How to Adjust Wire Size Using New Features in Revit MEP 2019
Revit determines the wire size for power lines based on specified dimensions, voltage drop calculations, and correction factors related to circuit protection. The software automatically adjusts the wire size to maintain a voltage drop of 3% in branch lines and 2% at the farthest outlet of the feeder line from the power supply. In other words, the line load dictates the required overcurrent protection (rated current), and once the overcurrent protection is specified, Revit calculates the necessary wire size for the live, neutral, and ground wires.
Rated Current of the Line (Protection)
Revit calculates the line load by summing the loads of all devices connected to the circuit and adjusts this based on ambient temperature using correction factors. However, Revit does not automatically assign the circuit’s rated current. After creating a circuit, you must specify its rated current according to its apparent current and load type, following applicable electrical standards. The ‘line instance’ parameter labeled “Rated” is used to set the required line protection size. This rated current then determines the appropriate wire size.
Voltage Drop Calculation
Voltage drop (VD) in the wiring circuit is calculated by Revit using the formula:
VD = (L × R × I) / 1000
- L = one-way length of the line (in feet)
- R = wire resistance (in ohms per kilofoot), based on the wire size impedance table for the specified wire type
- I = load current (in amperes)
Conductor Type
In the Electrical Settings dialog box, you can select wire types from the Wiring Type table to specify which wire types are allowed in the project. Multiple wire types can be assigned within the project settings. After specifying these, you can select the wire type via the Attributes palette. The first wire type listed in the table becomes the default and should represent the primary wire type used throughout the project.
Besides the built-in wire types provided by Revit, you can add new wire types, remove existing ones, or import wire types from other projects.
Live Wire Size Adjustment
The live wire size is initially determined by the rated current, wire type, rated temperature (60°C, 75°C, or 90°C), and correction factors. It is then adjusted as necessary to keep the voltage drop within 2% for the feeder circuit and 3% for branch circuits. Revit selects the live wire size based on these parameters using the basic wire size table.
Ground Wire Size Adjustment
The ground wire size is chosen according to the line’s rated current as specified in the ground wire size table. If multiple wiring conduits or cables are arranged in parallel—per applicable electrical codes—the size of each parallel equipment grounding conductor will be adjusted based on the rated amperage of the overcurrent device protecting the circuit.
Neutral Wire Size Adjustment
The neutral wire size can be adjusted based either on the live wire size or the unbalanced current. When adjusted according to the live wire size, a neutral load multiplier is applied to calculate the actual neutral wire size. This uses the neutral wire size table rather than the current-carrying capacity table and is designed to accommodate increased current caused by harmonic loads.
Harmonic loads are generated by switching power supplies found in many types of electrical equipment. These supplies create harmonic distortion in the current waveform, resulting in higher current flow than anticipated. The wire type settings also allow users to define a maximum wire size. Once this size is reached, the wire size will not increase further, but additional wires can be connected in parallel to support larger loads.
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