Try a full lesson: Checking a networked panel battery and NAC voltage drop calculation

Why this check matters at Level IV

At Level III you checked a battery calculation. At Level IV you approve a package for a networked system where every control unit, every remote power supply and every amplifier cabinet carries its own battery and its own calculation, and where one omitted load can sit unnoticed until a power outage. NICET performance measure 0303-7107 names power and battery calculations and component compatibility as what a Level IV shop drawing review must catch.

This free lesson works one node and one notification circuit from a fictional package, the way you would on a real review: inputs first, then arithmetic, then a comment the designer can act on.

Keep your own copy of NFPA 72 open. We cite chapters by number only and never reproduce the code's wording. Every number below is invented exercise data.

Checking power, battery and voltage drop calculations

The fictional project

A four-story laboratory building has three networked control units (nodes). Node 2 serves floors 3 and 4. It has a network interface card, a remote annunciator in the floor 3 lobby, 112 addressable devices on its signaling line circuit, two notification circuits of its own, and a remote power supply that runs four more notification circuits.

The design basis in the specification, which this exercise uses as given: 24 hours of standby, then 5 minutes of alarm, with a 20 percent margin added to the calculated amp-hours. Chapter 10 is where you would confirm what the code requires for a real project.

Step 1: check the inputs against the drawings

Before any arithmetic, compare the calculation's load list with the riser diagram and the data sheets. The submitted Node 2 calculation lists:

The riser diagram also shows a network interface card (data sheet: 0.040 A standby, 0.040 A alarm) and a remote annunciator (0.030 A standby, 0.060 A alarm) powered from Node 2. Neither is on the calculation. That is the first finding, and it came from comparing documents, not from arithmetic.

Step 2: rebuild the battery calculation

Corrected standby current: 0.180 + 0.0392 + 0.040 + 0.030 = 0.2892 A

Corrected alarm current: 0.320 + 0.060 + 2.100 + 0.040 + 0.060 = 2.580 A

• Standby amp-hours: 0.2892 A × 24 h = 6.941 Ah
• Alarm amp-hours: 2.580 A × (5 ÷ 60) h = 2.580 × 0.0833 = 0.215 Ah
• Total before margin: 6.941 + 0.215 = 7.156 Ah
• With the 20 percent margin: 7.156 × 1.20 = 8.59 Ah

The submittal shows 5.47 Ah and selects a 7 Ah battery. The correct requirement is 8.59 Ah, so the 7 Ah battery fails. A 12 Ah battery passes, provided the control unit's charger is listed to charge it; check the data sheet for the largest battery the charger supports.

Notice how the error hid. The missing standby load was only 0.070 A, but over 24 hours it is 1.68 Ah, and the margin was left off as well. Standby errors are multiplied by the longest duration in the calculation, so a small missing current matters more in standby than in alarm.

Step 3: check the remote power supply as its own system

The remote power supply has its own battery and needs its own calculation. Its data sheet gives 0.065 A standby and 0.090 A of its own draw in alarm. Its four circuits draw 5.600 A together in alarm.

• Standby: 0.065 × 24 = 1.560 Ah
• Alarm: (5.600 + 0.090) × 0.0833 = 0.474 Ah
• Total with margin: (1.560 + 0.474) × 1.20 = 2.44 Ah

The submittal shows a 7 Ah battery here, which passes. Also check that 5.69 A in alarm is within the supply's output rating, and within any spare capacity the specification requires.

Step 4: check the longest notification circuit

Circuit 3 on the remote power supply runs 380 ft one way to its farthest appliance and carries 14 horn-strobes. The appliance data sheet gives a maximum of 0.142 A each at the low end of its operating range and a minimum operating voltage of 16.0 V. The power supply's listing says to start voltage drop calculations at 20.4 V. The wire is 14 AWG at 3.07 ohms per 1,000 ft per conductor.

Lumped method, with all current treated as if it were at the far end:

• Current: 14 × 0.142 = 1.988 A
• Loop length: 2 × 380 = 760 ft
• Loop resistance: 0.760 × 3.07 = 2.333 ohms
• Drop: 1.988 × 2.333 = 4.64 V
• End voltage: 20.4 - 4.64 = 15.76 V

15.76 V is below the 16.0 V minimum, so the circuit fails as designed.

The submittal shows 19.36 V at the end and marks the circuit as passing. It started at 24 V nominal instead of 20.4 V. The arithmetic of the drop was right; the starting point was wrong. Because that starting point came from a template, every circuit in the package needs to be rechecked.

Step 5: decide what fixes are acceptable

The designer has three common options, and you should accept any that works:

• Larger wire. 12 AWG at 1.93 ohms per 1,000 ft: 0.760 × 1.93 = 1.467 ohms; 1.988 × 1.467 = 2.92 V drop; 20.4 - 2.92 = 17.48 V. Passes.
• Split the circuit. Two circuits of 7 appliances each over the same 380 ft: 0.994 × 2.333 = 2.32 V drop; 20.4 - 2.32 = 18.08 V. Passes, if the supply has a spare circuit.
• A correct point-by-point calculation. The lumped method is conservative. A calculation that places each appliance at its real distance may show the circuit passes as drawn. Accept it only if the distances match the drawings and the method follows the manufacturer's instructions.

What you should not accept is a change of the start voltage back to nominal, or a note saying the circuit "should be fine."

Step 6: write the comments

Numbered, specific and tied to the governing document:

1. Node 2 battery calculation omits the network interface card and the remote annunciator shown on the riser diagram, and omits the 20 percent margin in the specification. Corrected requirement is 8.59 Ah; the 7 Ah battery shown does not meet it. Revise, confirm the charger supports the selected battery, and resubmit. (Specification power section; NFPA 72 Chapter 10.)
2. All notification circuit voltage drop calculations start at 24 V. Recalculate from the start voltage the power supply's listing gives. Circuit 3 on the floor 4 remote power supply fails at 15.76 V against the 16.0 V appliance minimum. Revise and resubmit all circuits.

Stamp the calculations revise and resubmit. The floor plans can be reviewed separately so the job is not held up by the calculations alone.

What the exam tests here

Level IV questions give you a calculation with one wrong input or one missing step and ask what is wrong, what the correct value is, or what the reviewer should do. Practice:

• Matching the load list to the riser diagram before doing any arithmetic.
• Converting alarm minutes to hours and applying the margin to the total.
• Doubling the one-way length and starting from the listed calculation voltage.
• Treating each node and each power supply as its own battery calculation.
• Choosing a review action that fixes the method across the whole package, not just the one circuit you checked.

Where to look in your own books

• NFPA 72-2022 Chapter 10: power supplies, secondary power and battery capacity
• NFPA 72-2022 Chapter 7: what shop drawings and calculations must show
• NFPA 72-2022 Chapter 18: notification appliances and synchronization across power supplies
• NFPA 70-2020: the article on fire alarm systems, and the conductor properties table in its tables chapter

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