Residential wiring 101

Just as residential and commercial fire alarm requirements differ significantly, so do the rules governing their wiring. The wiring types and unique exceptions allowed for residential (one- and two-family homes) fire alarm systems are found in the National Electrical Code, Article 760. Before you hang anything anywhere-from panels to sensors and detectors to speakers and other accessories, you have to know the correct wiring requirements.

The first type of wire and the most common is FPL (Fire, Power-Limited). FPL is the general purpose rating needed for both commercial and residential fire alarm systems. At one time this wire was required to be made up of seven strands or less. Rather than make special stranded wire, the use of solid copper became the default product. Making the outer jacket/insulation red in color was just a marketing strategy. Reality is, wire/cables used in fire alarm systems can be any color and today conductors are made from both stranded and solid copper. Stranded (no limit to number of strands) is more flexible and can be routed through drilled holes and pulled through conduit more easily than its stiffer counterpart. It is also less likely to crack the terminal strip off circuit boards.

Multi-conductor fire rated cable is permitted to be as small as 26 gauge, but single conductors can be no less than 18 gauge. The 18-gauge limit is so it has enough cross section area to give it strength and not stretch or break easily. Manufacturers have made 18 the default wire gauge even for multi-conductor cables and is the most common size FPL sold. Today, we have grown accustomed to buying heavier conductors for notification-appliance circuits and longer SLC runs. What about using wire smaller than 18 gauge?

NEC 760.61 provides a cable listing hierarchy table which includes allowable substitutions for the standard FPL, FPLR or FPLP. The MP Multi-purpose designation was dropped from the list leaving only one permitted substitute called "communications" cable or CM. Communications cable, like FPL, is marked with the suffix letters for P-plenum (CMP) and R-riser (CMR). (Note that CMX and CMH communication cables are not listed by NEC 760.61 as substitutes for FPL cables.) Type CL2 cable (typically used for security alarm wiring) can be dual rated and marked with the additional CM designation. CM cable, like FPL cables, is available in orange, green, purple, black, white, gray, yellow, pink and RED, but CM's most common color is blue. Category 5e phone/data wire is a common example of CM-rated cable and comes in 22 and 24 gauge. Since CM is an approved equal for FPL all that is needed to allow its use for fire alarm systems is for the manufacturer to state that their equipment's terminals are suitable for use with 22- or 24-gauge wire. Using blue Cat 5e cable to wire your keypad/annunciators is perfectly legal and specifically allowed.

If your wire run will be used to transmit any fire alarm signals over an IDC or NAC, then FPL or a permitted substitute cable must be used. You may use CL2 from any security alarm device to the point ID module on addressable systems; but if the SLC is also used for fire alarm signals, then the SLC must be FPL/CM-rated wire. The secondary wiring from the transformer must also be FPL or one of the permitted substitute wiring cables.

Blast through the myths
NEC includes several exceptions for residential fire alarm system wiring.

First, we should clear up a misconception regarding FPLR (Riser) wiring in a home. FPLR wire is not required to be used in a residence, regardless of the height or size of the home (NEC, 2008, 760.154(b)(3)). A second misconception is that FPLP (Plenum) rated cable is required in a home- it is not. Neither is residential wiring required to be in conduit when passing through the width of an air duct space. This is the logic for that exception: in a home, air for heating and cooling throughout the house is commonly 'ducted' by using the space formed when drywall is attached to both sides of wall studs. Air returns are often made by using sheet metal to enclose the space formed by floor joists and the floor above. If the wooden floor joists caught fire, they would produce much more smoke than the little bit of FPL run through that space, so the FPL is of no consequence. However, the NEC does NOT permit running FPL/CM wiring exposed inside of, or along the length of these air-handling spaces (NEC, 2008, 300.22 (C)). This should never be a problem for experienced technicians since there are so many other spaces and routes available to run wiring inside a home.

Firestopping in a home isn't necessary, or required by code, since homes contain no fire barriers to be penetrated. There are a few other specific wiring rules that need to be followed. Your low-voltage alarm wiring should always be at least two inches away from other 110/220vac wiring in the home to avoid induced voltages and electrical "noise." The two-inch rule doesn't apply to non-metallic sheathed cable (Romex(r)). [This exception, found at NEC, 2008, 760.136 (G)(1) allows you to share the same stud that contains wiring for lights and outlets.]

Good workmanship means you will only drill a hole where permitted (do not weaken framing members) and then only large enough for your cable(s) to pass through without binding or sticking. When running cable along the side of framing members it must be kept back from any edge, at least 11/4 inches. If you notch the wood, the edge of the hole must be made at least 11/4 inches back from the edge, and you must use "a listed and marked" steel plate or sleeve to protect the wiring from nails and screws. These plates are cheap and plentiful from any electrical supply house and using them will keep you on good terms with the drywall contractors.

Following the manufacturer's installation instructions ensures the equipment will be installed in compliance with the codes and standards and operate as required by NFPA 72. Don't miss a chance to keep your local inspector 'in the loop.' You can educate them on code-compliant wiring methods by using your installations as shining examples of proper workmanship.

Greg Kessinger, CFPS, SET is SD&I's resident fire expert and regular contributor, reach him at greg@firelarm.org.

 

Loading