Short answer: Article 502 covers Class II locations, where combustible dust is the hazard, and Article 503 covers Class III locations, where easily ignitible fibers or flyings are the hazard (NEC 502.1) (NEC 503.1). In Class II, Division 1, wiring is generally threaded rigid or intermediate metal conduit, or listed cable types with fittings listed for the location (NEC 502.10). Where a raceway connects a dust-ignitionproof enclosure to one that is not, dust is kept out by a seal, or by a horizontal raceway at least 10 feet long, or a vertical raceway at least 5 feet long extending down from the dust-ignitionproof enclosure (NEC 502.15).
PSI lists Special Occupancies, Equipment and Conditions as a subject area of the Texas Journeyman exam: 6 scored NEC Knowledge questions and 3 scored Calculations questions. PSI does not say how many cover Class II or Class III locations. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Start from the classification
Class, Division, the dust groups and the Class I gas rules are on NEC 500 hazardous locations for apprentices. This page picks up where that one ends: once a question tells you the location is Class II (dust, Article 502) or Class III (fibers or flyings, Article 503), which wiring methods, seals and exposed-part limits apply.
Class II wiring methods
- Division 1. Threaded rigid metal conduit or threaded steel intermediate metal conduit, Type MI cable with termination fittings listed for the location, and certain other listed cable types named in the section. Boxes, fittings and flexible connections each have their own conditions in the section; where flexibility is needed, use only the flexible connections it lists, with the fittings it requires (NEC 502.10).
- Division 2. Everything allowed in Division 1, plus a wider list that includes EMT and dusttight wireways, with the conditions in the section (NEC 502.10).
Equipment rules depend on the division and on the type of equipment, so for a device question, go to the section for that device in Article 502 instead of assuming one rule for all equipment.
The Class II seal rule
A Class II seal does not need to be explosionproof. Its job is to keep dust out of a dust-ignitionproof enclosure. Where a raceway connects a dust-ignitionproof enclosure to one that is not, first check whether the other enclosure is in an unclassified location, because the section has an exception for that case. Otherwise, the section lists ways to keep dust out; each has conditions, so check them in your 2026 NEC (NEC 502.15):
- A permanent and effective seal.
- A horizontal raceway at least 10 feet long.
- A vertical raceway at least 5 feet long, extending downward from the dust-ignitionproof enclosure.
- Another arrangement the section allows; check the list in your 2026 NEC.
Sealing fittings used for this are accessible.
Class III wiring and exposed parts
Class III, Division 1 wiring includes rigid metal conduit, intermediate metal conduit, PVC, EMT, dusttight wireways and Type MC or MI cable with listed termination fittings, among the methods listed in the section, with dusttight boxes and fittings (NEC 503.10(A)(1)). For Division 2, compare the list in 503.10(B) with Division 1. On a question, find the division first, then check that the wiring method named is on that division’s list and meets its conditions. A distinctive Class III rule limits uninsulated exposed parts: the section sets a low voltage limit, lower in wet locations, and requires a protection technique suitable for the location. Read the exact values and conditions in 503.25.
Worked example: does this raceway need a Class II seal?
Each raceway connects a dust-ignitionproof enclosure to an enclosure that is not dust-ignitionproof. Both enclosures are inside the Class II location, and no sealing fitting is installed. The question is only whether the raceway arrangement qualifies under one of the options in 502.15.
- 12 feet of horizontal raceway. At least 10 feet horizontal. No seal required under the horizontal option.
- 4 feet of raceway running straight down from the dust-ignitionproof enclosure. Less than 5 feet vertical. A seal or another option in 502.15 is required.
- 6 feet of raceway running straight up from the dust-ignitionproof enclosure. The vertical option is for a raceway extending downward. This run does not qualify under that option; check the other options.
Check yourself
- A raceway runs 12 feet horizontally from a dust-ignitionproof enclosure to a general-purpose enclosure. Which 502.15 option can apply? The horizontal raceway option, at least 10 feet, subject to the conditions in the section.
- Does a Class II seal have to be explosionproof? No. It keeps dust out; it is not required to be explosionproof.
- Where do you look for the limit on uninsulated exposed parts in a Class III location? 503.25, which sets a voltage limit, a lower one for wet locations, and a protection technique requirement.
Disclosure: If you buy through the Amazon link below, Tsai Electro may earn a commission at no extra cost to you. As an Amazon Associate I earn from qualifying purchases.
Find Articles 502 and 503 faster on exam day
NFPA 70, National Electrical Code, 2026 Edition (softcover). Mark the seal options in 502.15 and write “10 ft horizontal, 5 ft down” beside them, so you find them fast and then check the conditions on each option. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What is the difference between Class II and Class III?
Class II is combustible dust, covered in Article 502. Class III is easily ignitible fibers or flyings, covered in Article 503.
Where are the Class II dust groups defined?
In Article 500, with the rest of the classification rules. Article 502 is where the wiring rules for Class II locations are.
When is a Class II raceway seal not needed?
When dust is kept out another way allowed by 502.15, such as a horizontal raceway at least 10 feet long or a vertical raceway at least 5 feet long extending down from the dust-ignitionproof enclosure.
Can EMT be used in Class II locations?
EMT is among the methods listed for Class II, Division 2, not Division 1. Check 502.10 for the conditions.
Are exposed bare parts allowed in a Class III location?
Only within the voltage limit in 503.25, with a lower limit in wet locations, and with a protection technique suitable for the location. Check the exact values in your 2026 NEC.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Articles 500, 502 and 503
