6.4 Notification Appliance Circuit Loading

Notification Appliance Circuit Loading

A notification appliance circuit (NAC) powers audible and visible appliances — horns, strobes, horn/strobes, speakers — during alarm. NAC loading is a favorite NICET FAS scenario because it blends field recognition with arithmetic. You must know which appliances are on the circuit, the current each draws at its selected candela (cd) setting, what the circuit and the power-supply output can deliver, and whether wire resistance leaves enough voltage at the last appliance.

Strobe current is not a single number. It rises with candela: a published horn/strobe might draw roughly 0.090 A at 75 cd but 0.135 A at 110 cd and more at 177 cd. Current also varies by manufacturer, voltage range (regulated vs full-wave/filtered), and whether synchronization is used. The exam gives a current table when exact values matter; your job is to pick the correct row for the model and setting and multiply by quantity.

Worked loading example: NAC-2 carries 18 horn/strobes at 75 cd, each 0.090 A. Circuit current = 18 × 0.090 = 1.62 A. If the NAC is rated 3.0 A, the ampacity check passes (1.62 A < 3.0 A) with ~1.38 A spare. But that does not end the analysis — voltage drop at the far end must still be verified (Section 6.6).

Isolating the Circuit and the Common Traps

For NICET FAS scenarios, isolate the circuit first. If NAC-1 serves the first floor and NAC-2 serves the second floor, a question about adding three appliances to NAC-2 must not include NAC-1 appliances in the circuit-current sum. A question about total power-supply load, however, may include both outputs when one supply feeds both.

A NAC loading workflow:

1. Identify the circuit or supply output being tested.
2. Count only the appliances on that circuit/output.
3. Select current per appliance from the provided data at the actual cd setting.
4. Multiply current × quantity for each group.
5. Sum the group loads.
6. Compare the total to the NAC rating and the supply-output rating (two separate ratings).
7. If asked, calculate voltage drop at the farthest appliance.
8. Include synchronization/control-module loads if the scenario shows them.

Trap 1 — lowballing candela: using the lowest cd current because it makes the circuit pass. If the exhibit shows 110 cd, use the 110 cd current, not 15 cd. Trap 2 — supply vs circuit: checking only total power-supply capacity while ignoring the per-output rating. A supply may have ample total capacity while one output is overloaded. Trap 3 — unit mixing: combining 0.120 A with 120 mA as if they were different scales; convert first.

Voltage drop is a separate concern. A NAC can be well under its amp rating yet still deliver too little voltage at the farthest appliance because of conductor resistance and distance. If the question supplies wire size, distance, current, and a minimum operating voltage, expect voltage drop to be the deciding check.

NICET Level II places loading in the layout/submittal domain, so a drawing question may ask which NAC schedule is incomplete. A complete schedule shows device counts, models and cd settings, per-device current, the NAC and supply-output ratings, conductor size and length, and a voltage-drop basis. Missing any of these is a documentation gap a reviewer should flag.

Finally, watch the operating mode. Some appliances draw a different current in synchronized versus non-synchronized mode, and speaker NACs are sized by wattage tap (e.g., 1/4 W, 1/2 W, 1 W) rather than a fixed current. Use the data sheet value that matches the configuration in the exhibit.

Two Ratings, Two Failure Modes, and Spare Capacity

Every NAC has two limits you must satisfy, and they fail in different ways:

1. NAC circuit rating (e.g., 3.0 A) — the maximum current the single output can supply. Exceed it and the panel either will not power up the circuit or trips its internal protection.
2. Power-supply output rating — the total all NACs on that supply may draw. A supply with three 3.0 A NACs is rarely rated for 9.0 A total; the aggregate is its own limit.

A derating margin is also good practice: many designers load a NAC to no more than about 80% of its rating to leave headroom for tolerance and future devices. On a 3.0 A NAC that is 2.4 A; the 1.62 A worked load sits comfortably below it. The exam may ask for spare capacity — rating minus load — which here is 3.0 − 1.62 = 1.38 A, enough for roughly 15 more 75 cd strobes electrically, though voltage drop may limit additions sooner.

Reading the NAC Schedule on a Submittal

On a Level II layout/submittal question, you may be handed a NAC schedule and asked whether it is complete or which circuit is overloaded. A defensible schedule lists, per circuit: device count and models, candela/wattage settings, per-device alarm current, the circuit total, the NAC rating, the supply-output rating, conductor AWG and run length, and the end-of-line voltage (the voltage-drop result). Spotting the missing column is exactly the plan-review skill NICET tests.

A schedule that shows current but omits the voltage-drop basis, or shows wire size but no length, cannot be verified — and "insufficient information to verify" is sometimes the correct answer. Train yourself to check the output rating and the circuit rating as separate gates, because a supply can be fine in total while one of its outputs is over its individual limit.