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Free sample · NICET FA III

Try a full lesson: Checking battery and voltage drop calculations

This is a complete lesson from the course (module 8 of 10, NICET task 3.3.1), followed by 10 practice questions with full explanations. Nothing to sign up for.

Why this task matters at Level III

At Level II you ran the battery, loading and voltage drop numbers yourself. At Level III you check numbers someone else ran and put your name on the result. Task 3.3.1 covers preparing and approving shop drawings, and the calculations are where a quiet error turns into a dead strobe at the acceptance test or a panel that drops out during an outage.

A reviewer does not redo every line by habit. A reviewer knows where calculations usually go wrong, rebuilds the critical ones from the data sheets, and writes comments a designer can act on without a phone call. This lesson works through three checks with invented numbers, then covers how to accept, comment on or return the package.

Keep your own copy of NFPA 72 open. We cite sections by number and never reproduce the code's wording.

Checking battery and voltage drop calculations

Before checking any arithmetic, check the inputs. Most rejected calculations fail on inputs, not on multiplication.

  • Every current figure should trace to a named data sheet and model.
  • Strobe currents should match the candela setting on the plans, at the low end of the operating voltage.
  • Every device on the plans should appear in the calculation. Count them.
  • The durations should match the system type. A typical system needs 24 hours of standby followed by 5 minutes of alarm. An emergency voice/alarm communications system needs 15 minutes of alarm instead. Read NFPA 72-2022 Chapter 10 in your own copy.
  • The margin should come from the code edition in force and the manufacturer's worksheet.

Worked calculation 1: checking a standby and alarm battery calculation

Exercise data (invented). A submitted calculation for one panel claims 6.89 Ah and selects a pair of 12 V, 7 Ah batteries. The data sheets give:

  • Panel: 0.180 A standby, 0.300 A alarm
  • Detectors and modules on the SLC: 0.095 A standby in total
  • Remote annunciator: 0.040 A standby, 0.060 A alarm
  • Notification appliances: 0 A standby, 3.20 A alarm
  • Margin for this exercise: 20 percent. Battery sizes offered: 7, 12 and 18 Ah.

Step 1. Standby current. 0.180 + 0.095 + 0.040 = 0.315 A

Step 2. Alarm current. 0.300 + 3.20 + 0.060 = 3.56 A

Step 3. Standby amp-hours. 0.315 A × 24 h = 7.56 Ah

Step 4. Alarm amp-hours. 5 minutes = 5 ÷ 60 = 0.0833 h. 3.56 A × 5 ÷ 60 = 0.297 Ah

Step 5. Total. 7.56 + 0.297 = 7.857 Ah

Step 6. Margin. 7.857 × 1.20 = 9.43 Ah

Step 7. Select. The next size up is 12 Ah. The submitted 7 Ah battery fails.

Finding the error. Remove the annunciator: standby 0.275 A × 24 = 6.60 Ah, alarm 3.50 A × 5 ÷ 60 = 0.292 Ah, total 6.89 Ah with no margin. That matches the submittal, so the comment is specific: annunciator omitted and no margin applied.

Double check. 0.315 × 24: 0.3 × 24 = 7.2, plus 0.015 × 24 = 0.36, so 7.56. 3.56 ÷ 12 = 0.2967. 7.857 × 1.2 = 7.857 + 1.571 = 9.428. Same result twice.

Worked calculation 2: checking end-of-circuit voltage and maximum run length

Exercise data (invented). One NAC carries 1.85 A of appliances, treated as a lumped load at the far end. Wire is 12 AWG solid copper at 1.93 ohms per 1,000 ft (NFPA 70-2020 conductor properties table). The last appliance is 310 ft from the power supply. The supply's end-of-battery output is 20.4 V and the appliances are listed down to 16.0 V. The submittal claims a maximum run of 1,120 ft.

Step 1. Loop length. 2 × 310 = 620 ft

Step 2. Resistance. 1.93 × 620 ÷ 1,000 = 1.197 ohms

Step 3. Voltage drop. 1.85 A × 1.197 ohms = 2.21 V

Step 4. End voltage. 20.4 - 2.21 = 18.19 V. That is above 16.0 V, so this circuit passes.

Step 5. Maximum run. Allowed drop = 20.4 - 16.0 = 4.4 V. Maximum resistance = 4.4 ÷ 1.85 = 2.378 ohms. Maximum loop = 2.378 ÷ 1.93 × 1,000 = 1,232 ft. Maximum one way = 616 ft.

Finding the error. Starting from 24 V instead: allowed drop 8.0 V, 8.0 ÷ 1.85 = 4.324 ohms, 4.324 ÷ 1.93 × 1,000 = 2,240 ft of loop, 1,120 ft one way. The submittal used nominal voltage. Its claimed maximum is almost double the real one, which matters the day someone extends this circuit.

Double check. 616 × 2 = 1,232 ft. 1.232 × 1.93 = 2.378 ohms. 2.378 × 1.85 = 4.40 V. 20.4 - 4.40 = 16.0 V. Correct.

Worked calculation 3: checking NAC load against a power supply rating

Exercise data (invented). A NAC power supply has four outputs rated 2.5 A each and a total output rating of 6.0 A. The submittal shows each circuit under 2.5 A and marks the supply as adequate. Appliance currents at the low end of the voltage range: 15 cd horn-strobe 0.066 A, 30 cd strobe 0.094 A, 75 cd horn-strobe 0.158 A, 110 cd strobe 0.205 A.

  • NAC 1: 10 at 15 cd = 0.660 A, plus 6 at 75 cd = 0.948 A. Total 1.608 A.
  • NAC 2: 12 at 30 cd = 1.128 A, plus 4 at 110 cd = 0.820 A. Total 1.948 A.
  • NAC 3: 14 at 75 cd = 2.212 A.
  • NAC 4: 8 at 15 cd = 0.528 A.

Each circuit is under 2.5 A. Now add them: 1.608 + 1.948 + 2.212 + 0.528 = 6.296 A. That is 0.296 A over the 6.0 A supply rating. The per-circuit check passed; the supply check fails.

Double check. 1.608 + 1.948 = 3.556. 3.556 + 2.212 = 5.768. 5.768 + 0.528 = 6.296. Same total. Comment: move NAC 4 to another supply or a panel NAC, then rerun the battery calculation for whichever supply takes it.

What the exam tests here. Practice finding the input error, not only the arithmetic one: a missing device, nominal instead of end-of-battery voltage, one-way length for loop length, a margin left off, or circuits checked one at a time against a supply that carries them all.

Check yourself

The free practice set below has questions in the style of the exam, with a full explanation for every answer.

Where to look in your own books

  • NFPA 72-2022 Chapter 10: secondary power capacity and duration
  • NFPA 72-2022 Chapter 18: notification appliance listing and installation
  • NFPA 72-2022 Chapter 7: shop drawing content
  • NFPA 70-2020, the conductor properties table: conductor resistance
  • IBC 2021, the section on submittal documents: construction documents submitted to the building department
  • The panel and power supply manuals: end-of-battery voltage, output ratings, battery worksheet

Practice set

Pick an answer, then check it. Every wrong option is explained too.

1. Installation

During acceptance, the supervisor listens at the far end of a corridor and finds the horns hard to hear over the HVAC noise. The circuit tested as working. What is the best next step?

2. Installation

The fire alarm drawings and the architectural drawings disagree about the location of a wall that holds a manual station. The crew has not yet installed the station. What should the supervisor do?

3. Installation

A record drawing set has hand-marked changes in pencil that cannot be read. What should the supervisor do?

4. Maintenance

A hospital wing is due for horn testing. Patients rest there. What should the supervisor do?

5. Maintenance

A panel shows an open circuit trouble on a notification circuit. The building is occupied. What is the first priority?

6. Maintenance

A report lists five failed devices and the technician's note says "to be fixed". What should the supervisor add before issuing it?

7. Submittal Preparation and System Layout

A room's detector layout meets the stated spacing, but a tall storage rack placed after design blocks airflow to two detectors. What should the reviewer ask?

8. Submittal Preparation and System Layout

A calculation lists a standby load of 0.25 A and an alarm load of 1.20 A. The stated criteria are 24 hours of standby, 5 minutes of alarm and a 20 percent margin. What is the required capacity?

9. Submittal Preparation and System Layout

A reviewer checks a notification circuit: 12 AWG at 1.93 ohm per 1,000 ft, 1.0 A at the far end and a 500 ft one-way run. The panel supplies 20.4 V at end of battery. What is the voltage at the last appliance?

10. Management and Supervision

The supplier says the panel will arrive two weeks late. Rough-in is on schedule. What should the supervisor do?

0 of 10 checkedCheck each answer to see your score.

Practice this module in the course: Accepting calculations, power and the submittal package

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  • 10modules of lessonsEvery task on the outline
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See all 10 modules
  1. Supervising the installation project
  2. As-builts and close-out documents
  3. Overseeing commissioning
  4. Managing periodic testing
  5. Resolving impairments and deficiencies
  6. Maintenance documentation and records
  7. Reviewing shop drawings: devices, coverage and code interfaces
  8. Accepting calculations, power and the submittal package
  9. Supervising work activities
  10. Supervising team members
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Questions are original and written to NICET's published content outline. Code is cited by section number so you can look it up in your own copy. Think something is wrong? Report it.