Wood Working Shop

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charlie b

charlie b

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Semi-Retired Electrical Engineer
Would wood that is collectively being tossed aside by a table saw, radial arm saw, band saw, table router, belt sander, and planer create the "combustible flyings" that NEC 500.5(D) includes in the definition of Class III locations? Would the presence of a central dust collection system (or separate dust collectors at each work station) change the situation? Is there a point (e.g., size of room, number of machines, hours per day of operation, etc.) beyond which a wood working shop should be treated as Class III?

The shop that leads me to ask these questions is built, but not yet in operation, and the owner is wondering if the correct wiring methods were used.
 
charlie b said:
Would wood that is collectively being tossed aside by a table saw, radial arm saw, band saw, table router, belt sander, and planer create the "combustible flyings" that NEC 500.5(D) includes in the definition of Class III locations? Would the presence of a central dust collection system (or separate dust collectors at each work station) change the situation? Is there a point (e.g., size of room, number of machines, hours per day of operation, etc.) beyond which a wood working shop should be treated as Class III?

The shop that leads me to ask these questions is built, but not yet in operation, and the owner is wondering if the correct wiring methods were used.
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I worked at a large sawmill that employed 150+ people. The 70+ acres had a huge amount pf equipment on it, including it's own 4160 v co generation plant. The only cassified area in the entire plant was the inside of the dust silos.
 
JMHO. I don't think small amounts of sawdust that are not allowed to pile up are a hazard.

The particle size is pretty big and it takes a fair amount of energy to ignite sawdust. A typical spark probably is not going to do it.

And if there is a good dust collection system there is no dust to worry about.
 
Dust collection systems are the real hazard in a big shop. Most large woodworking plants either install explosion protection or will (if they survive the big bang). Since there is normally a wide range of saw dust particle sizes generated, some will indeed be quite small. The limiting factor is usually the dust concentration in air. Dust collectors concentrate the dust, continuously in a cyclone and during bag house cleaning cycles. This is a situation for a knowledgeable professional.
 
While the Scope statement of Art 503 mentions explosions, the actual emphasis is fires. See 503.5. If an explosion is potentially likely, then it is dust (wood flour) that is the material of concern rather than fibers or flyings, and it is Class II, Group G rather than Class III.

Class III is fairly generous with wiring methods [503.10]. Enclosures and utilization equipment is usually specified to be ?dusttight? (Types 3, 3S, 5, 12,12K, or 13) because there is no Type for ?fibers or flyings-tight" :D and/or the emphasis is on the ?T-rating?. ?Dust-ignitionproof? (Type 9) is never specified since there is essentially no difference between Division 1 and 2. In general, good housekeeping practices can wipe out Class III locations entirely.

Dust collection is actually Class II, Group G as noted above and directly associated equipment should be identified as such. However, good housekeeping practices also greatly reduce Class II envelopes. See NFPA 499.
 
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