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Art. 500/501 — Classified Locations: Classes & Divisions

Class IFlammable gases/vapors. Div 1 = normal presence; Div 2 = abnormal (leaks only).
Class IICombustible dusts (grain/metal). Same Div 1/2 concept.
Class IIIIgnitable fibers/flyings—surface accumulations & handling areas.
SelectionEquipment must be identified for class/division & group; seals, enclosures, wiring methods vary accordingly.
501.15Sealing requirements: install explosionproof seals where required at boundaries and where the wiring method demands it—placement follows 501.15 notes.
T-CodesEquipment temperature codes (T-ratings) must be suitable for the ignitable material—verify against the area's gas/dust group and temperature class.

EXAM TRAP

Class vs Division

Class = material type (gas/dust/fiber). Division = likelihood (1 = normal, 2 = abnormal). Don’t mix them.

Identify Class **and** Division before selecting gear.

Seal placement follows listing & code notes.

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Art. 502/503 — Combustible Dusts & Ignitable Fibers

Dust LayersLayering dust is a hazard—housekeeping affects classification.
EnclosuresUse dust‑tight enclosures; keep gaskets and covers intact.
Wiring MethodRigid metal conduit or identified systems; flex only if identified for the location.
Surface TempEquipment surface temperature must stay below dust ignition temps.
Seals/BreathersUse identified dust-tight seals or breathers where pressure equalization is needed—don't defeat enclosure ratings.
MaintenanceHousekeeping and maintenance are part of safety: keep covers/gaskets intact and avoid opening enclosures in active dusty operations.

RULE OF THUMB

Dust wins

If dust can settle, plan for it. Choose dust‑tight equipment and fittings; minimize penetrations.

Surface temperature limits are critical in dusty spaces.

Use identified hubs/fittings; generic parts fail inspections.

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Art. 505/506 — Zone System (Gases & Dusts)

ZonesGases: Zone 0/1/2; Dusts: Zone 20/21/22 — likelihood‑based like Divisions but different cut lines.
MarkingsUse equipment marked/listed for the Zone/Group; you may see EPL (Equipment Protection Level) on labels.
DocsMaintain classification drawings; boundaries drive wiring methods and seals.
505.25Conversions: division-classified equipment may be acceptable in Zones when permitted—follow conversion rules and listing limitations.
Cable GlandsSelect Ex e / Ex d glands and wiring components that match the Zone and equipment type; terminations are a common inspection failure.

NEC REFERENCE

Zone labeling

Arts. 505/506 require gear identified for the Zone/Group. Match EPL and marks to each area, not the whole site.

Zones allow finer granularity than Divisions in many facilities.

Verify labels before install; substitutions can fail inspection.

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Art. 517 — Health Care Facilities

BranchesEssential system branches: Life Safety, Critical, Equipment—each with permitted loads.
ReceptaclesPatient care spaces have minimum counts and hospital‑grade devices where required.
BondingEquipotential bonding in patient care areas (verify listed assemblies and methods).
517.16Isolated grounding permitted in patient-care spaces but not a substitute for required bonding—follow 517 bonding methods.
Testing/MarkingHospital-grade receptacles, required counts, and periodic testing—document device locations and branch separation on as-builts.

TABLE

517 Quick Sheet — Essential System Branches

Branch	Purpose / Typical Loads	Examples
Life Safety	Egress, alarms, fire protection; operates during emergency	Egress lighting, fire alarm, exit signs
Critical	Patient care & task loads; continuity of care	Bed receptacles, monitors, task lighting
Equipment	Building services & mechanical systems	HVAC units, pumps, selected elevators

Keep separation and wiring ID per 517; follow the ATS/transfer scheme for each branch.

Life Safety and Critical circuits are managed separately.

Branch separation often reflected physically and by labeling.

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Art. 518/520 — Assembly & Theaters

CablesStage/production spaces allow specific portable cable types; others are prohibited.
Heat/SceneryConsider heat near drapes and scenery—location/shielding can be mandated.
DisconnectsStage lighting/hoists often require readily accessible disconnecting means.
Strain ReliefProvide listed strain reliefs at battens/trusses; avoid sharp bends and abrasion—stage cabling moves and flexes.
Portable Distro BondingBond and ground portable distribution per 520; keep neutrals isolated where required and verify GFCI where mandated.

CHART

Stage Area Cables — Allowed vs Banned

Cable Type	Status
SOOW / SEOOW — extra-hard usage	✅ Allowed
SC — stage cable	✅ Allowed
NM — nonmetallic-sheathed	❌ Prohibited
UF — underground feeder	❌ Prohibited

Use only listed extra-hard usage portable cables in stage spaces. See NEC 520 tables & notes.

Rigging & cabling must match 520 methods.

Plan strain reliefs & routing from day one.

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Art. 525 — Carnivals, Circuses, & Fairs

GFCIBroad GFCI requirements for receptacles and certain equipment—inspectors enforce hard.
ProtectionProtect portable cables from traffic, moisture, and physical damage—covers, elevation, or trenches.
BondingBond noncurrent-carrying metal parts of rides/structures—portable systems need the same continuity as permanent installs.
Cords/UseUse hard/extra-hard usage listed cords; protect from vehicles and foot traffic with ramps or elevation—wet-location fittings where needed.

JOBSITE HORROR STORY

Rain + Cable = Shutdown

A fair lost power after rain pooled over unprotected cable runs. Trip hazards, ground faults, and damaged insulation ended the night early.

Guard & elevate cables—foot traffic will find them.

Use listed GFCI devices and test them—don’t assume.

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Art. 545/550 — Manufactured Buildings & Mobile Homes

ListingFactory wiring is under listing; field connections must honor the manufacturer's instructions and label.
Service/FeederPedestal often = service. Conductors to the home often = feeder; bonding/neutral rules change.
Dwelling RulesSpacing and GFCI/AFCI mirror dwelling rules with extra location specifics.
550.33Four-wire feeder with isolated neutral to the home—bonding occurs only at service equipment, not downstream.
Protection/AccessProtect under-home wiring from damage; keep junctions accessible and use listed transition fittings at the pedestal.

EXAM TRAP

Service vs Feeder

Don’t bond neutrals downstream if it’s a feeder. Bonding point is at service equipment.

Neutral bonding at service; feeders keep neutral isolated.

Follow label details—outside the listing gets flagged.

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Art. 547 — Agricultural Buildings

CorrosionAmmonia & moisture are brutal—use corrosion‑resistant fittings/wiring methods.
BondingEquipotential planes may be required in livestock areas; verify details in 547.
EnclosuresUse drip‑proof/dust‑tight where needed; mind wash‑down zones.
GFCI/AFCIApply GFCI/AFCI requirements per 547 and Chapter 2—wet/corrosive zones commonly require GFCI protection.
LocationsWhen possible, locate sensitive equipment outside corrosive/wash-down areas; use corrosion-resistant materials if it must be inside.

RULE OF THUMB

Survive the wash

If it can’t survive wash‑down & chemicals, it doesn’t belong. Specify like the environment demands.

Select materials for chemical exposure & moisture.

Bonding reduces shock risk for animals & workers.

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Art. 555 — Marinas & Boatyards

GF/ELCIEquipment ground‑fault/ELCI protection is a major theme—people and water don't mix.
BondingBond all metal parts; corrosion & stray currents require solid bonding/grounding.
Marine RatedShore‑power inlets and cord sets must be listed for marine use.
ELCI/GFCIShore-power protection using ELCI/GFCI—expect ~30 mA trip for ELCI per listing; verify device ratings and coordination.
EquipotentialBond docks, pedestals, and conductive parts to reduce shock risk—corrosion control and continuity are both required.

NEC REFERENCE

Shock hazards

Art. 555 targets electric shock drowning: GF/ELCI protection, bonding continuity, and marine‑rated gear.

Use marine‑rated receptacles & enclosures.

Continuity and corrosion resistance matter.

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Art. 590 — Temporary Installations

DurationDefined short‑term use; remove at job completion. 'Temporary' is not forever.
ProtectionProtect cords/cables from damage; elevate or cover where subject to traffic.
GFCIRobust GFCI requirements for construction receptacles & portable distribution.
Listed GearUse listed temp power equipment and hard/extra-hard usage cords—no homemade boxes; weather ratings must match conditions.
Duration/RemovalDefine the time window and remove at completion; document the temp plan and route to avoid 'permanent temporary' installs.

TABLE

Temp Power Quick Sheet

Item	Rule
GFCI	Required on all construction receptacles
Protection	Cover/elevate cords & cables where subject to damage or traffic
Assemblies	Use only listed temporary power distribution equipment
Removal	Remove all temporary wiring when the job is complete

Use listed distribution gear with GFCI.

Protect cords from vehicles & foot traffic.

Chapter 5 — Special Occupancies

Art. 500/501 — Classified Locations: Classes & Divisions

Art. 502/503 — Combustible Dusts & Ignitable Fibers

Art. 505/506 — Zone System (Gases & Dusts)

Art. 517 — Health Care Facilities

Art. 518/520 — Assembly & Theaters

Art. 525 — Carnivals, Circuses, & Fairs

Art. 545/550 — Manufactured Buildings & Mobile Homes

Art. 547 — Agricultural Buildings

Art. 555 — Marinas & Boatyards

Art. 590 — Temporary Installations

Chapter 5 — Field Quick Hits

Class/Division vs Zone

517 Branches

Stage Spaces

Marina Shock Risk

Ag Durability

Temporary Power

Knowledge Check

In Class/Division classification, what does 'Division' describe?

Area seals in Class I locations are placed primarily to:

Which NEC Articles introduce the Zone system for hazardous locations?

Dust accumulation on surfaces can trigger classification even if airborne dust is minimal.

Equipment surface temperature ratings (e.g., T-codes) matter because:

In dust locations, which enclosure approach is typically expected?

Which branch supplies egress lighting and alarms in health care facilities?

Patient care spaces typically require which of the following?

Which cable type is typically prohibited in stage and theater spaces?

Stage lighting systems often require disconnecting means that are:

Carnivals/fairs have broad requirements for which protection?

Where is the neutral-to-ground bond typically located for a mobile home supply?

Agricultural buildings often require equipotential bonding in:

A primary safety focus of Article 555 is preventing:

Temporary wiring must be: