Grounding System Designer
Select a component from the palette, then click / tap the canvas to place it (auto-deselects after placing). Drag placed components to reposition. Click ports (colored dots on edges) to start a wire, then click another port to connect. Click a component to select it, then press Delete to remove. Wires route with right-angle bends. Loading an example also sets the sidebar method / system so Validate Design grades against the matching context.
Examples
Normal Current Path
Source → Phase conductor → Load → Neutral conductor → Source. Current returns via the neutral (grounded conductor).
Fault Current Path
No fault present. Select a fault location to visualize the fault return path.
Bond Location Status
Neutral Current Status
Fault Return Integrity
Likely Symptoms
Code / Concept Notes
Event Log
Phase (Hot)
Carries load current from source to utilization equipment.
Neutral (Grounded)
Returns normal load current to the source. Grounded at the service or SDS origin.
EGC (Equipment Grounding)
Safety path for fault current. Low impedance return to trip overcurrent device.
Correct Installation
Incorrect Installation
Key Differences
Select different configurations to see how correct and incorrect installations compare.
1. Normal Operation
Standard utility service, solidly grounded, proper N-G bond. All conductors functioning correctly.
2. Ground Fault at Load
Phase-to-ground fault at equipment. See how fault current returns via EGC to trip the breaker.
3. Missing N-G Bond
What happens when the main bonding jumper is removed? Fault current has no low-impedance return path.
4. Open Neutral
Neutral conductor is broken. Line-to-neutral voltages become unstable; neutral current may seek unintended metallic return paths.
5. HRG System
High-resistance grounded system. First ground fault is alarmed, not tripped. Limits fault current.
6. SDS Transformer
Separately derived system with system bonding jumper at the SDS source. GEC connection to the grounding electrode system required; a separate ground rod is not automatically required.
7. Generator (Switched Neutral / SDS)
Standby generator with switched (4-pole) neutral transfer. Generator is a separately derived system; system bonding jumper at generator.
8. Double N-G Bond
Bonded at both source and sub-panel. Normal neutral current splits onto EGC -- a code violation.
Grounding vs Bonding -- What's the Difference?
- Grounding = connection to earth (typically through a grounding electrode system). Establishes a voltage reference and stabilizes the system against voltage swings.
- Bonding = electrically connecting metallic parts together to maintain continuity. Creates the metallic effective ground-fault current path used to clear faults.
- Key insight: Earth alone is generally not relied on as a fault-return path. The intentionally bonded metallic path is what clears faults; earth grounding provides voltage reference, lightning / surge reference, and a common potential.
The N-G Bond (Main Bonding Jumper)
- Connects the grounded conductor (neutral) to the equipment grounding conductor at the source.
- Required at service entrance (NEC 250.24) or at separately derived system origin (NEC 250.30).
- Only ONE point of connection per system. Multiple bonds cause neutral current on EGC.
Separately Derived Systems
- A system with no direct electrical connection to the supply conductors of another system (NEC 250.30).
- Requires a system bonding jumper at the SDS source (or first disconnecting means as permitted) and a GEC connection to the grounding electrode system. This does NOT automatically mean driving a separate ground rod.
- The grounding electrode system can include building steel, metallic underground water pipe, concrete-encased electrode, or a ground ring, in addition to (or instead of) rods.
- Examples: isolation transformers; generators with switched neutral (4-pole transfer); some UPS configurations.
Why an Earth-Only Return Path Is Typically Inadequate
- Earth resistance varies widely with soil and electrode design (often several ohms to tens of ohms or more). A metallic EGC is usually a small fraction of an ohm.
- By Ohm's law, current divides among parallel paths in inverse proportion to their impedance. Most of the current will take the lowest-impedance path.
- Illustrative example: 120 V across a 25 Ω earth path ≈ 4.8 A. At that current, a typical 15 A or 20 A thermal-magnetic breaker may not operate on overcurrent alone in a useful time. Actual fault current depends on system voltage, source impedance, conductor impedance, electrode resistance, soil conditions, and bonding.
- That is why the intentionally bonded metallic path (the effective ground-fault current path, NEC 250.4(A)(5)) is what is relied on to clear faults -- not earth grounding alone.
Common Grounding Myths
MYTH: "The ground wire carries fault current to earth."
FACT: The EGC carries fault current back to the source (transformer/generator) to trip the overcurrent device. Earth is not the return path.
MYTH: "Adding more ground rods makes a system safer."
FACT: Additional ground rods lower earth electrode resistance but do NOT improve fault clearing. Only proper bonding clears faults.
MYTH: "Neutral and ground are the same thing."
FACT: They are bonded together at ONE point (the source). Beyond that point, neutral carries load current, EGC carries only fault current. Mixing them downstream creates hazards.
MYTH: "An ungrounded system can't shock you."
FACT: An ungrounded system still has distributed capacitance and insulation leakage that couple the phases to ground. A first ground fault is typically of small current but may not be cleared by OCP; a second ground fault on a different phase can become a phase-to-phase / line-to-line fault that can have substantial energy. Ground detectors are required by NEC 250.21(B) for ungrounded AC systems operating at 120 V up to 1000 V.
Quiz Challenges
Q1: Where should the N-G bond be located in a standard service entrance?
NEC 250.24(B) requires an unspliced main bonding jumper connecting the grounded conductor (neutral) to the equipment grounding conductor and the service-disconnect enclosure at the service disconnect (device requirements in NEC 250.28). NEC 250.24(A)(5) is the companion rule: the grounded conductor must not be reconnected to ground or equipment on the load side — only one point of connection per system.
Q2: What is the typical consequence of an additional N-G bond on the load side of the service / SDS bonding point?
With a second N-G connection, normal neutral current has two metallic paths back to the source: the neutral conductor and the EGC / bonded metallic paths. Some of the normal-operation neutral current splits onto the EGC / metallic system. This is "objectionable current" (NEC 250.6), can cause GFCI nuisance operation, and can raise touch voltage on bonded metal.
Q3: In a typical alarm-only HRG system, what happens on the first ground fault?
An HRG (high-impedance grounded neutral) system limits ground-fault current with a Neutral Grounding Resistor (NGR), typically 5 A or 10 A depending on design. The first ground fault is detected and alarmed, allowing service continuity while a disciplined fault-location process is followed. Specific behavior depends on the system design and protection settings (NEC 250.36).
Q4: What is the primary purpose of the Equipment Grounding Conductor (EGC)?
The EGC — which may be a wire (green or bare) or a metal raceway / cable armor per NEC 250.118 — provides a low-impedance fault-current path back to the source to operate the overcurrent protective device, and also bonds equipment to limit touch voltage. It carries essentially no current in normal operation (small leakage currents aside).
Q5: A separately derived system (SDS) typically requires which of the following?
NEC 250.30 (separately derived systems): the SDS needs a system bonding jumper at the SDS source or first disconnecting means as permitted, plus a grounding electrode conductor that lands on the grounding electrode system. The grounding electrode system can include the building steel, metal underground water pipe, concrete-encased electrode, etc.; driving a separate ground rod is not automatically required.
Q6: Why might GFCIs nuisance-trip in a system with a double N-G bond?
A GFCI monitors the difference between current on the hot and neutral conductors. With a double N-G bond, some neutral current returns via the EGC instead of the neutral wire, creating an imbalance that the GFCI interprets as a ground fault.
Q7: A 120V ground fault occurs but only ~4.8A flows. What is the most likely cause?
Illustrative example: with 120 V and ~25 ohms earth/incidental path resistance, I = V/R = 120/25 ≈ 4.8 A. This is far below the trip threshold of a typical 15 A or 20 A thermal-magnetic breaker. Without a low-impedance metallic return path (provided by the N-G bond), the breaker may not trip or may not trip promptly, and enclosures may remain energized. Actual fault current depends on system voltage, source impedance, conductor impedance, electrode resistance, soil conditions, and bonding.
Q8: Which conductor is intended to carry normal return current where used?
The neutral / grounded conductor is the conductor intended to carry normal return current where it is used. The EGC, GEC, and equipment bonding jumpers are not intended to carry normal load current; current on them during normal operation is generally objectionable (NEC 250.6). The main / system bonding jumper has nuanced language and is required at the service / SDS bonding point to establish the effective ground-fault current path.