NEC Grounding and Bonding Deep Dive Reference

Why this matters

Grounding + bonding is the single most-misunderstood section of the NEC (Article 250). Code says one thing; field practice has accumulated decades of folklore. Tech who understands the THEORY behind 250 inspects correctly + installs correctly + explains to inspector confidently. This is the field card.

Grounding vs bonding (the distinction)

Grounding = connection to earth:

• Ground rods, plates, Ufer
• Drains lightning, static
• Reference for voltage measurement

Bonding = electrical continuity between metallic parts:

• All metal connected together
• Provides low-impedance fault path
• Trips breaker on fault

Both required:

• Different purposes
• Often confused
• Code separates clearly

Equipment grounding conductor (EGC)

Purpose:

• Carries fault current back to source
• Trips OCPD (overcurrent protective device)
• Protects user from shock

Required at every:

• Outlet box
• Panel
• Equipment enclosure
• Non-current-carrying metal

Sized per:

• NEC Table 250.122
• Per OCPD rating

Grounding electrode system (GES)

Connection to earth (250.50):

• All available electrodes must be used
• Bonded together

Common electrodes:

• Metal underground water pipe (10+ ft underground)
• Concrete-encased electrode (Ufer, 20+ ft rebar)
• Ground ring (20+ ft #2 bare)
• Ground rod (8 ft, driven full depth)
• Plate electrode (rare modern)

Best: Ufer (concrete-encased):

• Lowest resistance typically
• Required in new construction (slab-on-grade)

Grounding electrode conductor (GEC)

Connects service to GES:

• Sized per 250.66 (per service conductor)
• Continuous (no splice except irreversible)
• Unbroken to electrode

Common sizes:

• 200A service = #4 copper minimum to ground rod
• #6 to Ufer (smaller allowed for concrete-encased)
• Per table

Ground rod requirements

Per 250.52:

• 8 ft minimum length
• 5/8 inch minimum diameter (steel)
• Driven flush OR below grade
• Resistance 25 ohms or less (else 2 rods 6+ ft apart)

Two rods standard:

• Easier than testing
• Per code as alternative
• Common installation

Bonding (250 Part V)

Service bonding:

• Neutral to ground bonded at FIRST disconnect only
• Bonding screw / bonding strap
• One location

Sub-panel bonding:

• Neutral isolated (separate bus)
• Ground bus bonded to enclosure
• Common code violation when not done

Equipment bonding:

• All metallic raceway, boxes, enclosures
• Continuous path
• Bonding bushings at concentric KOs

Concentric / eccentric knockouts

Issue:

• Multi-ring knockouts don't provide reliable bond
• Concentric rings between can be loose

Solution:

• Bonding bushings
• Bonding jumpers
• Per 250.97

Common miss:

• Standard locknut doesn't qualify
• Inspector flag

Bonding water + gas piping

Metal water pipe (250.104):

• Bonded to grounding system
• Sized per service
• Within 5 ft of entry

Metal gas pipe:

• Bonded if exposed to electrical fault
• Often via EGC of equipment served
• CSST gas requires specific bonding (per manufacturer + IFC)

Customer awareness:

• Plumbing replaced (PEX) loses ground path
• Need separate electrode

Fault current path requirements

Effective ground fault current path (250.4):

• Permanent + continuous
• Low impedance
• Adequate capacity
• Sized per fault current

Earth itself NOT acceptable:

• Common misconception
• Earth too high impedance
• Need metallic path back to source

GFCI vs grounding

GFCI (Ground Fault Circuit Interrupter):

• Senses imbalance between hot + neutral
• Trips on 4-6 mA imbalance
• Works WITHOUT ground (per 406.4(D)(3))
• Replaces 2-prong with 3-prong receptacle

AFCI (Arc Fault Circuit Interrupter):

• Senses arcing
• Different from GFCI
• Both required some locations

Grounded vs grounding

Grounded (white) = neutral:

• Current-carrying
• Returns current to source
• NOT for safety bonding

Grounding (green/bare) = EGC:

• Non-current-carrying
• Safety only
• Bonds metallic parts

Don't confuse:

• Cross-connection causes shock hazards
• Sub-panel sometimes wrongly bonded

Common code violations (250)

Sub-panel neutral bonded:

• Should be isolated
• Common error in DIY work

Missing bonding bushings:

• Concentric KOs
• Inspector flag

Loose ground connections:

• Causes intermittent issues
• Sometimes shock hazard

Undersized GEC:

• Per service rating
• Per table

Old 2-wire system improperly grounded:

• Pre-1962 sometimes
• Refer to GFCI replacement OR EGC retrofit

Customer's older home

Pre-1962 (commonly):

• 2-wire branch circuits (no EGC)
• Ungrounded outlets
• Sometimes BX cable as ground path (questionable)

Options:

• Rewire (expensive, comprehensive)
• GFCI replacements (per 406.4(D)(3))
• Selective upgrades
• Customer decision

Service entrance grounding

Common scenarios:

Overhead service:

• Drip loop
• GEC at meter base OR main panel
• Per utility

Underground service:

• Conduit to meter
• Similar grounding
• Sometimes utility-supplied ground

Mast service:

• Through roof
• Specific code requirements

Substation / commercial

Larger scale:

• Ground grid
• Multiple electrodes
• Engineered system
• Specialty

Common misunderstandings

"Ground rod makes it safe":

• Earth NOT fault path
• EGC + bonding is what protects

"More ground rods = better":

• Diminishing returns
• 2 rods OR 1 with proven low resistance

"Bonding water pipe grounds the system":

• Bonding adds water pipe to system
• Doesn't replace electrode

"GFCI needs ground":

• No - works on imbalance
• Allowed replacement for 2-prong

Customer education

Brief:

• "Ground is safety system"
• "Trips breaker on fault"
• Don't lecture

Testing

Continuity (EGC):

• Multimeter ohms
• Should read very low
• Verify path

Ground resistance (electrode):

• Specialty test equipment (3-pole fall-of-potential)
• 25 ohms or less per code
• Less common in field

Polarity (receptacle tester):

• Hot/neutral/ground correct
• Reverse polarity = code violation

Documentation

For grounding work:

• Electrode type + location
• GEC size + path
• Bonding completeness
• Per code

Specific scenarios

Customer's panel upgrade:

• New service per code
• Grounding electrode system updated
• Bonding verified

Customer's added sub-panel:

• Neutral isolated
• Ground bonded
• 4-wire feed (separate neutral + ground)

Customer's pool / hot tub:

• Equipotential bonding grid (per 680)
• Specialty
• Specific code

Customer's mobile / manufactured home:

• Specific grounding rules (per 550)
• Different from site-built

Cross-trade observations

HVAC tech:

• Sees equipment grounding
• Verify EGC present
• Bonding to equipment

Plumber:

• Replacing metal with PEX loses ground path
• Notify electrician
• Customer's continued grounding

References

• NEC Article 250 (Grounding and Bonding)
• IAEI grounding/bonding seminars
• Mike Holt grounding/bonding training
• Manuall internal: Common Grounding + Bonding, Electrical Box Sizing