Douglas Krantz - Technical Writer - Describing How It Works

What is the Difference Between Thermostat and Fire Alarm System Cables?

By Douglas Krantz | Electronics

What is the Difference Between Thermostat and Fire Alarm System Cables?

What is the Difference Between Thermostat and Fire Alarm System Cables?

Greetings Douglas,

What is the difference between LVT [Low Voltage Thermostat] and FAS [fire alarm system] cables, and what is the disadvantage of using LVT cables for fire alarm systems?

Thank You, SE

The differences between LVT (Low Voltage Thermostat) cables and FAS (Fire Alarm System) cables can be shown in two different categories: Technical and NFPA Code.

Technical - Voltage

The insulation on the Low Voltage Thermostat cables is usually rated by the manufacturer to be 30 volts, while the insulation on Fire Alarm System cables is rated by the manufacturers to be 300 volts. The rating voltage is for all voltages: for continuous DC voltages, like the voltage that comes from a battery, and also for short term transient voltages, like the peaks of voltage on AC signals or power.

Most Initiating Device Circuits (IDC), and most Notification Appliance Circuits (NAC) used with horns and strobes, at least on a 12-volt nominal and a 24-volt nominal fire alarm system keep these circuit voltages below 30 volts. So, in theory, the voltage would not be exceeded for using Low Voltage Thermostat cables.

Signaling Line Circuits (SLC), on a few fire alarm systems exceed that 30-volt threshold. Unless the manufacture says, in writing, that Low Voltage Thermostat cables may be used for Signaling Line Circuits, the LVT cables should not be used for a Signaling Line Circuit.

All 25-volt speaker circuits (about 35 volts peak) and 70-volt speaker circuits (about 99 volts peak) for Fire Alarm Systems exceed the 30-volt threshold for LVT cables, so Low Voltage Thermostat cables should never be used for speakers.

Any 24-volt AC (about 34 volts peak) transformer that is used exceeds the 30-volt threshold for LVT cables, so Low Voltage Thermostat cables should never be used with any 24-volt AC transformer.

Technical - Current and Wire Diameter

Low Voltage Thermostat wire sizes can be much smaller than what is specified in the printed literature provided by manufacturers of Fire Alarm Systems. Part of the reason is that the terminals for the Fire Alarm Systems won't grip smaller wires correctly, and won't provide "Gas-Tight" connections. As time goes on, tarnish buildup on the wire and the contacts will interfere with both signals and power, especially at lower voltages.

The minimum size wire that the manufacturer specifies should be the minimum size used.

Many fire alarm system circuits require wire that is larger than is available for Low Voltage Thermostat cables. Horns, Strobes, Speakers, etc. use lots of current. Unless the circuit for these is very short, and there are few devices, LVT cables just don't have the required current carrying capacity.

No matter what kind of wire is used, make sure to perform voltage loss calculations on each NAC or Speaker circuit. (OK, fire marshals want to see that you did it.)


The NFPA Code also addresses the voltage ratings and current carrying capacity of wires.

The NFPA Code is there, basically, to tell people, "Don't take shortcuts." But, the NFPA does try to make it easier for us to understand what we need to do to make wiring safe. As far as building wiring goes, the different types of power sources and wiring are divided out.

One way it's divided out is the separation between limitations in power, or no limitations power.

Power Limited - The overall power, the voltage, and the current available from the power source is kept low. The current is kept low so the building won't be burned down if wires are shorted together. The voltage is kept low so no one will be electrocuted if they make direct contact with a wire.

Power Un-Limited - The power from the power source is meant to run lights and electrical equipment. An uncontrolled short can burn down buildings, and the voltage is high enough to electrocute someone.

To keep the electricity from the Power Un-Limited power source from ever getting into the power limited wiring, the wires from the two types of power supplies have to be kept apart. If they have to be close, an extra barrier of some sorts has to be between them.

In the building wiring field, sometimes Power Un-Limited is known as "High Voltage Wiring" and Power Limited is known as "Low Voltage Wiring". The technical terms, though, are Power Un-Limited and Power Limited.


Another way that the power sources and wiring is divided out is in their classifications.

Power Source

Class II - The maximum peak voltage that the power source can put out is 100 volts. The currents have to be kept to a very low level.

Class III - The maximum peak voltage that the power source can put out is still 100 volts. The current, however, can be higher than Class II.

Both Class II and Class III power supplies are "Power Limited".

Wires Used with a Power Source

If it's a Class II system: The power source and wiring can be considered to be "Power Un-Limited" system. Here, the NFPA Code is concerned that any wire or cable used in a building is capable of handling, safely, any power source used in that classification.

If it's a Class II system: The wire has to be capable of handling 300 volts, but the size of the wire can be small.

If it's a Class III system: The wire has to be capable of handling 300 volts, but the wire has to be a little larger.

A fire alarm panel is a power source. As a power source, the power outputs on each circuit are usually equivalent to Class III, so any wiring attached to the circuit has to be capable of handling the voltage and current of a Class III power source.

LVT Wire and FAS Wire

A Low Voltage Thermostat cable is rated for 30 volts, so a Low Voltage Thermostat cable cannot be used, either technically or legally, in a fire alarm system. When it's rated by the manufacturer as LVT, it cannot be used with any Class II or Class III power source.

On the other hand, a Fire Alarm System cable is rated for 300 volts. The current carrying capacity of all FAS Cables can be used with any Class II power source.

Some of the smaller diameter (Gauge) FAS wires can't be used with a Class III power source. You can get an idea of what can be used by reading the fire alarm system manufacturer's panel installation manual or device installation sheet. By NFPA Code, the size wire shown in the installation manuals is the wire size that is allowed.

Douglas Krantz
Life Safety
This website uses cookies. See Privacy for details.
Make It Work Series of Books by Douglas Krantz
Want Regular Updates on Articles Like These?

No Charge - Unsubscribe Anytime