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Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.
A five-storey building has about 160 addressable detectors, call points and loop-powered sounders. The panel datasheet allows up to 250 addresses per loop. I want to know whether one loop is acceptable.
- What limits the device count apart from the address capacity?
- How many short-circuit isolators are needed?
- How do I check the cable length and voltage drop for loop sounders?
The address limit is rarely the governing one. In practice the loop is limited by the fault-tolerance rules (isolators), by the loop current and voltage drop of powered devices such as sounders, and by spare capacity. 160 devices on one loop can work, but only with enough isolators and a voltage-drop check in the alarm condition.
Step 1: Spare capacity
Design to about 80 % of the panel limit, so later additions do not need a new loop. Here 160 of 250 addresses (64 %) is fine. Many designers also split by building wing or fire compartment, so a loop failure is easy to understand on site.
Step 2: Isolators and fault tolerance
- EN 54-2 requires that a single fault in a transmission path does not remove more than 32 detectors and/or call points.
- BS 5839-1 adds that a single open or short circuit must not disable protection in an area larger than one zone (and a zone is limited to 2 000 m²).
- NFPA 72 uses pathway classes (Class A, X). Short-circuit isolation is commonly specified so that one fault takes out no more than one floor or about 50 devices; check the requirement in your edition and with the authority.
- For 160 devices: 160 / 32 = 5 segments at least, so at least 4 field isolators in addition to the panel’s loop-end isolators. In practice, put one at every zone or floor boundary, or use devices with built-in isolators.
Step 3: Voltage drop in alarm
Example: 1.5 mm² copper loop cable, R20 = 12.1 Ω/km per conductor, loop length 1 500 m, alarm current from loop sounders 150 mA. Assume the panel delivers 21 V at end of battery discharge and the devices need 17 V, a 4 V allowance. (Use your manufacturer’s actual figures.)
- Loop resistance with one end feeding (after a break, which the design must survive): 2 × 1.5 × 12.1 = 36.3 Ω.
- Whole load lumped at the far end: 0.150 × 36.3 = 5.4 V, which fails.
- Evenly distributed load: about half, 2.7 V, which passes. Real layouts are in between, so check with the manufacturer’s loop calculator.
- With 2.5 mm² (7.41 Ω/km): 2 × 1.5 × 7.41 = 22.2 Ω, lumped 3.3 V, which passes even in the worst case.
Step 4: Other limits
- Cable capacitance and inductance limits from the panel manufacturer, which matter for long loops or screened cable.
- Fire-resistant cable, for example to BS 8434 or the EN 50200 class required, is needed where the cable must keep working during a fire.
- Isolator count per loop, because each isolator adds resistance.
Common mistakes
- Filling a loop to 100 % of the addresses.
- Putting all sounders at the far end of one loop.
- Checking voltage drop only in quiescent condition, or only with the loop fed from both ends.
Related: Fire Alarm Standby Battery Calculation and Manual Call Point Placement.
For deeper study: Fire Detection and Alarm System Design course.
Check the edition of the code that applies to your project.
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