NAC Circuit Loading Calculator
Check whether your notification appliances actually fit on the panel or booster powering them. Every NAC supply has two independent limits — a per-circuit amp rating and a total alarm budget that is always smaller than the circuits combined — and this calculator checks both, with verified manufacturer ratings, a configurable headroom budget, sync-module loads, circuit-split suggestions, and a printable loading schedule. Free, no login.
Power source
Unused circuit — add appliances or leave empty.
Unused circuit — add appliances or leave empty.
Supply loading
Ratings per the supply’s UL 864 listing (manual figures). Two-level check: each circuit ≤ its rating AND Σ circuits + aux ≤ the total alarm budget. Headroom default 20% per IBC/IFC 907.5.2.3.2 (employee-work-area NACs) and common spec practice. Appliance currents are UL-max RMS (16–33 V window); match DC vs FWR to the supply.
Loading schedule header (project, panel, preparer)
NAC Circuit Loading Schedule
Notification circuit current loading — prepared with the NORDIX NAC Circuit Loading Calculator (nordixhq.com/tools/nac-circuit-loading-calculator)
| Project: — | Panel model / ID: — |
| Power source: Fire-Lite HPFF8 (booster) — 3 A/circuit, 8 A total, Class B | Prepared by: — Date: — |
| NAC 1 | Qty | mA each | A total |
|---|---|---|---|
| Horn/strobe 75 cd | 10 | 176 | 1.760 |
| NAC 1 total vs 3 A rating (2.40 A with 20% headroom) | 1.760 A — PASS | ||
| NAC 2 | Qty | mA each | A total |
|---|---|---|---|
| Horn/strobe 110 cd | 12 | 212 | 2.544 |
| NAC 2 total vs 3 A rating (2.40 A with 20% headroom) | 2.544 A — OVER HEADROOM | ||
| Sum of circuit loads | 4.304 A |
| Supply total vs 8 A rating (6.40 A with 20% headroom) | 4.30 A — PASS |
Circuit and supply ratings from the manufacturer’s installation manual (UL 864 listing figures). Appliance currents are manufacturer UL-max RMS across the regulated 16–33 V window — match the DC or FWR column to the supply type and verify against installed-device datasheets. Headroom 20% (IBC/IFC 907.5.2.3.2 requires ≥20% spare on NACs serving employee work areas; broader spare-capacity figures are project-specification practice). Current loading does not verify end-of-line voltage — submit voltage-drop calculations for each circuit. The panel’s listed installation instructions and the AHJ govern.
The two-level check
Sum the UL-max alarm current of every appliance on each circuit, then check two things independently: each circuit against its per-circuit rating, and the sum of all circuits plus aux loads against the supply’s total alarm rating. The second check is the one that gets missed, because on every real supply the per-circuit maxima add up to more than the total — an HPFF8 has four 3 A circuits but only 8 A total; an ES-50X has two 2.5 A circuits but only 2.7 A total shared with its annunciator bus. A design where every circuit passes individually can still overload the supply, and no spec sheet’s per-circuit number warns you. Some boosters add a third limit: the FCPS series is rated 6–8 A short-term (one hour, alarm) but only 4–6 A continuous, so standing loads like door holders check against the lower figure.
Worked example
Twelve 110 cd horn/strobes (System Sensor P2R, temporal high, DC) at 212 mA UL-max each:
- Circuit load: 12 × 0.212 A = 2.544 A.
- On a 2.5 A circuit (MS-9200UDLS): fail — over the rating by 44 mA.
- On a 3.0 A circuit (HPFF8, PSN-1000): within the rating, but over the 20% headroom budget of 2.4 A usable.
- Split across two circuits (6 + 6): 1.272 A each — passes with room to spare; the supply total (2.544 A of 8 A) is comfortable.
The default sheet above starts near this scenario — watch NAC 2’s badge change as you move appliances to NAC 3.
Reference: verified supply ratings
Every rating below was read from the manufacturer’s installation manual or datasheet — the figures the UL 864 listing fixes. Note the pattern: circuits × per-circuit rating always exceeds the total, and two small panels (ES-50X, SK 6700) can’t even feed one circuit at rating plus anything else.
| Supply | Circuits (B / A) | Per circuit | Total alarm | Notes |
|---|---|---|---|---|
| Fire-Lite MS-9200UDLS (panel) | 4 / 2 | 2.5 A | 6 A | 0.3 A per special-application aux output, inside the 6.0 A total |
| Fire-Lite MS-9600LS/UDLS (panel) | 4 / 2 | 3 A | 7 A | Aux outputs share the 7.0 A total; ≤1.5 A aux in standby |
| Fire-Lite ES-50X (panel) | 2 / 2 | 2.5 A | 2.7 A | ANN-BUS shares the 2.7 A total — the panel cannot feed both NACs at 2.5 A |
| Silent Knight 6700 (panel) | 2 / 2 | 2.5 A | 2.5 A | Total accessory load 2.5 A — one circuit at rating exhausts the panel |
| Silent Knight 6808 (panel) | 4 / 4 | 3 A | 6 A | — |
| Potter PFC-4064 (panel) | 4 / 4 | 3 A | 5 A | +1.0 A aux beyond the 5.0 A supply |
| Potter AFC-1000 / PFC-6800 (panel) | 6 / 6 | 3 A | 10 A | — |
| Fire-Lite FCPS-24FS6 (booster) | 4 / 4 | 3 A | 6 A (4 A cont.) | 6.0 A short-term (1 hr max) / 4.0 A continuous — continuous loads check the lower figure |
| Fire-Lite FCPS-24FS8 (booster) | 4 / 4 | 3 A | 8 A (6 A cont.) | 8.0 A short-term (1 hr max) / 6.0 A continuous |
| Fire-Lite HPFF8 (booster) | 4 / 4 | 3 A | 8 A | — |
| Fire-Lite HPFF12 (booster) | 4 / 4 | 3 A | 12 A | 2.0 A aux output inside the 12 A total |
| Potter PSN-1000 (booster) | 6 / 3 | 3 A | 10 A | — |
| Altronix AL1002ULADA (booster) | 4 / 4 | 2.5 A | 10 A | AUX load subtracts from the 10 A total; note 2.5 A/circuit, not 3 |
Headroom: what’s code and what’s practice
The one genuinely code-mandated spare capacity is IBC/IFC 907.5.2.3.2: notification circuits serving employee work areas must be designed with at least 20% spare capacity for future visible appliances — which in commercial work covers most circuits. Beyond that, the “80% rule” designers quote (a 1.5 A circuit treated as 1.2 A usable) is trade practice and project-specification language, not NFPA 72 or UL 864 text — engineering specs commonly require 20–25% spare on circuits, supplies, amplifiers, and batteries. NFPA 72’s own margin lives elsewhere: the ×1.25 battery aging factor (§10.6.7.2.14) and the 24-hour standby durations. This calculator defaults to a 20% headroom budget with that framing, shows it as an explicit “usable amps” line, and lets you set it to whatever your spec — or zero — requires.
Strobe synchronization and loading
NFPA 72 (2022) 18.5.5.7.2 requires visible appliances to flash in synchronization when more than two are in any field of view — which is nearly every corridor. Panel built-in sync protocols don’t change circuit capacity (the 2.5/3 A ratings apply as printed), but external sync modules do add their own draw and carry their own limits: a Wheelock DSM is rated 3 A per circuit, serves two Class B circuits (or one Class A) per module, daisy-chains to 20 modules, and adds 35 mA of module current — the calculator’s add-menu includes it. Circuit capacity is current-limited, not device-count-limited, in every manual we verified: “how many strobes per circuit” is always an amps question, which is exactly what this tool answers.
Frequently asked questions
How many horn strobes can I put on one NAC circuit?
It's an amps question, not a count question: sum the UL-max alarm current of every appliance and stay inside the circuit's rating — typically 2.5 or 3 A — less your headroom. At 110 cd, a 212 mA horn/strobe means eleven devices fit a 2.5 A circuit raw (2.332 A) but only nine fit a 20%-headroom budget of 2.0 A; at 15 cd you'd fit three times as many. Candela drives everything, which is why rules of thumb like 'seven per circuit' are only right for one candela at one rating. This calculator does the sum per circuit and badges each one.
What is the current rating of a NAC circuit?
Typically 2.5 or 3 A per circuit, set by the panel's UL 864 listing and printed in its manual: Fire-Lite MS-9200UDLS is 2.5 A, MS-9600 and most boosters (FCPS, HPFF, Potter PSN-1000) are 3 A — but Altronix's AL1002ULADA is 2.5 A, so never assume. The per-circuit number is only half the story: the supply also has a total alarm rating shared across all circuits and aux loads, and it is always less than the circuits combined. This calculator carries verified figures for the common panels and boosters and checks both levels.
Why did my design overload the panel when every circuit was under its rating?
Because the total budget binds before the circuit ratings do. An ES-50X has two 2.5 A circuits but only 2.7 A of total system power shared with its annunciator bus — load both circuits to 1.4 A and every circuit passes while the panel fails. An MS-9200UDLS has four 2.5 A circuits inside a 6.0 A total; an HPFF8 has four 3 A circuits inside 8 A. This two-level structure is universal, it's the actual field failure mode, and checking it is the core of this calculator.
Is there a code rule that NAC circuits can only be loaded to 80%?
Not in NFPA 72 or UL 864 — the 80% figure is trade practice borrowed from the NEC's continuous-load concept, plus project specifications that commonly demand 20–25% spare on circuits, supplies, and batteries. The one genuinely code-mandated spare capacity is IBC/IFC 907.5.2.3.2: NACs serving employee work areas must be designed with at least 20% spare for future visible appliances — which covers most circuits in commercial buildings. This calculator defaults to a 20% headroom budget on that basis and lets you change or zero it.
What is a continuous vs short-term rating on a NAC power supply?
Some boosters carry two totals: the Fire-Lite FCPS-24FS6 delivers 6.0 A short-term — defined as one hour maximum, fine for alarm loads — but only 4.0 A continuously. Standing loads like door holders, aux-powered devices, and anything energized around the clock must fit the continuous figure; notification appliances that only run in alarm may use the short-term one. Where a supply in the library distinguishes the two, this calculator shows a separate continuous-loads check.
Do sync modules change how much I can put on a circuit?
Panel built-in synchronization doesn't — the manuals state the same 2.5/3 A ratings with sync enabled. External sync modules add two things: their own draw (a Wheelock DSM adds 35 mA at 24 V) and their own limits — a DSM is rated 3 A per circuit, serves two Class B circuits or one Class A per module, and daisy-chains to a maximum of 20 modules. Remember the code driver: NFPA 72 (2022) 18.5.5.7.2 requires synchronization whenever more than two visible appliances share a field of view.
What do I do when the appliances don't fit the circuits?
In order: regroup — the calculator tells you the minimum circuits the load needs at your headroom; drop candela where the coverage calc allows it (a 110→75 cd change cuts a device from 212 to 176 mA); move load to unused circuits; and when the supply total itself is the wall, add a NAC power extender (booster) — a 4-circuit, 8–12 A box triggered by an existing NAC — placing it closer to the appliances, which usually fixes voltage drop at the same time. On addressable systems, also check you have SLC points and control modules available to trigger the new booster.
If my circuits pass loading, is the design done?
No — loading is one of three checks in the submittal package. A circuit can pass its amp rating and still fail voltage drop: 2 A on 400 ft of 14 AWG drops nearly 5 V, taking a 20.4 V source below the 16 V appliance floor. And the total alarm load drives the battery calculation, where the capacity must survive 24 hours of standby plus 5 or 15 minutes of alarm with the ×1.25 aging factor. Run all three — this calculator, the NAC voltage drop calculator, and the battery calculator share one device library so the numbers can't drift between sheets.
Method: appliance UL-max RMS alarm currents (16–33 V regulated window; DC vs FWR columns differ by device — match the supply) summed per circuit and per supply against the manufacturer’s listed ratings. Ratings table read from manufacturer manuals/datasheets 2026-07-19; verify against the shipping manual revision for your hardware. Headroom default 20% per IBC/IFC 907.5.2.3.2 (employee-work-area NACs) and common specification practice. Wire ampacity (18 AWG = 6 A per NEC Table 402.5) is never the binding limit at these ratings. Passing loading does not verify end-of-line voltage or battery capacity — run all three calculations. The panel’s listed installation instructions and the AHJ govern.
Complete the submittal package with the NAC Voltage Drop Calculator and Fire Alarm Battery Calculator, or see the NAC current draw chart for the appliance current tables.