SLC Loading Calculator
Plan an addressable loop against the limits that actually bind: the panel's real point-pool structure (99 detectors + 99 modules is not 198 fungible addresses), a spare-capacity policy drawn from real specification language, the loop's wire resistance and length limits — which trade places as the governing constraint depending on gauge — and the Class A/B/X and T-tap rules. Prints as a loop schedule, the missing third calc sheet in the submittal package. Free, no login.
Loop devices
Loop schedule header (project, loop ID, preparer)
Loop loading
4 isolators — no addresses consumed. Fire-Lite limit: ≤25 devices between isolators (≤7 with relay/sounder bases).
Point caps and wire limits per the panel’s listed documents (DF-60601 / 51309:R6). Spare policy is specification practice — NFPA 72 has no spare-point rule. Classes per NFPA 72 12.3; single-fault zone rule 23.6.1 (2016+). Wire: NEC Table 8 pair basis.
SLC Loop Loading Schedule
Signaling line circuit loading — prepared with the NORDIX SLC Loading Calculator (nordixhq.com/tools/slc-loading-calculator)
| Project: — | Loop: SLC 1 — Fire-Lite MS-9200UDLS, Class B |
| Prepared by: — Date: — | Spare policy: 20% (specification practice) |
| Device | Pool | Qty | Standby mA |
|---|---|---|---|
| Addressable photoelectric smoke (SD355) | detector | 74 | 22.2 |
| Addressable duct smoke (PAD100-DUCT) | detector | 6 | 1.8 |
| Addressable pull station (BG-12LX) | module | 6 | 1.8 |
| Monitor module (MMF-300) | module | 30 | 12 |
| Control module (CMF-300) | module | 24 | 9.4 |
| Relay module (CRF-300) | module | 10 | 2.7 |
| Fault isolator module (I300) | isolator | 4 | 1.6 |
| Detector pool | 80 of 99 (81%) — usable at policy 79 — OVER SPARE POLICY |
| Module pool | 70 of 99 (71%) — usable at policy 79 — PASS |
| Wire — 14 AWG, 4000 ft (farthest branch end) | 24.6 Ω of 40 Ω — max 6,515 ft (resistance governs) — PASS |
| Loop standby current (to battery calculation) | 51.5 mA |
Basis: panel point capacities and wire limits per the manufacturer’s listed documents (DF-60601 / 51309:R6); pool structure (separate detector and module address pools) as published. Wire resistance measured through both conductors (NEC Ch. 9 Table 8 pair basis, 75°C). T-taps permitted on Class B only. Spare capacity is a specification requirement where invoked — NFPA 72 contains no spare-point rule. Single-fault zone rule per NFPA 72 23.6.1(2016+ editions; each floor is a zone). The panel’s installation manual and the AHJ govern.
The pool structure is the whole game
Most panels don’t have one loop capacity — they have two. A Fire-Lite MS-9600 loop takes 159 detectors and 159 modules in separate address ranges: 160 smoke detectors fail on a loop with 150 module addresses sitting empty. Addressable pull stations, monitor, control, and relay modules all draw from the module pool; smokes, heats, and duct detectors from the detector pool. Potter panels and small Fire-Lite panels (ES-50X) instead use one shared pool — any combination up to the cap. This calculator carries each panel’s published structure, which the only other web tool in existence reduces to a single number you type yourself. On top of capacity, wire limits are per-manufacturer and must never be merged: Fire-Lite allows 40 Ω of loop resistance with a 10,000 ft cap at 12 AWG and no capacitance limit; Silent Knight 50 Ω, 12,500 ft, and 0.5 µF; Potter 50 Ω total with 10 Ω between isolators. The binding constraint flips — at 14 AWG the 40 Ω limit governs (6,514 ft), at 12 AWG the length table does (10,000 ft before the 10,363 ft the resistance would allow).
Worked example: the asymmetric spare-policy failure
The default loop above: 150 devices on an MS-9200UDLS with a 20% spare policy.
- 80 detector addresses used of 99 (80.8%); 70 module addresses of 99 (70.7%) — raw capacity passes on both pools.
- The 20% spare policy makes 79 addresses usable per pool: the module pool passes at 70, but the detector pool fails at 80 — one pool over, one under, invisible to any single-number capacity check.
- The four isolators consume no addresses at all (Fire-Lite: none until you pass 100 per loop) — but they do bound the fault: at most 25 devices between isolators, 7 where relay or sounder bases are used.
Reference: verified loop capacities and wire limits
Every figure read from the manufacturer’s datasheet or wiring manual. Note the Silent Knight 6700 is 50+50 in SK mode (the 99+99 panel is the 6808), and SK/SD protocols never mix on one panel.
| Panel | Points per loop | Pool structure | Loop Ω | Max @ 12 AWG | Cap. limit |
|---|---|---|---|---|---|
| Fire-Lite MS-9200UDLS | 99 det + 99 mod | Separate pools | 40 Ω | 10,000 ft | none |
| Fire-Lite MS-9600LS/UDLS (per loop) | 159 det + 159 mod | Separate pools | 40 Ω | 10,000 ft | none |
| Fire-Lite ES-50X | 50 any mix | Shared pool | 40 Ω | 10,000 ft | none |
| Fire-Lite ES-200X | 99 det + 99 mod | Separate pools | 40 Ω | 10,000 ft | none |
| Silent Knight 6700 (SK mode) | 50 det + 50 mod | Separate pools | 50 Ω | 12,500 ft | 0.5 µF |
| Silent Knight 6808 (SK mode) | 99 det + 99 mod | Separate pools | 50 Ω | 12,500 ft | 0.5 µF |
| Potter AFC-1000 / PFC-6800 (per loop) | 127 any mix | Shared pool | 50 Ω | — | 0.5 µF |
| Notifier NFS-320 (FlashScan) | 159 det + 159 mod | Separate pools | 40 Ω | 10,000 ft | none |
Classes, T-taps, and the zone rule
Class B has no redundant path and permits unlimited T-taps — resistance is checked from the panel to the farthest branch end with all tapped length counted. Class A adds the redundant return (out and return routed separately, 12.3.8) and prohibits T-taps; Class X adds survival of a single short and also prohibits T-taps, met in practice with isolators before and after each device. Since the 2016 edition, NFPA 72 23.6.1 requires that a single pathway fault not take out devices in more than one zone — and each floor is a zone — which is where isolator-per-floor practice comes from (the rule doesn’t exist in the 2010 edition, and the often-quoted “a short shall not affect more than 50 addressable devices” appears in none of the 2010–2019 texts). Spare points are the same story as amplifier spare capacity: NFPA 72 has no requirement; the 20–25% figures come from project specifications (UFGS 28 31 76 requires 25%, VA guidance 20%) — this calculator defaults to 20% and labels it practice.
Frequently asked questions
How many devices can go on an SLC loop?
It depends on the panel — and usually on two separate limits, not one. A Fire-Lite MS-9200UDLS takes 99 detectors AND 99 modules in separate address pools; an MS-9600 loop takes 159+159; a Notifier NFS-320 the same. Potter panels (127 per loop) and small panels like the ES-50X (50 points) instead use one shared pool in any combination. The trap: 99+99 is not 198 fungible addresses — 100 smoke detectors overflow the detector pool even with the module pool nearly empty. This calculator carries each panel's published pool structure and counts your devices against the right pools.
Do detectors and modules really use separate addresses?
On most large addressable panels, yes. Fire-Lite, Notifier, and Silent Knight (in SK protocol) keep detector addresses and module addresses in independent ranges — and pull stations, monitor modules, control modules, and relay modules all consume MODULE addresses, while smokes, heats, and duct detectors consume DETECTOR addresses. Potter's PAD100 protocol and Fire-Lite's ES-50X use a single shared pool instead. Protocol also matters: a Silent Knight 6808 is 99+99 in SK mode but 127 shared in SD mode, and the two protocols can never mix on one panel.
How long can an SLC loop be?
Two independent limits govern, and which one binds flips with wire gauge. Fire-Lite allows 40 ohms of loop resistance (measured through both conductors) plus a length table — 10,000 ft at 12 AWG, 8,000 at 14, 4,875 at 16, 3,225 at 18 for LiteSpeed on twisted pair. At 14 AWG the resistance limit governs (about 6,514 ft); at 12 AWG the table does (10,000 ft, less than the ~10,360 ft that 40 ohms would allow). Silent Knight uses 50 ohms, its own table up to 12,500 ft, and a 0.5 µF capacitance cap; Potter 50 ohms total with 10 ohms max between isolators. Never mix one manufacturer's limits with another's — this calculator keeps them separate and tells you which constraint governs.
Can I T-tap an SLC loop?
On Class B, yes — Fire-Lite's SLC wiring manual permits unlimited T-taps on 2-wire Class B configurations, with resistance checked from the panel to the farthest branch end and every tapped branch counting toward total length. On Class A the same manual is explicit: T-taps are NOT allowed — the redundant return path only works if the loop is one continuous run — and Class X prohibits them too. Selecting a class in this calculator surfaces the right rule and changes what the wire-length input means (farthest branch vs total loop).
Where do isolators go, and do they use addresses?
The code driver is NFPA 72 23.6.1 (new in the 2016 edition): a single pathway fault must not take out devices in more than one zone, and each floor is a zone — hence isolator-per-floor practice on Class B risers. On Fire-Lite, isolator modules and bases consume no SLC address at all until a loop exceeds 100 isolators (then 2 addresses each beyond 100), but engineering limits apply: at most 25 addressable devices between isolators, dropping to 7 when relay or sounder bases are used. Potter isolators take no address but consume the panel's parallel 'power unit' budget, with 10 ohms max wire between isolators. The often-repeated 'a short can't affect more than 50 devices' rule appears nowhere in the 2010–2019 code texts.
Does NFPA 72 require spare capacity on SLC loops?
No. Full-text search of the 2010, 2016, and 2019 editions finds no spare-point requirement — every 'spare' in the code concerns supervising-station hardware, the same pattern as the amplifier spare-capacity myth. What is real is specification practice: UFGS 28 31 76 (the military unified spec) requires 25% spare capacity on circuits, VA guidance calls for 20% initiating-device growth, and institutional specs commonly land at 20–25%. This calculator makes spare percentage a visible policy input defaulting to 20%, labeled as spec practice — never as code. The one web tool that existed before this one hardcodes '80% per NFPA 72,' a misattribution.
Do NAC power extenders and boosters use SLC addresses?
Yes — that's usually how the panel triggers them. A booster monitored and triggered over the loop consumes a control-module address (Potter's PAD100-NAC is explicitly one address), and reversing relays or monitor modules for its trouble contacts add more. On addressable systems this is frequently the real constraint when adding notification capacity — the forum wisdom is that you run out of SLC points and module addresses before you run out of power. When the NAC loading calculator sends you shopping for an extender, come back here and count its module address against the loop.
How does SLC loading affect the battery calculation?
Every loop device draws standby current around the clock — about 0.3 mA for a typical addressable smoke or pull station, 0.27–0.4 mA for modules and isolators — so a loaded 159+159 loop adds roughly 95 mA of standby load, which over 24 hours is more than 2 amp-hours before the aging factor. Some loop cards also publish their own current budget (the MS-9600 loop runs 100 mA normal, 400 mA short-circuit), so a fully loaded loop plus sounder bases deserves a check. This calculator totals the loop's standby current from the shared device library — carry it into the battery calculator's standby column.
Method: point capacities, pool structures, loop resistance/length/capacitance limits, T-tap permissions, and isolator engineering limits per each manufacturer’s listed documents (Fire-Lite 51309:R6 + datasheets; Silent Knight LS10146-001SK-E; Potter 5403602/AFC; Notifier DN-7112) — read 2026-07-19; verify against the shipping revision. Wire resistance on the NEC Chapter 9 Table 8 (75°C) pair basis, measured through both conductors as the manuals specify. Circuit classes per NFPA 72 12.3; single-fault zone rule 23.6.1 (2016 and later editions). Loop standby currents are the shared device-library values. This is a design aid — the panel’s installation manual, its listing, and the AHJ govern.
The suite closes the loop: detector counts come from the detector spacing calculator, NAC extenders from the NAC loading calculator each cost a module address here, and the loop’s standby current feeds the battery calculator. Pathway classes are charted on the pathway class chart.