FireCAD Speaker Schedule Report - User Guide
Overview
The Speaker Schedule Report is a point-to-point electrical analysis tool designed specifically for 25V and 70V audio speaker circuits. This report is the audio circuit equivalent of the Point-to-Point Report, calculating voltage drop between each speaker and converting power loss into decibel (dB) loss to show acoustic signal attenuation. This ensures speakers will produce adequate audio output at their installed locations.
What is a Speaker Schedule?
A Speaker Schedule report shows every speaker on an audio circuit in sequential order (from amplifier to end-of-line device), calculating:
- Wattage consumption at each speaker
- Voltage levels throughout the circuit
- Decibel (dB) loss from voltage drop (audio signal attenuation)
- Wire resistance and voltage drop between speakers
- Power summary showing circuit capacity vs. usage
Key Distinction: This report uses the same voltage drop calculation method as Point-to-Point Reports, but converts the electrical power loss into decibel loss for audio circuit analysis.
When to Use This Report
Use the Speaker Schedule report when:
- Designing 25V or 70V audio speaker circuits
- Verifying audio output levels (dB) will meet code requirements
- Troubleshooting voltage drop issues on speaker circuits
- Documenting speaker circuit performance for AHJ approval
- Checking if speakers will receive adequate voltage/power
- Analyzing circuits with mixed speaker wattages and tap settings
Report Structure
Each circuit generates a separate worksheet containing:
1. Circuit Header
- Project name
- Panel and circuit identification
- Report title (e.g., "PANEL-1 SPEAKER-1 SPEAKER SCHEDULE")
2. Wattage Summary Section
| Field | Description |
|---|---|
| Max. Circuit Watts | Maximum wattage limit for the audio circuit |
| Total Circuit Watts | Sum of all speaker wattages on the circuit |
| Spare Circuit Watts | Remaining wattage capacity (Max - Total) |
| Spare Circuit Watts % | Percentage of unused capacity |
| Max. Card Watts (optional) | Maximum wattage for the amplifier card |
| Total Card Watts (optional) | Total watts from all circuits on the amplifier |
| Spare Card Watts (optional) | Remaining amplifier capacity |
| Spare Card Watts % (optional) | Percentage of unused amplifier capacity |
Color Coding:
- 🟢 Green: Values within acceptable limits (good)
- 🔴 Red: Values exceeding limits or below minimum (bad)
- ⚪ Gray: Calculated/output values
3. Power Summary Section
| Field | Description |
|---|---|
| Starting Calc. Voltage | Voltage at circuit origin (typically 25V or 70V) |
| Max. Voltage Drop | Total voltage drop from start to end-of-line |
| Voltage Drop % | Percentage of voltage lost |
| Min. Operational Voltage | Minimum voltage required for speakers to operate |
| End Of Line Voltage | Actual voltage at the last speaker |
| Max. dB Loss | Total decibel loss from start to end of circuit |
| Wire Resistance (Ω/kFt) | Resistance per 1000 feet for selected wire gauge |
| Total Circuit Length | Total wire length including overage percentage |
| Total Circuit Resistance | Total wire resistance for round-trip circuit |
4. Device Table
The report includes 13 default columns showing detailed information for each speaker:
Default Columns:
1. Device Label: Speaker address/location label
2. Part No: Manufacturer part number
3. Description: Speaker description
4. Candelas: Light intensity rating (for speaker/strobes only)
5. Watts: Power consumption in watts (tap setting)
6. Remaining Watts: Total watts from downstream speakers
7. Distance From Previous: Wire distance to previous speaker
8. Resistance From Previous (Ω): Wire resistance to previous speaker
9. Voltage Drop From Previous: Voltage lost in wire segment
10. Voltage At Device: Voltage available at this speaker
11. Total Voltage Drop: Cumulative voltage drop from circuit start
12. dB Loss From Previous: Decibel loss in this wire segment
13. Total dB Loss: Cumulative dB loss from circuit start
Calculation Methods Explained
1. Watts To Amps Conversion
Why This Conversion is Necessary
Voltage drop calculations require current (Amperes) because of Ohm's Law: V = I × R. However, speakers are rated by power consumption (Watts), not current. This conversion bridges the gap between speaker specifications and electrical circuit analysis.
The fundamental relationship:
Power (Watts) = Voltage (Volts) × Current (Amps)
P = V × I
Therefore:
Current (Amps) = Power (Watts) / Voltage (Volts)
I = P / V
Understanding Constant Voltage Audio Systems
25V and 70V audio systems are constant voltage distribution systems where the amplifier maintains a fixed RMS voltage on the circuit, regardless of the number of speakers connected (within the amplifier's power capacity).
Key Characteristics:
- 25V systems: Amplifier outputs 25V RMS continuously
- 70V systems: Amplifier outputs 70V RMS continuously
- Speakers connect in parallel across the constant voltage line
- Each speaker draws current based on its wattage tap setting
- Total current = sum of all individual speaker currents
This is fundamentally different from traditional audio systems where voltage varies with signal level.
Speaker Tap Settings
Most commercial speakers for fire alarm voice evacuation have multiple tap settings - internal transformers that allow you to select the power level:
Common 25V Speaker Taps:
- 0.25W, 0.5W, 1W, 2W, 4W, 8W
Common 70V Speaker Taps:
- 0.5W, 1W, 2W, 4W, 8W, 15W, 30W
Each tap setting draws a different current:
For 25V speakers:
0.25W tap: I = 0.25 / 25 = 0.010 A (10 mA)
0.5W tap: I = 0.5 / 25 = 0.020 A (20 mA)
1.0W tap: I = 1.0 / 25 = 0.040 A (40 mA)
2.0W tap: I = 2.0 / 25 = 0.080 A (80 mA)
4.0W tap: I = 4.0 / 25 = 0.160 A (160 mA)
For 70V speakers:
0.5W tap: I = 0.5 / 70 = 0.007 A (7 mA)
1.0W tap: I = 1.0 / 70 = 0.014 A (14 mA)
2.0W tap: I = 2.0 / 70 = 0.029 A (29 mA)
4.0W tap: I = 4.0 / 70 = 0.057 A (57 mA)
8.0W tap: I = 8.0 / 70 = 0.114 A (114 mA)
Notice: 70V systems draw less current for the same wattage, which allows smaller wire gauges or longer circuit runs.
Practical Implications for Circuit Design
Higher wattage taps = More current = More voltage drop:
Example Circuit: 10 speakers, 400 feet of 18 AWG wire (6.5 Ω/kFt)
Scenario 1: All speakers at 1W tap (25V)
Total Watts: 10 × 1.0W = 10.0W
Total Current: 10.0W / 25V = 0.40A
Wire Resistance: 6.5 × 2 × 400 / 1000 = 5.2Ω
Voltage Drop: 5.2Ω × 0.40A = 2.08V
End Voltage: 25V - 2.08V = 22.92V ✅ (Good!)
Scenario 2: All speakers at 2W tap (25V)
Total Watts: 10 × 2.0W = 20.0W
Total Current: 20.0W / 25V = 0.80A
Wire Resistance: 6.5 × 2 × 400 / 1000 = 5.2Ω
Voltage Drop: 5.2Ω × 0.80A = 4.16V
End Voltage: 25V - 4.16V = 20.84V ⚠️ (Marginal)
Scenario 3: All speakers at 4W tap (25V)
Total Watts: 10 × 4.0W = 40.0W
Total Current: 40.0W / 25V = 1.60A
Wire Resistance: 6.5 × 2 × 400 / 1000 = 5.2Ω
Voltage Drop: 5.2Ω × 1.60A = 8.32V
End Voltage: 25V - 8.32V = 16.68V ❌ (Too low!)
Key Lesson: Selecting appropriate speaker taps is critical. Use the lowest tap setting that meets sound pressure level (SPL) requirements to minimize current draw and voltage drop.
Why Not Just Use Watts Directly?
You might wonder: "Why convert to amps at all? Why not calculate voltage drop directly from watts?"
Answer: Voltage drop is governed by Ohm's Law (V = I × R), which requires current and resistance. Wire resistance doesn't directly interact with power - it interacts with current flow. The conversion is a necessary step in the calculation chain:
Step 1: Watts → Amps (using circuit voltage)
Step 2: Amps + Wire Resistance → Voltage Drop (using Ohm's Law)
Step 3: Voltage Drop → dB Loss (using logarithmic formula)
Formula Summary
Current (A) = Speaker Watts / Starting Voltage
For Speaker Schedule reports:
- Converts each individual speaker's watts to current
- Used to calculate voltage drop in each wire segment
- Accounts for "Remaining Watts" (downstream speakers)
Example:
- Speaker: 2.0W (25V tap)
- Starting Voltage: 25V
- Current: 2.0 ÷ 25 = 0.08A (80 mA)
2. Wire Resistance Calculation
Resistance (Ω) = Wire Resistance (Ω/kFt) × 2 × Distance (Ft) / 1000
The ×2 factor accounts for round-trip current flow (out and back).
Example:
- Wire: 18 AWG (6.5 Ω/kFt)
- Distance: 75 feet
- Resistance: 6.5 × 2 × 75 / 1000 = 0.975Ω
3. Voltage Drop Calculation
Voltage Drop (V) = Resistance (Ω) × Remaining Current (A)
Uses Ohm's Law with remaining downstream load.
Remaining Current = Total current from all speakers downstream from this point
Example:
- Resistance: 0.975Ω
- Remaining Current: 0.48A (6 speakers × 0.08A each)
- Voltage Drop: 0.975 × 0.48 = 0.468V
4. dB Loss From Previous Device
dB Loss = 20 × Log₁₀(Voltage At Previous / Voltage At Device)
Calculates audio signal attenuation between speakers.
Example:
- Voltage at previous speaker: 25.0V
- Voltage at current speaker: 24.53V
- dB Loss: 20 × Log₁₀(25.0 / 24.53) = 0.33 dB
5. Total dB Loss
Total dB Loss = 20 × Log₁₀(Voltage At Device / Starting Voltage)
Cumulative audio attenuation from circuit start.
Example:
- Starting voltage: 25V
- Voltage at speaker: 24.1V
- Total dB Loss: 20 × Log₁₀(24.1 / 25) = -0.63 dB
Complete Example - 25V Speaker Circuit
Circuit Configuration:
- Amplifier: Audio Amplifier Panel
- Circuit: SPKR-1
- Starting Voltage: 25V (constant voltage audio system)
- Wire: 18 AWG (6.5 Ω/kFt)
- Wire Overage: 15%
- Max Circuit Watts: 100W
- Min Operational Voltage: 19V
Devices (All Speakers):
1. SPK-1 (Ceiling Speaker): SP-C25, 2.0W tap, 75ft from amplifier
2. SPK-2 (Ceiling Speaker): SP-C25, 2.0W tap, 65ft from SPK-1
3. SPK-3 (Wall Speaker): SP-W15, 1.5W tap, 80ft from SPK-2
4. SPK-4 (Wall Speaker): SP-W15, 1.5W tap, 70ft from SPK-3
5. SPK-5 (Ceiling Speaker): SP-C10, 1.0W tap, 60ft from SPK-4
6. SPK-6 (Ceiling Speaker): SP-C10, 1.0W tap, 55ft from SPK-5
7. EOL (End of Line): EOL-R, 0W, 40ft from SPK-6
Calculations:
Speaker 1 (SPK-1):
Distance: 75 ft
Resistance: 6.5 × 2 × 75 / 1000 = 0.975 Ω
Remaining Current: (2+2+1.5+1.5+1+1) / 25 = 9.0W / 25V = 0.360 A
Voltage Drop: 0.975 × 0.360 = 0.351 V
Voltage at Device: 25.0 - 0.351 = 24.65 V
dB Loss: 20 × Log₁₀(25.0 / 24.65) = 0.25 dB
Speaker 2 (SPK-2):
Distance: 65 ft
Resistance: 6.5 × 2 × 65 / 1000 = 0.845 Ω
Remaining Current: (2+1.5+1.5+1+1) / 25 = 7.0W / 25V = 0.280 A
Voltage Drop: 0.845 × 0.280 = 0.237 V
Voltage at Device: 24.65 - 0.237 = 24.41 V
dB Loss: 20 × Log₁₀(24.65 / 24.41) = 0.17 dB
Speaker 3 (SPK-3):
Distance: 80 ft
Resistance: 6.5 × 2 × 80 / 1000 = 1.040 Ω
Remaining Current: (1.5+1.5+1+1) / 25 = 5.0W / 25V = 0.200 A
Voltage Drop: 1.040 × 0.200 = 0.208 V
Voltage at Device: 24.41 - 0.208 = 24.20 V
dB Loss: 20 × Log₁₀(24.41 / 24.20) = 0.15 dB
Speaker 4 (SPK-4):
Distance: 70 ft
Resistance: 6.5 × 2 × 70 / 1000 = 0.910 Ω
Remaining Current: (1.5+1+1) / 25 = 3.5W / 25V = 0.140 A
Voltage Drop: 0.910 × 0.140 = 0.127 V
Voltage at Device: 24.20 - 0.127 = 24.07 V
dB Loss: 20 × Log₁₀(24.20 / 24.07) = 0.09 dB
Speaker 5 (SPK-5):
Distance: 60 ft
Resistance: 6.5 × 2 × 60 / 1000 = 0.780 Ω
Remaining Current: (1+1) / 25 = 2.0W / 25V = 0.080 A
Voltage Drop: 0.780 × 0.080 = 0.062 V
Voltage at Device: 24.07 - 0.062 = 24.01 V
dB Loss: 20 × Log₁₀(24.07 / 24.01) = 0.04 dB
Speaker 6 (SPK-6):
Distance: 55 ft
Resistance: 6.5 × 2 × 55 / 1000 = 0.715 Ω
Remaining Current: 1.0W / 25V = 0.040 A
Voltage Drop: 0.715 × 0.040 = 0.029 V
Voltage at Device: 24.01 - 0.029 = 23.98 V
dB Loss: 20 × Log₁₀(24.01 / 23.98) = 0.02 dB
End of Line (EOL):
Distance: 40 ft
Remaining Current: 0 A (no speakers downstream)
Voltage Drop: 0 V
Voltage at Device: 23.98 V
Total Voltage Drop: 1.02 V (4.08%)
Total dB Loss: -0.15 dB
Final Report Table:
| Device | Part No | Description | Watts | Remaining | Dist | Resistance | V Drop | Voltage | Total Drop | dB Loss | Total dB |
|---|---|---|---|---|---|---|---|---|---|---|---|
| SPK-1 | SP-C25 | Ceiling Speaker | 2.0 | 9.0 | 75 | 0.975 | 0.35 | 24.65 | 0.35 | 0.25 | -0.25 |
| SPK-2 | SP-C25 | Ceiling Speaker | 2.0 | 7.0 | 65 | 0.845 | 0.24 | 24.41 | 0.59 | 0.17 | -0.42 |
| SPK-3 | SP-W15 | Wall Speaker | 1.5 | 5.0 | 80 | 1.040 | 0.21 | 24.20 | 0.80 | 0.15 | -0.57 |
| SPK-4 | SP-W15 | Wall Speaker | 1.5 | 3.5 | 70 | 0.910 | 0.13 | 24.07 | 0.93 | 0.09 | -0.66 |
| SPK-5 | SP-C10 | Ceiling Speaker | 1.0 | 2.0 | 60 | 0.780 | 0.06 | 24.01 | 0.99 | 0.04 | -0.70 |
| SPK-6 | SP-C10 | Ceiling Speaker | 1.0 | 1.0 | 55 | 0.715 | 0.03 | 23.98 | 1.02 | 0.02 | -0.72 |
| EOL | EOL-R | End of Line | 0.0 | 0.0 | 40 | 0.520 | 0.00 | 23.98 | 1.02 | 0.00 | -0.15 |
Summary Results:
- ✅ Total Circuit Watts: 9.0W (well under 100W limit)
- ✅ End Of Line Voltage: 23.98V (above 19V minimum)
- ✅ Max. dB Loss: -0.15 dB (excellent - well under -1.5 dB limit)
- ✅ Voltage Drop: 4.08% (excellent - typically max 10-15%)
Understanding 25V vs. 70V Systems
25V Systems (Constant Voltage Audio)
- Typical Starting Voltage: 25V RMS
- Common Applications: Fire alarm voice evacuation, background music, paging
- Typical Power Range: 0.25W to 10W per speaker
- Wire Gauge: Usually 18-16 AWG for most installations
- Max dB Loss: -1.5 dB typical, -1.0 dB for critical voice evacuation
70V Systems (Constant Voltage Audio)
- Typical Starting Voltage: 70V RMS
- Common Applications: Large buildings, long wire runs, high-power speakers
- Typical Power Range: 0.5W to 30W per speaker
- Wire Gauge: Can use smaller wire due to lower current
- Max dB Loss: Same as 25V (-1.5 dB typical)
Advantage of 70V: Lower current for same wattage allows smaller wire gauge or longer runs.
Frequently Asked Questions
Q1: Why does my starting voltage show 25V instead of 24V?
25V and 70V are constant voltage audio distribution systems, not to be confused with:
- 24V DC fire alarm notification circuits (NAC circuits)
- These are different circuit types with different purposes
- 25V/70V systems are specifically for audio/speaker distribution
- Some panels may use 20.4V for audio circuits
Q2: What's the difference between Speaker Schedule and Point-to-Point Reports?
Speaker Schedule:
- For 25V/70V audio speaker circuits only
- Uses wattage-based calculations
- Converts voltage drop to dB loss for audio analysis
- Shows acoustic signal attenuation
Point-to-Point:
- For standard 24V DC notification circuits (horns, strobes, horn/strobes)
- Uses current (Amps) based calculations
- Shows voltage drop only (no dB conversion)
- No acoustic analysis needed
Both use the same voltage drop calculation method - Speaker Schedule just adds the dB loss conversion.
Q3: What is an acceptable dB loss?
Typical Limits:
- Voice Evacuation Systems: -1.0 dB maximum
- Standard Speaker Circuits: -1.5 dB maximum
- General Audio/Paging: -2.0 dB may be acceptable
Best Practice: Target -1.0 dB or better to allow safety margin.
Q4: Can I use Speaker Schedule for horn/strobe circuits?
No. Speaker Schedule is specifically for audio speaker circuits. Use:
- Speaker Schedule: Audio speakers on 25V/70V circuits
- Point-to-Point: Horn/strobe/notification appliances on 24V DC circuits
Q5: What does "Remaining Watts" mean?
"Remaining Watts" represents the total power load from all speakers downstream from that point in the circuit. It's used to calculate equivalent current for voltage drop:
Remaining Current = Remaining Watts / Circuit Voltage
This is the audio circuit equivalent of "Remaining Current" in Point-to-Point reports.
Technical Notes
Relationship to Point-to-Point Reports
The Speaker Schedule uses identical voltage drop calculations to Point-to-Point Reports:
1. Calculate current from power: I = W / V
2. Calculate wire resistance: R = ρ × 2 × L
3. Calculate voltage drop: ΔV = R × I
The key difference: Speaker Schedule adds one more step:
4. Convert voltage drop to dB loss: dB = 20 × Log₁₀(V₁ / V₂)
This conversion is necessary because audio signal attenuation is measured in decibels, not volts.
Decibel Loss Formula
dB = 20 × Log₁₀(V₁ / V₂)
Why logarithmic?
- Human hearing perceives sound logarithmically
- -3 dB = 50% sound pressure (sounds "half as loud")
- -6 dB = 25% sound pressure
- -10 dB = ~30% perceived loudness
Related Reports
- dB Loss Reports: Lump sum version of Speaker Schedule (aggregated by part number)
- Point-to-Point Reports: Same calculation method for standard NAC circuits (no dB conversion)
- Lump Sum Reports: Aggregated current-based for standard NAC circuits
- Battery Calculation: Backup power requirements for all circuits
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