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Electrical Wiring FAQ (Part 2 of 2)
Section - Smoke detector guidelines

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	Many (most?) building codes now require the installation of
	smoke detectors in homes.  In fact, this has been made
	retroactive in many municipalities.

	There are many different types of smoke detectors.  Ionization,
	photo-cell, battery-powered, AC-powered etc.  The only thing
	we're concerned with here, is AC versus battery powered, other
	than to comment that most building codes are based around
	ionization detectors, photocell units being usually for
	somewhat more specialized purposes.  All things being equal, in
	a residential setting with the "ordinary fire", an ionization
	detector will detect smoke before a photo-cell will - indeed,
	in some fires, the smoke is almost invisible, and less likely
	to trip a photo-cell.

	There is another type of fire detectors - "heat detectors".
	These work usually by a small piece of special metal melting at
	110F or so.  These are much better at avoiding false trips.
	But they usually take much longer to trip than a smoke detector, and
	should usually only be considered for triggering sprinkler
	devices (where the consequences of a false trip are quite
	severe).  Heat detectors should not be used as primary fire

	Most building codes that mandate detectors mandated AC-powered
	ones for new construction.  This is because the statistics show
	that, in houses equipped with smoke detectors, a lot more
	people were getting killed in houses with battery-only
	detectors that had dead batteries than were getting killed in
	houses where the breakers tripped and killed an AC-only
	detector.  It's also worth noting that some battery detectors
	are quite sensitive about battery condition.  Some even refuse
	to work if the battery is zinc-carbon (standard cheap battery)
	instead of alkaline (more expensive).

	Our building code discourages the installation of smoke
	detectors on circuits used for other purposes.  This means that
	only a main-panel breaker trip can kill the detectors.  A
	main-panel trip is unlikely even in a fire started by an
	electrical fault until well after the fire has really engulfed the

	These codes also usually require that the AC detectors be
	interconnected so that if one triggers, they all sound the
	alarm.  This is usually done by an additional wire between the

	The above suggests that the best way of doing things is to have
	one circuit dedicated for smoke detectors, and you run 14-3
	between each of the detectors - the red wire being the "gang
	trip" control.

	If you're still concerned about losing power and thereby losing
	your detectors, we suggest either the use of detectors that run
	off AC power with battery backup, OR, adding battery detectors
	into a system that's already adequately covered with AC detectors.

	Battery-only detectors should only be considered a stopgap
	measure in putting detectors into a house that doesn't have any
	detectors at all, or adding redundancy into a system that already
	has AC detectors.

	We also suggest that, if you have battery detectors, you make
	changing the battery a yearly (or semi-yearly) scheduled event.
	Some people change the batteries on their birthdays.  Others
	change the batteries during a "daylight/standard time change"
	maintenance pass.

	In Canada, the day before the standard/daylight time change
	(a Saturday) now seems to be officially called "smoke detector
	battery day" ;-)

	We don't recommend waiting for the detector to tell you that the
	battery is dead, unless you manually test the detector monthly.

User Contributions:

Report this comment as inappropriate
Dec 21, 2011 @ 12:00 am
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
Report this comment as inappropriate
Dec 24, 2011 @ 12:12 pm
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.
Report this comment as inappropriate
Feb 24, 2012 @ 11:11 am
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
Report this comment as inappropriate
Nov 26, 2012 @ 9:21 pm
Does a grounding electrode facilitate the operation of a OCPD, to clear a ground fault ?
Report this comment as inappropriate
Mar 18, 2013 @ 10:10 am
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
Report this comment as inappropriate
Jan 26, 2014 @ 10:10 am
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: Copper wire characteristics table
Next Document: Other links

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Last Update March 27 2014 @ 02:11 PM