Specifications

78
Diagrams are for reference only.
Consult the appropriate installation sheet for wiring details.
TM
REFERENCE SECTION
25 OR 70 VRMS NAC WIRE LENGTH25 OR 70 VRMS NAC WIRE LENGTH
25 OR 70 VRMS NAC WIRE LENGTH25 OR 70 VRMS NAC WIRE LENGTH
25 OR 70 VRMS NAC WIRE LENGTH
25 Vrms 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:
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.
For example, the maximum allowable wire length for an audio circuit consisting of a
40 W, 25 Vrms amplifier driving thirty 1-watt speakers, using 18-guage wire would be
95 ft.
Max. length =
59.25 x Amplifier Output²
Wire Resistance x Circuit Load
B-1 and B-2 gives the maximum allowable wire lengths for various wire sizes and
loads. Use B-1 when designing circuits for amplifiers set for 25 Vrms output. Use B-2
when designing circuits for amplifiers set for a 70 Vrms output.
94.95 =
59.25 x 25
13 x 13