System calculations
EST3 Installation and Service Manual B.17
25 or 70 Vrms NAC wire length
The maximum allowable wire length is the farthest distance that
a pair of wires can extend from the amplifier to the last speaker
on the notification appliance circuit without losing more than 0.5
dB of signal. Calculating the maximum allowable wire length
using this method ensures that each speaker operates at its full
potential.
Several factors influence the maximum allowable wire length:
• Wire size
• Output signal level of the amplifier driving the circuit
• Number of speakers installed on the circuit
To calculate the maximum allowable wire length for a 0.5 dB
loss, use the following formula:
Max length =
Wire resistance X Circuit load
59.25 X Amplifier output
2
where:
• Amplifier output is the signal level in Vrms supplied by the
amplifier driving the circuit
• Circuit load is the total watts required by the audio circuit
• Wire resistance is the resistance rating of the wire per 1000
ft pair, see Table B-5.
For example, the maximum allowable wire length for an audio
circuit consisting of a 30 W, 25 Vrms amplifier driving thirty
1-watt speakers, using 18-guage wire would be 95 ft.
94.95 =
13 X 30
59.25 X 25
2
Table B-5: Wire resistance ratings
Wire Size Resistance per 1,000 ft pair
(ohms)
18 AWG (0.75 sq mm) 13.0
16 AWG (1.0 sq mm) 8.0
14 AWG (1.50 sq mm) 5.2
12 AWG (2.5 sq mm) 3.2
Table B-6 and Table B-7 give the maximum allowable wire
lengths for various wire sizes and loads. Use Table B-6 when
designing circuits for amplifiers set for 25 Vrms output. Use
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