Choosing the right fire alarm cable affects how reliably power and signals travel through a system. Two common options are 18/2 and 16/2 cable. Both use two conductors, but their wire sizes differ. That difference changes resistance and voltage drop. It also impacts current capacity and cable weight.
A short circuit may work well with one size. A longer run may call for another. The right choice should also match the fire alarm equipment and cable listing. It should also comply with local code requirements. A careful selection keeps the circuit within its electrical limits.
It also supports dependable operation during an alarm event. This is something we will discuss in this article. It will help you get a better idea of it.
Understanding the Numbers (AWG and Conductor Count)
The numbers printed on a fire alarm cable tell you useful information before installation begins. The first number identifies the conductor size under the American Wire Gauge system. The number after the slash identifies how many conductors are inside the cable. These details affect electrical performance and installation choices.
AWG (American Wire Gauge)
AWG stands for American Wire Gauge. It identifies the diameter of a conductor. A lower AWG number means a larger conductor. Therefore, 16 AWG copper has a larger cross-sectional area than 18 AWG copper. Larger conductors generally have lower electrical resistance.
That characteristic becomes important as circuit length and electrical load increase. NFPA 70 permits 18 AWG and 16 AWG copper conductors for certain non-power-limited fire alarm circuit applications when applicable requirements are met.
The "/2" Designation
The "/2" tells you that the cable contains two conductors. A typical 18/2 fire alarm cable has two 18 AWG conductors. A 16/2 cable has two 16 AWG conductors.
The conductor count does not identify the wire size. It simply tells you how many conductors are included. Different factors can still vary between products. This includes:
- Cable construction
- Insulation
- Shielding
- Jacket type
- Listing
Key Differences Between 18/2 and 16/2 Fire Alarm Cables
The gauge difference affects several electrical properties that matter during system design. Resistance is especially important because it increases voltage loss along a circuit. Circuit length and load also affect the result. The following factors should be checked before selecting a cable for a fire alarm installation.
Electrical Resistance
Conductor resistance affects how much electrical energy is lost as current travels through a cable. Smaller conductors have higher resistance. For example, one current fire alarm cable specification lists DC resistance at about 6.669 ohms per 1,000 feet for 18 AWG and 4.181 ohms per 1,000 feet for 16 AWG.
The actual resistance depends on conductor material, temperature, construction, and manufacturer specifications. Always use the resistance value published for the exact cable being installed.
Voltage Drop
Voltage drop occurs as current moves through the resistance of the conductors. Long cable runs can therefore lose more voltage than short runs. Higher current also increases the voltage drop.
Fire alarm circuit calculations should account for conductor resistance, circuit length, and connected load. NFPA documentation provides voltage-drop calculation methods that use current, resistance, and circuit length.
A cable with lower resistance can reduce voltage loss. The final circuit voltage must still meet the requirements of the connected equipment.
Current Capacity
Current capacity refers to how much current a conductor can safely carry under specified conditions. The permitted value depends on factors such as:
- Conductor size
- Insulation rating
- Ambient temperature
- Installation method
- Applicable electrical code
A 16 AWG conductor has a greater copper cross-sectional area than an 18 AWG conductor. However, cable selection should never rely on conductor size alone. The cable listing, equipment requirements, circuit classification, and applicable code rules must also be checked.
How To Choose Between 18/2 And 16/2 Fire Alarm Cable?
The correct cable starts with the circuit requirements rather than the cable size alone. Check the equipment load, cable distance, voltage-drop calculation, installation environment, and required listing. The project specifications may also identify an approved cable type. These checks narrow the choice and prevent electrical problems during installation.
When to Choose 18/2 Fire Alarm Cable
An 18/2 cable can be suitable when the circuit requirements fit the conductor's electrical limits, and the selected product carries the required fire alarm listing. Shorter runs and lower loads may reduce voltage-drop concerns. The exact application should still be verified against the manufacturer's data and applicable code requirements.
The following points are useful when reviewing an 18/2 option. They focus on practical installation factors rather than assuming that every 18/2 cable has the same construction. Product specifications should always have the final say.
- Construction: Features two 18 AWG copper conductors with insulation and a fire alarm-rated outer jacket based on the selected product.
- Shorter Runs: Works well for circuits where calculated voltage drop remains within the equipment's permitted operating range.
- Lower Load: May suit circuits with modest current requirements when the cable listing and equipment specifications permit its use.
- Installation: Its smaller conductor size can make handling and routing easier in suitable fire alarm applications.
- Specification Check: Requires confirmation of cable type, temperature rating, voltage rating, and listing before installation.
When to Choose 16/2 Fire Alarm Cable
A 16/2 cable provides a larger conductor than 18/2. Its lower resistance can be useful when a circuit has greater length or a higher electrical load. It may also provide more electrical margin for voltage-drop calculations. The cable still has to match the equipment and installation requirements.
Several practical checks should guide the selection of 16/2 cable. These points help keep the decision tied to the actual circuit rather than relying on conductor size as the only factor.
- Construction: Features two 16 AWG copper conductors with insulation and a fire-alarm-rated outer jacket based on the selected product.
- Longer Runs: May be useful when circuit length creates a greater voltage-drop concern, and calculations support the larger conductor.
- Higher Loads: Provides lower conductor resistance, which can support circuits carrying greater loads within approved limits.
- Physical Size: Has a larger conductor than 18 AWG, so routing and termination space should be checked during installation.
- Specification Check: Requires confirmation of listing, insulation type, temperature rating, voltage rating, and equipment compatibility.
Choose the Right Fire Alarm Cable With TS Cables
The right conductor size depends on circuit length, electrical load, voltage drop, cable listing, and installation conditions. 18/2 fire alarm cable may suit qualifying circuits with lower demands, while 16/2 fire alarm cable provides lower resistance for circuits that require it.
TS Cables provides suitable fire alarm cable options for dependable installations. Choose the right cable for your project today.
FAQs
Is 16/2 cable better than 18/2 for fire alarms?
Neither size is automatically better. The appropriate choice depends on circuit length, load, voltage-drop calculations, equipment requirements, cable listing, and applicable electrical codes.
Does 16/2 fire alarm cable have lower resistance?
Yes. A 16 AWG copper conductor generally has lower resistance than an 18 AWG conductor. Manufacturer specifications provide the exact resistance for each cable.
Can 18/2 cable be used for long fire alarm runs?
It may be used when the cable is properly listed, and voltage-drop calculations confirm adequate voltage at connected devices throughout the circuit.
Does the "/2" mean two wires?
Yes. The "/2" designation generally identifies a cable containing two insulated conductors. It does not identify the conductor gauge or cable listing.
Should cable size be selected by an installer?
The selection should follow the system design, equipment specifications, applicable codes, and manufacturer's requirements. Qualified professionals should verify the final installation.
Further Reading
Explore our other cable and fire alarm blogs for practical information on cable selection, installation requirements, electrical performance, and system planning.
