“Conductors and equipment on the supply side of the power source shall be installed in accordance with the appropriate requirements of Part II, and Chapters 1 through 4. Transformers or other devices supplied from power-supply conductors shall be protected by an overcurrent device rated not over 20 amperes. Exception: The input leads of a transformer or other power source supplying power-limited fire alarm circuits shall be permitted to be smaller than 14 AWG, but not smaller than 18 AWG, if they are not over 12 in. long and if they have insulation that complies with 760.49(B).”
So, the answer is, one foot.
Q. When may a circuit serving a residential fire alarm system in a home be allowed to omit protection by an AFCI? (NOTE: An AFCI is defined in the NEC as “A device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.”)
A. You will find this answer in NEC article 210.12, “Arc-Fault Circuit-Interrupter Protection,” after the requirement for installing AFCI protection in dwellings:
“(B) Dwelling Units. All 120-volt, single phase, 15- and 20-ampere branch circuits supplying outlets installed in dwelling unit family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas shall be protected by a listed arc-fault circuit interrupter, combination-type, installed to provide protection of the branch circuit.”
Exception. “Where a branch circuit to a fire alarm system installed in accordance with 760.41(B) [NPLFA] and 760.121(B) [PLFA] is installed in RMC, IMC, EMT, or steel armored cable, Type AC, meeting the requirements of 250.118, with metal outlet and junction boxes, AFCI protection shall be permitted to be omitted.”
Q. Does NFPA 72 permit residential smoke alarms to be on regular house wiring/circuits (also used for power and lighting) and also fed through arc-fault circuit interrupters?
A. This answer can also be found at 29.6.3 “AC Primary Power Source.” Here in the 2010 edition of NFPA 72 it states the following with our answer found in (5).
“The ac power source shall comply with the following conditions:
(5) Operation of a switch (other than a circuit breaker) or a ground-fault circuit-interrupter shall not cause loss of primary (main) power. Smoke alarms powered by AFCI protected circuits shall have a secondary power source.”
Yes, and since an AFCI will cause the loss of AC power while detecting a potential for a fire ignition, smoke alarms on AFCI circuits must have battery back-up provided.
Q. Does NFPA 70 allow commercial fire alarm systems to be fed through ground-fault circuit interrupters or arc-fault circuit interrupters?
A. In NPFA 70, 2008, under “760.121 Power Sources for PLFA Circuits” you will find the well-known requirement for ‘dedicated branch circuit,’ that agrees with the same requirement found in NFPA 72. However, NFPA 70 adds an additional stipulation for GFCI and AFCI protection when it states “(B) Branch Circuit. An individual branch circuit shall be required for the supply of the power source. This branch circuit shall not be supplied through ground-fault circuit interrupters or arc-fault circuit interrupters.” So, the answer to this is “no”.
Q. Does NFPA 72 require either residential fire alarm systems or 110VAC smoke alarms to be wired on a dedicated (AC primary power) circuit?
A. As mentioned in the first paragraph of this article, you are not required to place your fire alarm control unit or interconnected smoke alarms on a dedicated circuit. According to NFPA 72, 2010, 29.6.3, “AC Primary Power Source. The AC power source shall comply with the following conditions: (4) AC primary (main) power shall be supplied either from a dedicated branch circuit or the un-switched portion of a branch circuit also used for power and lighting.”
This means that it isn’t required to be dedicated. So, it may be dedicated, but should it be dedicated? Many say, and some codes now require, that this circuit not be wired on a dedicated circuit. The reason is, that the resident will not notice (or care) if the dedicated circuit breaker has tripped and will not investigate. However, if the same circuit was also used to operate the refrigerator, or living room lights or television, the homeowner would notice right away and be more inclined to get it fixed.