NEC 110.18 - Arcing Parts: Or, "How to Keep Your Sparks From Starting a Bonfire"

The "No Kidding" Translation

Look, we all love a good light show – Fourth of July, rock concerts, your buddy's angle grinder hitting a nail in a stud – but some electrical equipment just naturally throws sparks like a wedding throws rice. We're talking disconnect switches under load, motor starters, circuit breakers doing their thing, and anything else that goes "ZAP" during normal operation.

Here's the deal: If your equipment is gonna arc, spark, flame up, or spit molten metal (yeah, molten metal – we're basically metal workers with better insurance), you've got TWO jobs:

1. Lock it up – Put that sparky bastard in an enclosure
2. Keep it away from burny stuff – Separate and isolate it from anything combustible

Notice it says "in ordinary operation." This isn't about fault conditions – we ALL know things can get spicy when something goes wrong. This is about equipment that literally sparks just doing its job. Like that one guy on every crew who causes drama just by showing up.

Key Takeaways (The Stuff That Matters)

• "Arcing parts" means equipment that produces arcs, sparks, flames, or molten metal during normal operation (not just when things go sideways)
• Must be enclosed OR separated/isolated – Your choice, but pick one
• Combustible materials are the enemy – wood, paper, that cardboard you were gonna use as a drop cloth (don't lie, we've all done it)
• This applies to ORDINARY operation – we're not talking about fault conditions here
• Common culprits: contactors, motor starters, switches operated under load, brushes in motors, knife switches, and old-school fuse pullers

Real-World Scenarios (Where This Rule Saves Your Bacon)

Scenario 1: The Sawdust Switchgear Special

You're wiring a woodshop – sawdust EVERYWHERE. The owner wants you to mount a 100A fused disconnect right on the wall next to where he rips lumber. You know that pulling those fuses under any kind of load is gonna create a light show that would make Tesla jealous.

The Wrong Move: Mount it and walk away. Sawdust + arcs = somebody's insurance rates are about to get interesting.

The Right Move: Either put that disconnect inside a proper enclosure (which it should be anyway) or mount it well away from the sawdust production zone. Better yet, use a non-fused disconnect with a OCPD elsewhere. Bonus points: spec a circuit breaker instead so nobody's pulling anything under load.

Scenario 2: The Open Motor Starter Incident

Industrial job, older facility. They've got an ancient open-style motor starter mounted on a plywood backboard (because apparently someone in the '70s thought that was fine). The contactor chatters and sparks every time that motor kicks on – which is about 50 times a day.

The Problem: That plywood is combustible. Those sparks are... well, sparks. You see where this is going.

The Fix: Either replace it with an enclosed starter, or if they're married to their vintage equipment, mount it on non-combustible material and get it away from any flammable storage. And yeah, that means moving the paint thinner that's been sitting six inches away for "only a few months."

Scenario 3: The Farm Service Call

You're doing a service call at a farm (my condolences to your boots). They've got a knife switch for the barn that's mounted right by the hay storage because "it's always been there." Every time they throw that switch, it arcs. You know it, they know it, the mice living in the hay DEFINITELY know it.

What You Tell Them: "I can write you up a quote to move this, or you can keep playing Russian Roulette with your hay and see who wins – you or physics. And just so you know, physics is undefeated."

Then you relocate that switch to a non-combustible location or get it properly enclosed. And you document EVERYTHING because farm insurance adjusters have seen some stuff.

What to Study (Exam Fodder)

For Licensing Exams, Focus On:

1. Definition of "arcing parts" – Equipment that arcs/sparks during normal operation, not fault conditions
2. Two options for compliance: Enclose OR separate/isolate
3. What counts as combustible material – Wood, paper, textiles, flammable liquids, etc.
4. Common equipment this applies to:
   • Motor starters and contactors
   • Knife switches
   • Fused disconnects (when operated under load)
   • Brush-type motors
   • Arc welders (when they're welding, obviously)

Test Question Patterns You'll See:

• "A motor starter that produces arcs during normal operation shall be _____." (Answer: enclosed or separated from combustible material)
• Scenario questions about mounting equipment near wood, stored materials, or in specific occupancies
• True/False about whether this applies to normal operation vs. fault conditions

Pro Tip for Test Day: If the question asks about equipment that sparks and there's combustible stuff nearby, the answer is ALWAYS going to involve either enclosing it or moving it away. The Code doesn't mess around with fire hazards.

The Bottom Line (Foreman's Final Word)

This section is beautifully simple: Sparks + flammable stuff = bad news.

Keep your arcing equipment either buttoned up in proper enclosures or away from anything that can burn. And before you think "nah, it's fine" – remember that insurance investigators love finding electrical causes for fires because then they can deny claims or sue electricians. Don't be that guy.

Your equipment manual will tell you if it produces arcs during normal operation. Read it. (I know, reading manuals is like flossing – we all know we should do it, but...)

When in doubt: Enclose it. Isolate it. Don't create a fire hazard that'll have you explaining to a jury why you thought mounting a spark machine next to kindling was a good career move.

Now get out there and keep those sparks where they belong – contained, controlled, and nowhere near anyone's combustibles.

Stay safe, stay legal, and for the love of Mike, keep the fireworks where they belong – outside on the Fourth of July, not inside your electrical installations.