I'm going to use the term "resistance of the wire" because of the factors that are involved with the type of wire that is used.
Resistance of the Wire
As far as the resistance goes, the current through the wire is negligible; the current is so low that it does not affect voltage losses in the wire.
As far as what size of wire should be used, the Installation Manual for the fire alarm system should be referenced. The Installation Manual shows the size of wire that should be used under the screw terminals of the Signaling Line Circuit (the SLC or Addressable Circuit). That is the size of the wires that should be used in the whole SLC (addressable circuit).
The Installation Manual also shows the wiring practices that should be used. Class II or Class III wiring practices, or else power-limited wiring practices are common requirements.
Capacitance of the Wire and Voltage Lag
A capacitor is defined as "Two Conductors Separated by an Insulator".
In a SLC, the positive wire is a conductor, and the negative wire is a conductor. The lining around the wires is the insulation. For practical purposes, the two conductors, along with the insulation separating the conductors is a capacitor.
A great concern is the capacitance of the SLC. When the capacitance of the wire is greater, it takes more time for the available current to "fill up" the capacitor. The "more time" is voltage lag. When the lag is too long, the data becomes corrupted.
The Installation Manual for the fire alarm system shows the maximum capacitance allowable for the SLC. The capacitance of the cable can be found in the Data Sheets that are available from the wire manufacturer.
There are a number of factors affecting the capacitance of the wires in the cable.
- The more total wire-pair (more total cable, including all t-taps) used in the circuit, the greater the capacitance; the less total wire-pair (more total cable, including all t-taps) used in the circuit, the lower the capacitance
- The larger the wire (conductors), the greater the capacitance; the smaller the wire (conductors), the lower the capacitance
- The closer the wires are together (caused by thinner insulation), the greater the capacitance; the further apart the wires are from each other (caused by thicker insulation), the lower the capacitance
- The greater the dielectric constant of the insulation around the wire, the greater the capacitance; the lessor the dielectric constant of the insulation around the wire, the lower the capacitance
- Shielding around the wire pair is another capacitor added to the already two-wire capacitor, and the added shield increases the capacitance of the cable. . . a lot
The maximum allowable capacitance of the SLC (SLC-capacitor) is shown in the Installation Manual for the fire alarm system. Exceeding the capacitance will corrupt the data.
If the Installation Manual shows that shielded wire should not be used, believe the Installation Manual and don't use shielded wire. Shielding can easily corrupt the data.
If the Installation Manual shows that shielded wire should be used, there are other factors involved, and shielded wire should be used. In this case, not shielding the wire will corrupt the data.
Corrupting the data means, essentially, that the circuit is useless.
Wire Length
Often, the Installation Manual will go into the distance requirements of the fire alarm system's SLC.
When the Installation Manual shows information about maximum cable length for the SLC, exceeding the length will corrupt the data. Don't exceed the length shown in the Installation Manual.
Douglas Krantz
