Capstone: laying out devices for a school wing

Ravi is on holiday for two weeks when the architect for Willowbank School's summer addition sends the final floor plans, and the school board wants the fire alarm layout for the new wing within a week, so the electrical contractor can price it. Donna asks Sasha to prepare the first draft of the device layout herself, with Gerry checking her work and Ravi reviewing it when he returns. It is the job this course has prepared her for: choosing every initiating device and notification appliance for a real wing of a real building, placing each one by the rules, and producing a device schedule that the designer, the contractor and the authority can follow. This capstone follows her through it, space by space, and ends with the issues Ravi's review finds.

The wing and the brief

The new wing is one story, sprinklered throughout, with a ceiling 10 ft high in most spaces. It contains eight classrooms of 30 ft by 26 ft, a library of 49 ft by 40 ft, a music room with a shed ceiling rising from 10 ft to 20 ft, the gym of 100 ft by 66 ft with its sloped roof at 23 to 26 ft, a staff workroom, two washrooms, a mechanical room with the air handler for the wing, and a corridor 10 ft wide and 210 ft long joining them. The corridor has four exits to the outside, and the gym has two more of its own. The existing panel will be replaced with an addressable panel that serves both the old and new parts of the school, so the wing's devices are addressable.

The brief from the school board and the authority sets the system's design: a general evacuation system under the building code for an educational occupancy, with smoke detection in the corridor and in spaces where the code or the board's standards require it, heat detection where smoke detectors would cause nuisance alarms, manual fire alarm boxes at every exit, horn-strobes throughout, sprinkler waterflow and tamper monitoring, a duct smoke detector to shut down the new air handler, and connections to the door holders on the corridor's fire doors. The installation follows NFPA 72.

Step 1: initiating devices, space by space

Sasha starts with the corridor, because it is the escape route. It is 10 ft wide with a smooth flat ceiling, so detectors on its centerline can be spaced up to about 40 ft apart by the geometry of Lesson 11, with the ends within half that. Over 210 ft, that gives six detectors, with the end detectors near the end walls; she adds one more at each pair of fire doors, because the door holders there should release from a detector on either side of the doors, as the sequence of operation calls for. The classrooms and library are sprinklered and occupied by day, and under the brief they get no smoke detectors. The music room, the workroom and the washrooms likewise need none under the brief, except a heat detector in the workroom's kitchenette, because the board's standard asks for one where there is a stove.

The gym's sloped roof and high ceiling suit beam detection, and the board's standard asks for detection in large assembly spaces. Using the beam listing from Lesson 3, two beams running the length of the gym, each about 16 ft from a side wall, cover its width. In the mechanical room, a duct smoke detector goes in the air handler's supply duct, downstream of the fan and filters, and Sasha checks the duct air speed against the detector's listed range using the air handler's schedule. Because the room is hot and dusty, a heat detector goes there too, as the brief allows. At the sprinkler riser, the wing's floor control assembly gets a waterflow switch and a tamper switch, each on its own input module. A manual station goes at each of the six exits.

Step 2: notification appliances

Sasha uses the ambient levels Ravi measured in Lesson 9 for the noisy spaces and typical values for the others. Every classroom, the library, the music room, the workroom and both washrooms get a horn-strobe, because the doors are normally closed and the strobes are needed for accessibility. The corridor gets horn-strobes spaced along its length. The gym gets the six horn-strobes from Ravi's earlier gym design, with the sound system muted on alarm. She sizes each room's strobe from the room table in Lesson 10, using the longer side of each room, and she checks each space's required audible level against the horn's output at the farthest point.

# Draft device schedule for the new wing (NFPA 72 layout)
import math
S = 30                                 # smoke detector spacing on smooth flat ceilings (ft)
strobe_table = [(20, 15), (28, 30), (40, 60), (44, 75), (54, 95), (59, 110), (63, 135), (68, 185)]                   # (max room side ft, minimum cd) for one wall strobe
def cd_for(side):
    return next((c for s, c in strobe_table if side <= s), None)
def corridor_count(length, width):
    r = 0.7 * S
    sc = 2 * math.sqrt(r * r - (width / 2) ** 2)
    return math.ceil(length / sc)
spaces = [  # name, length, width, smoke detection?, notes
    ("Corridor", 210, 10, True, "plus 2 at fire doors"),
    ("Classroom (x8)", 30, 26, False, ""),
    ("Library", 49, 40, False, ""),
    ("Music room", 40, 33, False, ""),
    ("Workroom", 23, 16, False, "heat detector at kitchenette"),
]
total_det = 0
for name, L, W, smoke, note in spaces:
    det = (corridor_count(L, W) + 2) if smoke and name == "Corridor" else 0
    total_det += det
    side = max(L, W)
    cd = cd_for(side) if name != "Corridor" else 15
    strobe = f"1 wall strobe {cd} cd" if cd else "several strobes (too large for one)"
    print(f"{name:16s} smoke detectors {det:2d}  {strobe:34s} {note}")
print(f"corridor smoke detectors in total: {total_det}")

Corridor         smoke detectors  8  1 wall strobe 15 cd                plus 2 at fire doors
Classroom (x8)   smoke detectors  0  1 wall strobe 60 cd                
Library          smoke detectors  0  1 wall strobe 95 cd                
Music room       smoke detectors  0  1 wall strobe 60 cd                
Workroom         smoke detectors  0  1 wall strobe 30 cd                heat detector at kitchenette
corridor smoke detectors in total: 8

The schedule is a draft: corridor strobes are spaced along the corridor by their own rule, not by the room table, so the script just marks them at 15 cd for the count, and the gym and mechanical room are listed separately in her full schedule. The library's longer side of 49 ft needs 95 cd for a single strobe; she shows two 60 cd strobes on opposite walls instead, if the multi-strobe table allows it, because the shelving would hide part of the room from a single strobe.

Step 3: the audibility check

For each space, Sasha checks the level at the farthest point from its horn against the requirement. In a classroom, the horn on the wall is about 30 ft from the far corner; a horn rated at 85 dBA at 10 ft gives about 75 dBA there, which meets the requirement for a classroom with an ambient of about 55 dBA. In the music room during band practice, the ambient reaches about 85 dBA, so a single horn can't reach the required level without being painful close up; she adds a second horn and flags the room for Ravi, suggesting that the music room's sound system be muted on alarm as in the gym. The washrooms are small and reflective, and one horn-strobe each is ample.

# Audibility check at the farthest point of each space (one horn unless noted)
import math
L_ref, d_ref = 85.0, 10          # horn rating (dBA at ft)
spaces = [  # name, distance to farthest point (ft), ambient (dBA), horns
    ("Classroom", 30, 55, 1),
    ("Library", 33, 45, 2),
    ("Music room (band)", 40, 85, 1),
    ("Music room (band)", 23, 85, 2),
    ("Workroom", 23, 50, 1),
]
for name, d, amb, n in spaces:
    one = L_ref - 20 * math.log10(d / d_ref)
    level = one + 10 * math.log10(n)
    need = amb + 15
    ok = "meets" if level >= need else "SHORT"
    print(f"{name:18s} horns {n}: {level:5.1f} dBA at {d} ft, need {need:.0f} -> {ok}")

Classroom          horns 1:  75.5 dBA at 30 ft, need 70 -> meets
Library            horns 2:  77.6 dBA at 33 ft, need 60 -> meets
Music room (band)  horns 1:  73.0 dBA at 40 ft, need 100 -> SHORT
Music room (band)  horns 2:  80.8 dBA at 23 ft, need 100 -> SHORT
Workroom           horns 1:  77.8 dBA at 23 ft, need 65 -> meets

The check shows the music room still short with two horns. The real answer, as in the gym, is to remove the source of the noise: a relay that silences the music room's amplified sound on alarm, so the ambient during an alarm is the room's acoustic noise rather than the band's speakers. Live instruments will stop when people hear and see the alarm; the strobes make sure they see it.

Step 4: manual stations, sprinkler devices and controls

The six exits get manual fire alarm boxes, each within 5 ft of the exit door with its handle at 45 in, inside NFPA 72's range, and she checks that no point in the wing is more than 200 ft of travel from a box. None of the openings is wider than 40 ft, so one box per exit is enough. The riser's waterflow switch gets a retard set so the alarm is indicated within 90 seconds, and the tamper switch on the floor control valve is individually addressed. The duct detector's output shuts down the air handler through a control module, and the corridor detectors at the fire doors release the door holders through a second module. Each control function is a row in the sequence of operation.

Ravi's review

When Ravi returns, he reviews the layout with Sasha and the drawings side by side. Most of it stands. He finds four issues, which she corrects. First, the corridor's two fire-door detectors each sit right beside a supply diffuser on the ceiling plan, which the mechanical drawings show; they must move to keep the required distance. Second, the music room's horn locations put one directly beside the conductor's podium, which would be painful; he moves it and accepts the sound-system mute as the main fix. Third, the workroom's heat detector is a rate-of-rise type, and the kitchenette has a toaster oven and a kettle near it; he changes it to fixed-temperature. Fourth, she has the gym's beams mounted on the roof joists; he moves them to the masonry end walls, as in Lesson 3.

He also points out something she couldn't have known: the authority's plan reviewer for the school board has asked, on other projects, for smoke detection in the library because of its large fuel load and the long time it is unoccupied after school. That isn't in the brief, so he raises it with the board as an option rather than adding it. The board decides to add it, and Sasha lays out the library's detectors on a 30 ft grid: four detectors, each within half the spacing of the walls, clear of the diffusers and the light fixtures.

What the layout shows

The finished layout has, for the wing, nine corridor smoke detectors, four library smoke detectors, two beam detectors in the gym, two heat detectors, a duct detector, a waterflow switch, a tamper switch, six manual stations, one at each exit, and horn-strobes in every room, the corridor and the gym, each with its candela setting and horn volume recorded. Each device has an address and a label on the device schedule, and every control function has its row. The layout goes to the authority with Ravi's stamp, and to the electrical contractor for pricing.

Sasha's draft also shows what a layout can't do on its own. Every rule in this course assumes the building is built as drawn. If the contractor moves a diffuser, adds a bulkhead under a detector, or a teacher stacks boxes to the ceiling in a corner, the protection changes. That is why the layout is followed by installation inspections, verification and periodic testing, which are the subjects of FAT-220, FAT-230 and FAT-330. Ravi's last comment on her draft is: 'Good layout. Now go and make sure they build it.'