Power, Loading and Circuits (NFPA 72 Ch. 10 & 12, NEC Art. 760)
NFPA 72-2022 Ch. 10 (equipment, power supplies) · NFPA 72-2022 Ch. 12 (circuits and pathways) · NFPA 72-2022 Ch. 7 (documentation and calculations) · NFPA 70-2020 Art. 760 and Art. 770 · NFPA 70-2020 Ch. 3 (wiring methods, raceways, boxes) and Ch. 9 (Tables 1, 4, 5, 8) · Outline tasks 2.1.2, 2.3.4
What NICET expects at Level II for this topic
This pack covers task 2.3.4 (determine power supply and loading requirements) and task 2.1.2 (install fire alarm infrastructure: circuits, pathways, raceways). A Level II technician works with limited supervision, so the questions hand you loads, wire sizes, distances, or a pathway and raceway situation and ask for a number or a ruling: What battery? What gauge? What voltage reaches the last appliance? Is this riser compliant, and what is the fix? Each calculation states the factors it wants (for example the 25% margin or the starting voltage). Use the values in the question and look up the tables in your own books.
Power supplies (NFPA 72-2022 Ch. 10)
| Item | Rule | Where |
|---|---|---|
| Primary power | Dedicated branch circuit; disconnect has a red marking and its location is identified at the control unit; access limited to authorized persons; no GFCI or AFCI | §10.6.5; NEC 760.41(B), 760.121(B) |
| Secondary power | Dedicated storage battery, or an automatic-start engine-driven generator plus dedicated batteries with at least 4 h of capacity | §10.6.7 |
| Capacity | 24 h standby, then 5 min of alarm. Voice (EVACS) and in-building mass notification: 15 min at maximum connected load | §10.6.7.2 |
| Transmitting equipment | Equipment that sends signals to a supervising station needs its own secondary supply if it is not powered from the control unit: 24 h standby, then capacity to transmit for 5 min | §10.6.7.2 |
| Margin | Added on top of the calculated Ah. Industry summaries put it at 25% (factor 1.25) in the 2022 edition, 20% before. The question will state the factor | NFPA 72 Ch. 10 (battery calculations) |
| Batteries and charger | Rechargeable batteries are listed or component-recognized; the charger keeps them fully charged; if the batteries are not in or next to the control unit, their location (and the charger's) is permanently identified at the control unit. Never exceed the battery size the charger is listed for | §10.6.10 |
| Equipment | Listed for the purpose; devices powered from a circuit must be compatible with the control unit; operating range around nameplate voltage | §10.3 |
| Documentation | Battery and voltage-drop calculations belong in the submittal | §7.2.1 |
Battery calculation
Ah = (standby A × 24 h + alarm A × 5/60 h) × margin. For voice or mass notification use 15/60 h. Add every load of every device: control unit, SLC devices, annunciators, NAC appliances, relays. Count each load only in the condition where it draws current: a door holder that stays energized on battery power and releases in alarm goes in the standby total only. Then choose the next commercial battery size that the panel charger is listed for. Example with a stated 1.25 margin: standby 0.40 A and alarm 3.0 A give (9.6 + 0.25) × 1.25 = 12.3 Ah. You can also work backward: a given battery divided by 1.25, less the alarm Ah, divided by 24 h, is the largest standby current allowed. A remote NAC power extender, or a communicator in its own enclosure, needs its own calculation, because the main panel batteries do not support it.
Loading: NAC, SLC, IDC and power supplies
- NAC: add the alarm currents of all appliances at the stated voltage and compare with the circuit rating. Then add all the circuits on the supply and compare with the supply rating: four 2.5 A outputs do not make a 10 A supply unless the manufacturer says so.
- SLC: address capacity, loop resistance, and loop capacitance come from the manufacturer. Spare capacity (for example 25%) comes from the specification, not from NFPA 72. Addressed isolator and module devices count as addresses.
- IDC: the number of two-wire detectors is limited by the standby current and device count in the compatibility listing. Round down.
Voltage drop
Resistance method: VD = I × (Ω/kft × 2L ÷ 1000). K method: VD = 2 × K × I × L ÷ CM with K = 12.9. L is the one-way length. For different wire sizes in series, add the drops of each segment. For appliances spread along the circuit, either lump the whole load at the far end (the conservative method) or add the drop of each segment using only the current that flows through that segment. For an SLC length limit, divide the manufacturer's loop resistance by 2 × Ω/kft (both conductors). The voltage at the last appliance is the starting voltage minus the drop, and it must be at least the appliance's listed minimum.
| AWG | cmil | Solid Ω/kft | Stranded Ω/kft |
|---|---|---|---|
| 18 | 1620 | 7.77 | 7.95 |
| 16 | 2580 | 4.89 | 4.99 |
| 14 | 4110 | 3.07 | 3.14 |
| 12 | 6530 | 1.93 | 1.98 |
| 10 | 10,380 | 1.21 | 1.24 |
Starting voltage: use the lowest voltage the NAC output can supply in any condition, from the manufacturer's data (about 20.4 V on a 24 V system at battery end of discharge; an unregulated source also varies with the ac line). The minimum operating voltage of an appliance is a listing issue: it comes from the marking and data sheet (for example 16 V), not from NFPA 72.
Pathways and survivability (NFPA 72 Ch. 12)
| Class | What it tolerates |
|---|---|
| A | Redundant return path: keeps operating past a single open; return routed separately (§12.3.8) |
| B | No redundancy: stops at the open; a T-tap leaves branches unmonitored |
| C, N | Integrity verified by end-to-end communication; N adds redundant network paths |
| D, E | Not monitored for integrity (D is fail-safe) |
| X | Like A, plus a short circuit, using isolators |
Survivability is a different question: Level 0 none; Level 1 fully sprinklered building plus metal raceway or armored cable; Level 2 two-hour protection (CI cable, cable system, rated enclosure, or approved alternative); Level 3 sprinklered building plus a Level 2 method; Level 4 (new in 2022) one-hour protection (§12.4). The levels are not a ranking, and a class is not a level. Faults must be annunciated within 200 s (§12.6.1).
Article 760 and wiring methods (NEC 2020)
| Topic | Key value | Section |
|---|---|---|
| PLFA supply side | The 120 V feed is wired with Chapter 1 to 4 methods | 760.127 |
| Plenum / riser cable | FPLP / FPLR; FPLP satisfies both | 760.154 |
| Plenum raceways | Metal types only (EMT, IMC, RMC without coating, FMC); no PVC, ENT, or LFNC | 300.22(C) |
| EMT | Support every 10 ft and within 3 ft of boxes; 360° of bends between pull points | 358.30, 358.26 |
| FMC | Within 12 in. of boxes, then every 4½ ft | 348.30 |
| PVC | 1/2 to 1 in.: every 3 ft; expansion fittings at 1/4 in. or more | Table 352.30, 352.44 |
| EMT outdoors, in concrete | Fittings and boxes listed for wet locations outdoors; concretetight fittings when embedded in concrete | 314.15, 358.42 |
| Underground raceway | Wet location: use THWN-type conductors; cover under a street, driveway, or parking lot is 24 in. | 300.5(B), Table 300.5, Table 310.104(A) |
| Warm-to-cold raceway | Seal the raceway where condensation is a problem, such as a freezer | 300.7(A) |
| Free conductor | 6 in. of free conductor in a box, and 3 in. outside an opening smaller than 8 in. | 300.14 |
| Grounding | EMT is a permitted EGC; EGC size from the breaker rating (20 A: 12 AWG) | 250.118, Table 250.122 |
| Firestopping | Listed through-penetration system installed as listed | 300.21 |
Fill calculations
- Conduit: more than two conductors or cables: 40% of the Table 4 area; add the Table 5 areas of the conductors. A multiconductor cable counts as one conductor with its area taken from its diameter (Note 9). Round up only when the decimal is 0.8 or more (Note 7).
- Wireway: the sum of all conductor areas may not exceed 20% of the interior area (§376.22(A)), not the 40% of a conduit.
- Box: volume allowance per conductor from Table 314.16(B) (14 AWG 2.00; 12 AWG 2.25 in³). All grounding conductors together count once, all internal clamps count once, and each device yoke counts twice.
Finding it in your printed books
- NFPA 72 index: look up "Batteries," "Power supplies," "Secondary power supply," "Pathway," "Survivability," "Monitoring for integrity," and "Circuits." Tab §10.6, §12.3, §12.4, §12.6, and §7.2.1.
- NEC index: look up "Fire alarm systems," "Raceways," "Electrical metallic tubing," "Flexible metal conduit," "Boxes, fill," and "Grounding, equipment." Tab Article 760, Table 760.154, §300.22, Table 300.5, and Article 376.
- NEC back of the book: Chapter 9 Tables 1, 4, 5, 8 and the Notes to Tables, Annex C, Table 314.16(A) and (B), Table 250.122.
- Remember: 10.x means NFPA 72 Chapter 10, 12.x means Chapter 12, and an asterisk marks explanatory Annex A text.
What to tab in your books
| Section | Why |
|---|---|
| NFPA 72 §10.3 | Equipment listed for the purpose, compatibility, operating range |
| NFPA 72 §10.6.5 | Primary power: dedicated branch circuit, red disconnect, location identified at the control unit |
| NFPA 72 §10.6.7 | Secondary power: battery or generator plus 4 h battery |
| NFPA 72 §10.6.7.2 | Capacity: 24 h + 5 min; voice and mass notification 15 min; transmitting equipment 5 min |
| NFPA 72 §10.6.10 | Batteries: listing, charger, remote location identified at the control unit |
| NFPA 72 §7.2.1 | Documentation: battery and voltage-drop calculations |
| NFPA 72 §12.3 | Pathway classes A, B, C, D, E, N, X |
| NFPA 72 §12.3.8 | Separate routing of Class A / X outgoing and return conductors |
| NFPA 72 §12.4 | Pathway survivability Levels 0 to 4 |
| NFPA 72 §12.6.1 | Monitoring for integrity: 200 s |
| NEC 2020 Art. 760 / §760.127 | Fire alarm circuits; supply side of the PLFA source |
| NEC 2020 Table 760.154 and 760.154(A) | Cable applications and substitutions (FPL, FPLR, FPLP) |
| NEC 2020 §300.22(C) | Wiring methods in plenum spaces |
| NEC 2020 Ch. 9 Tables 1, 4, 5, 8 and Notes | Conduit fill, areas, conductor resistance, Notes 4, 7, 9 |
| NEC 2020 Table 314.16(A) and (B) | Box volume and conductor volume allowances |
| NEC 2020 §358.30, §358.26, §348.30, Table 352.30 | Support and bends: EMT, FMC, PVC (expansion fittings: §352.44) |
| NEC 2020 §250.118 and Table 250.122 | Permitted EGCs and EGC size by breaker rating |
| NEC 2020 §300.5(B), Table 300.5, Table 310.104(A) | Underground raceways are wet locations; cover depths; conductor types |
| NEC 2020 §314.15, §358.42 and §300.7(A) | Fittings in wet locations and in concrete; seals between warm and cold areas |
| NEC 2020 §376.22, §300.14 and §300.21 | Wireway fill (20%); free conductor length; firestopping of penetrations |
Chapter vocabulary
- Standby (quiescent) load
- Current the system draws in its normal, non-alarm condition; it is carried for 24 hours in the battery calculation.
- Alarm load
- Current the system draws with the alarm notification appliances operating; it is carried for 5 minutes (15 for voice) in the battery calculation.
- Capacity margin
- Factor applied to the calculated ampere-hours to allow for battery aging (1.25 in the 2022 edition per industry summaries).
- Ampere-hour (Ah)
- Unit of battery capacity: current in amperes multiplied by time in hours.
- End-of-discharge voltage
- Lowest battery voltage at which the panel still operates, about 20.4 V on a 24 V system; the worst-case starting voltage.
- Regulated power supply
- Supply that holds its output voltage constant over its rated load and input range.
- Unregulated power supply
- Supply whose output voltage follows the ac line voltage and the load.
- Voltage drop
- Voltage lost in the circuit conductors: current times conductor resistance.
- Circular mil (cmil)
- Unit of conductor cross-sectional area used in the K-method voltage drop formula.
- NAC power extender
- Remote power supply that adds notification appliance circuits; it needs its own loading and battery calculation.
- Dedicated branch circuit
- Branch circuit that serves only the fire alarm equipment, with no GFCI or AFCI protection.
- Compatibility listing
- Manufacturer and listing information showing which devices may be connected to a given control unit circuit.
- Pathway class
- NFPA 72 designation (A, B, C, D, E, N, X) of how a pathway is monitored and what faults it tolerates.
- Survivability level
- NFPA 72 designation (0 to 4) of how well a pathway is protected from fire; independent of the pathway class.
- Circuit integrity (CI) cable
- Cable listed to keep operating for a stated time (for example 2 hours) in a fire.
- Short-circuit isolator
- Device that opens a shorted section of an SLC so the rest of the loop keeps operating.
- Plenum
- Space used for environmental air, such as above a return-air ceiling; requires plenum-rated cable or metal raceway.
- Riser
- Vertical cable run through more than one floor; requires riser-rated cable or better.
- Firestop
- Listed system that seals a penetration of a fire-rated assembly so the rating is maintained.
- Equipment grounding conductor (EGC)
- Conductor or raceway path that carries ground-fault current back to the source so the breaker opens.
- Pull point
- Box or conduit body where conductors can be pulled; no more than 360 degrees of bends are allowed between pull points.
- Volume allowance
- Cubic inches that Table 314.16(B) assigns to each conductor, clamp, or device when calculating box fill.
- Wireway
- Sheet-metal trough with a hinged or removable cover used to house and protect conductors; the conductors may fill no more than 20 percent of its interior area.
- Concretetight fitting
- EMT coupling or connector made so that concrete cannot enter the raceway; required where EMT is embedded in concrete.
44 practice questions cover this chapter, each with an explanation and the code section. Get lifetime access or try the free practice exam.
More Level II study guides
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- Power, Loading and Circuits
- Detector and Notification Layout
- Control Functions, Interfaces and Commissioning
- Periodic Testing and Records
- Impairments, Deficiencies and Troubleshooting
- IBC 2021 and Building Context
- Work Plans, Contracts and Coordination
- Level II tabbing guide for your printed code books
- Strategy with printed code books
- Key numbers (NFPA 72 · NEC · OSHA)
- Formulas and conversions
- Fire alarm glossary for the NICET exams