around a four-conductor feeder.
Final review checklist
Before issuing the diagram, mark each statement true or not applicable:
- Every grounded system has a named source and one deliberate system bonding location.
- No downstream neutral is connected to an enclosure, EGC, raceway, or electrode after that location.
- Every exposed conductive enclosure has a fault-current path back to its source.
- Grounding electrode and equipment grounding conductors are drawn as different functions.
- Transfer equipment shows whether the neutral is switched.
- Detached structures show both the feeder EGC and the local grounding electrode system where required.
- The applicable NEC edition, local amendment set, and AHJ review status are stated.
If those answers are visible, the diagram is doing its job: it lets a reviewer follow normal current and fault current without guessing. To draft the topology before engineering review, describe the source, disconnects, feeder conductors, separately derived systems, and neutral-switching state in ChatDiagram’s electrical diagram maker. Then verify every bond against the actual equipment instructions and adopted code.