NFPA 72 calculation brief
Voltage Drop Calculation for Fire Alarm NAC Circuits
Voltage Drop Calculation for Fire Alarm NAC Circuits explains the calculation inputs, formula context, field checks, and limits engineers should confirm before using results in a project workflow.
Page role
Use this page for a defined task before moving to the next project record.
- Audience
- Engineers, designers, technicians, contractors, and reviewers using a calculator-linked technical brief.
- Job
- Voltage Drop Calculation for Fire Alarm NAC Circuits supports fire alarm notification appliance circuit workflow by turning verified inputs into a calculator result, reference check, and worksheet handoff instead of a standalone explanation.
- Evidence
- Appliance count, Listed current at selected setting, Power supply rating, Spare capacity criterion, Source voltage
- Output
- Confirm appliance current comes from listed data at the selected candela or device setting. Review NAC spare capacity before using the circuit result in battery or voltage-drop records.
- Next
- Use /worksheets/fire-alarm-battery-nac before treating this topic as complete.
Calculator-first route
- 1Confirm the worksheet context before using this brief as a calculation input.
- 2Verify required inputs and linked reference data before opening the calculator.
- 3Run the related calculator, review warnings, and compare the output against the review checks.
- 4Carry only reviewed values into the worksheet, authority record, or final project package.
Technical brief workflow
Use this page as a calculator-linked technical brief: confirm the role, verify inputs, run checks, and move to the next tool.
| Brief phase | Engineering detail | Evidence or next step |
|---|---|---|
| Calculator role | Voltage Drop Calculation for Fire Alarm NAC Circuits supports fire alarm notification appliance circuit workflow by turning verified inputs into a calculator result, reference check, and worksheet handoff instead of a standalone explanation. | NAC Circuit Load Calculator, Fire Alarm NAC Voltage Drop Calculator, Fire Alarm Battery Capacity Calculator |
| Inputs to verify | Appliance count, Listed current at selected setting, Power supply rating, Spare capacity criterion, Source voltage | AWG Wire Resistance Table for Fire Alarm Circuits, US Code Adoption and AHJ Checkpoints |
| Review checks | Confirm appliance current comes from listed data at the selected candela or device setting. | NFPA 72 |
| Next step | Start from fire alarm notification appliance circuit workflow when this topic controls a design or field-review decision. | Use /worksheets/fire-alarm-battery-nac before treating this topic as complete. Open /calculator/fire-alarm/nac-circuit when the topic needs numeric screening. Check /reference/awg-wire-resistance before transferring reviewed values into the project package. |
Input checklist
- 1Appliance count
- 2Listed current at selected setting
- 3Power supply rating
- 4Spare capacity criterion
- 5Source voltage
Next steps
- 1Start from fire alarm notification appliance circuit workflow when this topic controls a design or field-review decision.
- 2Use /worksheets/fire-alarm-battery-nac before treating this topic as complete.
- 3Open /calculator/fire-alarm/nac-circuit when the topic needs numeric screening.
- 4Check /reference/awg-wire-resistance before transferring reviewed values into the project package.
Technical review checks
- 1Confirm appliance current comes from listed data at the selected candela or device setting.
- 2Review NAC spare capacity before using the circuit result in battery or voltage-drop records.
- 3Check voltage drop for the same circuit length and conductor size before submittal reliance.
- 4Document any product data or AHJ basis that changes circuit loading.
Technical explanation
Formula, variables, example, interpretation, and use limits for the calculation topic.
Formula
Voltage drop = circuit current x conductor resistance x round-trip length
NAC voltage drop checks whether notification appliances still receive acceptable end-of-line voltage after conductor resistance is applied across the circuit path.
Variables
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| I | circuit current | A | Total current on the notification appliance circuit. |
| R | conductor resistance | ohms/1000 ft | Resistance for the selected AWG copper conductor. |
| L | one-way route length | ft | Measured route length from source to end of line. |
| V_src | source voltage | V | Circuit source voltage used for the screen. |
| V_eol | end-of-line voltage | V | Voltage remaining at the farthest device. |
NAC voltage at end of line
Assumptions
- 1A NAC draws 1.2 A.
- 2The route length is 300 ft one way.
- 3The conductor resistance is 1.59 ohms per 1000 ft and the source is 24 V.
Steps
- 1Convert 300 ft one-way route length to 600 ft round-trip conductor length.
- 2Compute conductor resistance as 1.59 x 600 / 1000 = 0.954 ohms.
- 3Multiply 1.2 A by 0.954 ohms to get about 1.14 V drop.
- 4Subtract the drop from 24 V to get about 22.86 V at the end of line.
- 5Compare end-of-line voltage against listed appliance operating requirements.
Result
The screen shows about 22.86 V at the end of line. That value is useful only when the device current, selected candela setting, conductor size, and route length match the actual circuit.
Interpretation
- 1Higher current and longer route length increase voltage drop.
- 2Larger conductors reduce voltage drop but must still match installation and project constraints.
- 3End-of-line voltage should be judged against the listed operating range for the selected appliances.
Common mistakes
- 1Using physical room distance instead of routed one-way conductor length.
- 2Forgetting the round-trip conductor path when applying resistance.
- 3Using appliance current from the wrong candela or voltage setting.
- 4Checking voltage drop without also checking NAC power supply load and spare capacity.
Limits and sources
- 1This screen does not replace manufacturer voltage-drop worksheets or listed appliance data.
- 2Class, topology, synchronization modules, and power supply behavior can change the final circuit review.
- 3Final fire alarm design remains subject to adopted code, product listings, and AHJ review.
- 4Use conductor resistance data for the selected wire size and material.
- 5Use listed appliance current and voltage range from current product data.
Where this fits in the workflow
Use this brief inside the fire alarm notification appliance circuit workflow. It should answer a calculation handoff question, not act as general reading. Start from the related worksheet and open the linked calculator only after the input source is known.
Inputs to verify
Before running a tool, verify: Appliance count, Listed current at selected setting, Power supply rating, Spare capacity criterion, Source voltage. Check units, equipment listings, pipe or conductor data, and whether the value comes from measured field data or an early design assumption.
Calculator sequence
Use the related calculator set for this topic: /calculator/fire-alarm/nac-circuit, /calculator/fire-alarm/voltage-drop, /calculator/fire-alarm/battery-sizing. Review warning states and compare the output with the linked reference basis before transferring it into a worksheet.
Project record handoff
Retain the calculator inputs, result, formula context, warning state, reference row, and worksheet handoff note together. The next project record is /worksheets/fire-alarm-battery-nac.
Limits before project use
Confirm appliance current comes from listed data at the selected candela or device setting. Treat the output as a screening value. If the result controls design, has a narrow margin, affects equipment selection, or affects AHJ approval, it should be reviewed in the full design package by a qualified professional.
Worksheet handoff
Calculation tools
Next records
Voltage Drop Calculation for Fire Alarm NAC Circuits
Open related tools
Before filing
- 1Start from fire alarm notification appliance circuit workflow when this topic controls a design or field-review decision.
- 2Use /worksheets/fire-alarm-battery-nac before treating this topic as complete.
- 3Open /calculator/fire-alarm/nac-circuit when the topic needs numeric screening.
- 4Check /reference/awg-wire-resistance before transferring reviewed values into the project package.