- The Site Sheet
- Posts
- ⚡ Code Corner: 🔸 NEC 503.10(B) – Boxes and Fittings in Class II Locations
⚡ Code Corner: 🔸 NEC 503.10(B) – Boxes and Fittings in Class II Locations
Sharpen your knowledge with a quick dive into the NEC.
Chris Martinez
June 14, 2025
📘 What NEC 503.10(B) says:
Boxes and fittings installed in Class II, Division 1 hazardous (dust) locations must be dust-ignitionproof and approved for Class II use. For Class II, Division 2 locations, standard dust-tight enclosures are permitted if they exclude dust.
🔍 Why it matters:
Dust can be just as dangerous as flammable gases in certain environments—especially combustible metal or grain dusts. If dust enters a box and ignites from a spark or arc, it can cause a serious explosion. That’s why properly rated enclosures are critical in these areas.
💡 Pro Tip:
In Division 1 areas, only use enclosures specifically listed as dust-ignitionproof. In Division 2, you still need a tight seal, but you have more flexibility—so always confirm the classification first.
❓ Quick Code Quiz: Chapter 5 – Hazardous Locations
Choose the correct answer:
Question:
You’re installing boxes in a Class II, Division 1 location. What is the minimum requirement for those boxes?
A) Rain-tight enclosures
B) Dust-tight enclosures
C) Dust-ignitionproof boxes
D) Weatherproof enclosures
Answer below 👇
✅ Correct Answer: C) Dust-ignition proof boxes
Reference:
📘 NEC 2023 – Article 503.10(B)(1):
“Boxes and fittings shall be dust-ignitionproof and approved for Class II locations.”
This applies to Class II, Division 1 hazardous (combustible dust) locations, where enclosures must be capable of containing an internal ignition and preventing combustible dust from entering.
🔍 For further clarification:
Class II, Division 1: Dust is present under normal conditions in sufficient quantities to ignite.
Class II, Division 2: Dust is normally not present, but may be present under abnormal conditions (e.g. equipment failure).
Class II = Combustible dust
Class III = Ignitable fibers/flyings