Voltage Drop Calculation Basis

Voltage drop is calculated for circuits based upon the following formula:

$$Voltage\>Drop = {{Load \times Cable\>Resistance \times (Cable\>Length \times 2)} \over 1000}$$

For class A circuits, voltage drop will be calculated along the initial and redundant paths, assuming single-open faults after the last device and first device on the circuit, respectively. Calculations for both paths will be displayed in the voltage drop schedule, separated by a slash. Circuits that are set to upsize automatically will use the worst-case scenario.

Per UL 864 and NFPA 72 Handbook 18.3.2.3, $Voltage\>Drop$ should be subtracted from 85% of the nominal voltage to calculate the voltage at the device, assuming power is being supplied from a battery at the end of its useful life. Using the default FireBIM customization, voltage drop will be calculated for a 24V system using a starting voltage of 20.4V. The nominal voltage and starting voltage can be modified using the System voltage and Starting circuit voltage options in the Project Options command.

The variables used in the voltage drop calculation differ based upon the Calculation method ▾ setting in the Project Options command:

Point-to-Point

Using the point-to-point method, voltage drop is calculated at each device on the circuit.

info

FireBIM determines device ordering on circuits using wires drawn between those devices on plan views with the Revit Systems→Wire commands. These wires must be drawn to ensure the correct device ordering, which will impact point-to-point voltage drop values and can affect cable length calculations. Visit Draw Wires for Circuits on Plan Views for more information.

$Load$ is the sum of the Alarm Load values on the device and each device downstream of it on the circuit as set in the Family Edit or Device Edit command.

The default $Cable\>Resistance$ values are based upon NEC Table 8 and can be modified using the Cable Types command.

$Cable\>Length$ is the length of cable to each device from the previous device, converted to feet if needed. The value is multiplied by $2$ to account for the return cable. Cable length calculation methods can be set for the panel in the Panel Edit command and for individual circuits in the Circuit Edit command.

note

If the circuit length is set to Revit calculated length or Fixed in the Circuit Edit command, the end-of-line voltage drop will be calculated for that circuit instead.

End-of-Line

Using the end-of-line method, voltage drop is calculated once using the total load of devices and the total cable length for the circuit. Results are typically more conservative than with point-to-point calculations.

$Load$ is the sum of the Alarm Load values for all devices on the circuit as set in the Family Edit or Device Edit command.

The default $Cable\>Resistance$ values are based upon NEC Table 8 and can be modified using the Cable Types command.

$Cable\>Length$ is the total length of cable from each device to the panel, converted to feet if needed. The value is multiplied by $2$ to account for the return cable. Cable length calculation methods can be set for the panel in the Panel Edit command and for individual circuits in the Circuit Edit command.