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Electrical Wiring FAQ (Part 2 of 2)
Section - What is Romex/NM/NMD? What is BX? When should I use each?

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Top Document: Electrical Wiring FAQ (Part 2 of 2)
Previous Document: General outlet placement rules/line capacities
Next Document: Should I use plastic or metal boxes?
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	Romex is a brand name for a type of plastic insulated wire.
	Sometimes called non-metallic sheath.  The formal name is NM.
	This is suitable for use in dry, protected areas (ie: inside
	stud walls, on the sides of joists etc.), that are not subject
	to mechanical damage or excessive heat.  Most newer homes are
	wired almost exclusively with NM wire.  There are several
	different categories of NM cable.

	BX cable -- technically known as armored cable or "AC" has a
	flexible aluminum or steel sheath over the conductors and is
	fairly resistant to damage.

	TECK cable is AC with an additional external thermoplastic
	Protection for cable in concealed locations: where NM or AC cable
	is run through studs, joists or similar wooden members, the outer
	surface of the cable must be kept at least 32mm/1.25" (CEC & NEC)
	from the edges of the wooden members, or the cable should be protected
	from mechanical injury.  This latter protection can take the form of
	metal plates (such as spare outlet box ends) or conduit.

	[Note: inspector-permitted practice in Canada suggests that armored
	cable, or flexible conduit can be used as the mechanical protection,
	but this is technically illegal.]

	Additional protection recommendations: [These are rules in the
	Canadian codes.  The 1993 NEC has many changes that bring
	it close to these rules.  These are reasonable answers to the
	vague "exposed to mechanical damage" in both the NEC and CEC.]

	    - NM cable should be protected against mechanical damage
	      where it passes through floors or on the surface of walls
	      in exposed locations under 5 feet from the floor.
	      Ie: use AC instead, flexible conduit, wooden guards etc.
	    - Where cable is suspended, as in, connections to furnaces
	      or water heaters, the wire should be protected.  Canadian
	      practice is usually to install a junction or outlet
	      box on the wall, and use a short length of AC cable
	      or NM cable in flexible conduit to "jump" to the appliance.
	      Stapling NM to a piece of lumber is also sometimes used.
	    - Where NM cable is run in close proximity to heating
	      ducts or pipe, heat transfer should be minimized by
	      means of a 25mm/1" air space, or suitable insulation
	      material (a wad of fiberglass).
	    - NM cable shall be supported within 300mm/1' of every box
	      or fitting, and at intervals of no more than 1.5m/5'.
	      Holes in joists or studs are considered "supports".
	      Some slack in the cable should be provided adjacent to
	      each box.  [while fishing cable is technically in violation,
	      it is permitted where "proper" support is impractical]
	    - 2 conductor NM cable should never be stapled on edge.
	      [Knight also insists on only one cable per staple, referring
	      to the "workmanship" clause, but this seems more honoured
	      in the breach...]
	    - cable should never be buried in plaster, cement or
	      similar finish, except were required by code [Ie: cable
	      burial with shallow bedrock.].
	    - cable should be protected where it runs behind baseboards.
	    - Cable may not be run on the upper edge of ceiling joists
	      or the lower edges of rafters where the headroom is more
	      than 1m (39").

	Whenever BX cable is terminated at a box with a clamp, small
	plastic bushings must be inserted in the end of the cable to
	prevent the clamps forcing the sharp ends of the armor through
	the insulation.

	Whenever BX cable is buried in thermal insulation, 90C
	wire should be selected, but derated in current carrying
	capacity to 60C.

	BX is sometimes a good idea in a work shop unless covered by
	solid wall coverings.

	In places where damage is more likely (like on the back wall of
	a garage ;-), you may be required to use conduit, a
	UL- (or CSA-) approved metal pipe.  You use various types of
	fittings to join the pipe or provide entrance/exit for the

	Service entrances frequently use a plastic conduit.

	In damp places (eg: buried wiring to outdoor lighting) you will
	need special wire (eg: CEC NMW90, NEC UF).  NMW90 looks like
	very heavy-duty NMD90.  You will usually need short lengths of
	conduit where the wire enters/exits the ground.  [See underground
	wiring section.]

 	Thermoplastic sheath wire (such as NM, NMW etc.) should not be
 	exposed to direct sunlight unless explicitly approved for that

	Many electrical codes do not permit the routing of wire through
	furnace ducts, including cold air return plenums constructed
	by metal sheeting enclosing joist spaces.   The reason for this
	is that if there's a fire, the ducting will spread toxic gasses
	from burning insulation very rapidly through the building.
	Teflon insulated wire is permitted in plenums in many areas.
 	Canada appears to use similar wire designations to the US,
 	except that Canadian wire designations usually include the
 	temperature rating in Celsius.  Eg: "AC90" versus "AC".
	In the US, NM-B is 90 degrees celcius.

	NOTE: local codes vary.  This is one of the items that changes
	most often.  Eg: Chicago codes require conduit *everywhere*.
	There are very different requirements for mobile homes.
	Check your local codes, *especially* if you're doing anything
	that's the slightest out of the ordinary.

	Wire selection table (incomplete - the real tables are enormous,
	uncommon wire types or applications omitted)

	Condition			Type	CEC	NEC

	Exposed/Concealed dry		plastic	NMD90	NM
					armor	AC90	AC

	Exposed/Concealed damp		plastic	NMD90	NMC
					armor	ACWU90

	Exposed/Concealed wet		plastic	NMWU90
					armor	ACWU90
	Exposed to weather		plastic	NMWU
						TW etc.
					armor	TECK90
	Direct earth burial/		plastic	NMWU*	UF
	Service entrance			RWU
					armor	RA90
	[* NMWU not for service entrance]

User Contributions:

In a fire protection circuit, circuts are shown witha no example 6,8,4etc. what it mean?these circuits are connected between smode detector,junction box etc
My daughter dropped a small necklace behind her dresser. The necklace crossed a plug terminal and shorted the receptacle.
I bought a new receptacle and installed the same. I still have no power I suspect there could be a bigger problem,this is aluminum wiring.
I've killed the breaker and call an electrician but am curious as to what happened.P.s. there is a dimmer switch on the same circuit.
Regarding new construction wiring and running 12/2 and 14/3 wire in the same box.

I have multiple switches to lights. Ran 12/2 and 14/3 into switch box and inspector wrote correction needed.

What should I have done instead?

thank you
Does a grounding electrode facilitate the operation of a OCPD, to clear a ground fault ?
Assuming you are installing two switches in a two switch box, you probably should have used 14/2 and 14/3 instead of replacing 14/2 with 12/2. If you are only installing one switch in a one switch box, you should only have one cable in the box.
P k
I prefer to use nothing smaller than12 awg /the smallest sized wire on a circuit determines the allowable ampacity
Ex: 15 amp-14awg. 12awg-20amp only rule for thumb other factors such as continuous load,heating and others if you do not know the safe NEC rules then please call a qualified journeyman Electrician better be safe

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Top Document: Electrical Wiring FAQ (Part 2 of 2)
Previous Document: General outlet placement rules/line capacities
Next Document: Should I use plastic or metal boxes?

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Last Update March 27 2014 @ 02:11 PM