Speaker Circuit Calculator
Design voice-evacuation speaker circuits the way the submittal has to read: per-circuit tap schedules summed against the amplifier's capacity with reserve shown at both 80% (the practice specs demand) and 100% (what the code actually allows), speaker-line dB loss by wire gauge with the maximum run at your loss budget, and an audibility spot-check against the NFPA 72 floors — printed as an audio circuit calculation sheet in the format plan reviewers already know. Free, no login.
Amplifier
Speakers tapped at (W)
Speakers tapped at (W)
Amplifier loading
Audibility spot-check
Free-field estimate: listed dBA at 10 ft for the tap, −6 dB per distance doubling; doors cost ~10–25 dB. Where a ≥60 s maximum ambient exists, +5 dB over that may govern instead — measure.
Tap sum ≤ amplifier watts (80% gate = spec practice, NOT an NFPA rule — NFPA 72 has no amplifier spare requirement). Loss: 20·log₁₀(Z/(Z+R)), NEC Table 8 pair resistance. Audibility: 18.4.4.1 / 18.4.6.1; ambients per Table A.18.4.4; UL 1480 dBA @ 10 ft.
Audio circuit sheet header (project, amplifier ID, preparer)
Audio Circuit Calculation
Speaker circuit / amplifier loading — prepared with the NORDIX Speaker Circuit Calculator (nordixhq.com/tools/speaker-circuit-calculator)
| Project: — | Amplifier: AMP-1 — 100 W, 25 Vrms |
| Prepared by: — Date: — | Loss limit: −3 dB per circuit; wire per NEC Ch. 9 Table 8 (75°C) |
| Circuit | 0.25 W | 0.5 W | 1 W | 2 W | 4 W | 8 W | 15 W | Load (W) | AWG | Length (ft) | Loss (dB) | Max len (ft) | R (Ω) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1S1 | 8 | 10 | 5 | 4 | 20 | 16 | 825 | −1.99 | 1,318 | 8.07 | |||
| 1S2 | 12 | 6 | 18 | 400 | −0.5 | 2,765 | 6.22 |
| Total connected load | 26 W |
| 80% of amplifier rating | 80 W |
| Reserve capacity based on 80% of maximum | 54 W |
| Actual reserve capacity based on 100% of available power | 74 W |
| Result | WITHIN 80% PRACTICE |
Method: constant-voltage tap loading — each speaker draws its tap wattage; Σ taps ≤ amplifier rating (the 80% gate is submittal/spec practice; NFPA 72 sets no amplifier spare-capacity percentage). Line loss 20·log₁₀(Z/(Z+R)) with Z = V²/ΣW and NEC Chapter 9 Table 8 (75°C) pair resistance. EVACS secondary power: 24 h standby + 15 min at maximum connected load (NFPA 72 10.6.7.2.1.2); audibility per 18.4.4/18.4.5/18.4.6 with ambients per Table A.18.4.4; 520 Hz sleeping-area signals require amplifier + speaker combinations listed for the waveform (18.4.6.3, 24.4.4.2). The panel’s listed installation instructions and the AHJ govern.
How constant-voltage speaker circuits work
A 25 or 70.7 Vrms speaker line is a constant-voltage distribution system: every speaker hangs in parallel through its line-matching transformer, and each draws its tap wattage regardless of how many share the line. The design rules follow directly: the sum of tap watts must not exceed the amplifier’s rating; the tapped load reflects an impedance Z = V² ÷ ΣW; and the wire run loses signal as 20·log₁₀(Z ÷ (Z + Rwire)) — which is why 70.7 V allows exactly 8× the wire length of 25 V at the same gauge and loss. One thing this calculator will not tell you: that NFPA 72 requires spare amplifier capacity — because it doesn’t. Full-text verification of the code found no amplifier loading percentage anywhere; the 80% rule is submittal and specification practice (the classic AHJ form requires showing it, so this tool computes reserve both ways), and the code’s real 20% lives in the battery amp-hour margin. Backup amplifiers are likewise an Annex A recommendation, not a mandate — though amplifier monitoring is required.
Worked example: the AHJ form’s own circuit
The default circuit above reproduces the sample from a published AHJ audio-calculation form:
- Circuit 1S1: 8 speakers at ¼ W + 10 at ½ W + 5 at 1 W + 4 at 2 W = 20.0 W of taps.
- 25 V line → Z = 625 ÷ 20 = 31.25 Ω; 825 ft of 16 AWG (9.78 Ω/1000 ft pair) → R = 8.07 Ω.
- Loss = 20·log₁₀(31.25 ÷ 39.32) = −1.99 dB — inside the form’s −3 dB ceiling; the same circuit could run to ~1,318 ft before hitting it.
- On a 100 W amplifier the 26 W total is 26% loaded — reserve 54 W at the 80% gate, 74 W at the rating.
The audibility floors (NFPA 72 Chapter 18)
Public mode: at least 15 dBA above the average ambient — or 5 dBA above any maximum lasting 60 seconds or more, whichever is greater — measured 5 ft above the floor (18.4.4.1). Private mode drops to +10 (18.4.5.1). Sleeping areas: the same, but never below 75 dBA at the pillow (18.4.6.1), with intervening doors in place — and the tone that does the waking must be the 520 Hz low-frequency signal from equipment listed for it (18.4.6.3, invoked for voice systems by 24.4.4.2); ordinary speaker taps generally can’t be credited for 520 Hz, which is why sounder bases appear alongside speakers in dwelling units. Total output is capped at 110 dBA (18.4.1.2). Speakers are rated in dBA at 10 ft under UL 1480 — subtract 6 dB per distance doubling, and note real tap ladders deviate from the ideal +3 dB per doubling, so the presets here use the listed per-tap values. The code’s own design ambients:
| Occupancy (Table A.18.4.4) | Average ambient | Public-mode floor (+15) |
|---|---|---|
| Business / office | 54 dBA | 69 dBA |
| Educational | 45 dBA | 60 dBA |
| Industrial | 88 dBA | 103 dBA |
| Institutional | 50 dBA | 65 dBA |
| Mercantile / retail | 40 dBA | 55 dBA |
| Mechanical rooms | 91 dBA | 106 dBA |
| Places of assembly | 60 dBA | 75 dBA |
| Residential | 35 dBA | 50 dBA |
| Storage | 30 dBA | 45 dBA |
| Underground / windowless | 40 dBA | 55 dBA |
Reference: listed tap ladders
UL 1480 dBA at 10 ft per tap — note the deviations from the ideal +3 dB per doubling, and UL’s 3-dBA rating milestones (a speaker measuring 89.9 publishes as 87). Use the datasheet ladder for the model you’re installing.
| Speaker | ¼ W | ½ W | 1 W | 2 W | 4 W | 8 W |
|---|---|---|---|---|---|---|
| Wheelock E70 (¼–2 W) | 81 | 83 | 86 | 89 | — | — |
| Wheelock ET70WP (¼–8 W) | 80 | 83 | 86 | 88 | 91 | 93 |
Frequently asked questions
How do you calculate speaker circuit loading on a fire alarm amplifier?
Sum the tap wattages. On a 25 or 70.7 V constant-voltage line every speaker draws its transformer tap setting regardless of how many share the circuit, so eight speakers at ¼ W plus ten at ½ W plus five at 1 W plus four at 2 W is exactly 20 watts of load. That sum must stay within the amplifier's rated output, and submittal practice — required on the classic AHJ audio-calculation form — is to show loading against 80% of the rating with the reserve capacity spelled out both ways. This calculator builds that per-circuit tap schedule and prints the sheet.
Does NFPA 72 require spare amplifier capacity?
No — and this is one of the most repeated myths in the trade. Full-text verification of NFPA 72 (2010 and 2019 editions) finds no amplifier spare-capacity percentage anywhere; every 'spare' in the code concerns supervising-station equipment. The 20% people half-remember is the battery amp-hour margin (10.6.7.2.1.1). The 80% loading rule is real, but it's project-specification and submittal practice — commercial audio practice (Rane) is even more conservative at 1.5× — so this calculator shows reserve at both 80% and 100% and labels the 80% gate as practice, never as code.
What's the difference between 25 volt and 70.7 volt speaker circuits?
Wire length, mostly. The tapped load reflects an impedance Z = V²/W, so at 70.7 V the impedance is exactly 8 times higher than at 25 V for the same wattage — which means 8 times the allowable wire run at the same gauge and loss. 70.7 V is the standard for long commercial runs (the number itself comes from an old UL rule: 70.7 Vrms = 100 V peak, the threshold that once required conduit); 25 V persists in schools and jurisdictions with stricter voltage rules. Most fire alarm speakers and panels are jumper-selectable between both.
How do I calculate dB loss on a speaker circuit?
Loss = 20·log₁₀(Z ÷ (Z + R)), where Z is the load impedance (V² ÷ total tap watts) and R is the round-trip wire resistance. Example from a published AHJ form: 20 W of taps on a 25 V line gives Z = 31.25 Ω; 825 ft of 16 AWG (9.78 Ω per 1000 ft of pair) adds 8.07 Ω, for a loss of 1.99 dB. Design budgets: 0.5 dB is hi-fi grade, 1.5 dB is the fire-alarm-acceptable figure from the System Sensor applications guide, and 3 dB is the ceiling the AHJ form enforces. The calculator computes the actual loss and the maximum run length at your budget for every circuit.
How loud does a fire alarm speaker need to be?
Public mode: at least 15 dBA above the average ambient sound level — or 5 dBA above any maximum lasting 60 seconds or more, whichever is greater — measured 5 ft above the floor (NFPA 72 18.4.4.1). Private mode is +10 dBA. Sleeping areas must also never fall below 75 dBA at the pillow (18.4.6.1), measured with doors closed, and total output is capped at 110 dBA. NFPA's own design ambients (Table A.18.4.4): offices 54 dBA (so 69 required), mechanical rooms 91 (so 106 — right at the cap), residential 35. Speakers are rated in dBA at 10 ft under UL 1480; subtract 6 dB per doubling of distance, and expect a door to cost 10–25 dB.
What speaker tap should I use?
The lowest tap that hits the required level at the farthest listener — then verify, don't guess up. A typical speaker runs 81 dBA at 10 ft on the ¼ W tap climbing to about 89 at 2 W, but real ladders deviate from the ideal +3 dB per doubling, so use the datasheet's listed per-tap values (and note UL publishes ratings only in 3-dBA steps). The published design doctrine favors more speakers at lower taps over fewer loud ones — better intelligibility, less reverberation, and less amplifier load. Taps exist precisely so output can be raised in the field after acceptance testing.
Do fire alarm speakers work for sleeping areas and the 520 Hz requirement?
Only as a listed combination. Sleeping areas require the 520 Hz low-frequency signal (NFPA 72 18.4.6.3, applied to voice systems by 24.4.4.2), and the code's annex is explicit that it can be produced by an amplifier-and-loudspeaker system — but the combination must be listed for the waveform. In practice only specific amp + speaker pairings carry that listing (Potter's EVAX panels list particular models), ordinary speaker taps can't simply be credited, and the common design is 520 Hz sounder bases in the units alongside corridor speakers. The 520 Hz waveform also demands meaningfully more amplifier power — budget for it.
How do speaker circuits affect the battery calculation?
Two ways. First, duration: a voice-evacuation system's secondary supply must run 24 hours of standby and then 15 minutes at maximum connected load (NFPA 72 10.6.7.2.1.2) — 15 minutes, not the 5 used for horn systems. Second, current: the amplifier draws from 24 VDC roughly in proportion to its tapped load (a Fire-Lite worksheet adds about 0.066 A per tapped watt on top of the amp's base current). Take the amplifier's standby and alarm currents from its battery-calculation sheet into our battery calculator with the voice duration selected — the two sheets travel together in the submittal package.
Method: constant-voltage tap loading (Σ tap watts vs amplifier rating; the 80% gate is submittal/spec practice — NFPA 72 contains no amplifier spare-capacity requirement, verified against full code text; the code’s 20% is the battery margin). Line loss from Z = V²/ΣW and NEC Chapter 9 Table 8 (75°C) pair resistance — the basis AHJ forms use; published 20°C tables run longer. Audibility per NFPA 72 (2019/2022) 18.4.4/18.4.5/18.4.6 with Table A.18.4.4 design ambients — free-field estimates; the +5-over-60-s-max branch and final compliance are measured in the field. Intelligibility (STI) is a separate requirement — see the intelligibility reference. Speaker circuits are audio circuits: DC NAC voltage-drop math does not apply, and vice versa. EVACS secondary power = 24 h + 15 min at maximum connected load (10.6.7.2.1.2). The amplifier’s listed installation instructions and the AHJ govern.
Complete the package with the Fire Alarm Battery Calculator (15-minute voice duration), or see the speaker tap & dB chart and voice intelligibility reference for the tables. Horn/strobe circuits are DC — use the NAC Circuit Loading and NAC Voltage Drop calculators instead.