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Return Path Lengths for Class A Circuits

How does FireBIM calculate the return path length for class A circuits? Is there a way to modify that length?​

The return path length calculation is based upon the Circuit Length ▾ setting for the circuit in the Circuit Edit command. The methods for calculating and modifying that length are described below for each setting.

Straight Line / Right Angles​

The Straight line setting calculates a direct return path from the last device on the circuit back to the panel. Similarly, the Right angles setting will calculate the shortest path along the building axes from the last device back to the panel.

If you need to calculate the length for a longer return path while still using either of these settings, you can determine the additional length and add it to the circuit using the Circuit Make-Up Length ▾ field. This method will require some manual measurement, but allows you to use FireBIM's circuit length calculations for the rest of the circuit.

For voltage drop calculations, when voltage drop is calculated along the redundant path, the make-up length will be added to the return path. Visit the Voltage Drop Calculation Basis article for more information.

Revit Calculated Length​

The Revit calculated length setting uses the same value as the Revit Length parameter for the circuit, which you can modify by selecting a device on the circuit and running the Revit Electrical Circuits→ Edit Path command. If Path Mode ▾ is set to All Devices, Revit will trace a path through each device that ends at the last device on the circuit, and you can make additional changes as needed. For more information, visit the Autodesk documentation for editing a circuit path.

For voltage drop calculations, because Revit only reports the total circuit length and does not give FireBIM a way to detect device ordering or where the last device sits along that length, you must determine the worst-case scenario for voltage drop and modify the Revit circuit path to match that scenario. In addition, the end-of-line calculation method will be used for Revit-calculated circuits.

Fixed​

The Fixed setting uses the circuit length you enter, ignoring any other values or calculations. If you want to measure and input the entire length of the circuit manually, including the return path, use this setting.

For voltage drop calculations, because FireBIM cannot determine how a fixed length should be divided among devices on the circuit or where the last device sits along that length, you must determine the worst-case scenario for voltage drop and set the circuit length to match that scenario. In addition, the end-of-line calculation method will be used for fixed length circuits.