Revit calculates the wire sizes for power circuits, based on the size specified for circuit protection, voltage drop calculation, and correction factor. Wire sizes are automatically sized to maintain a voltage drop of less than 3 percent for branch circuits, and 2 percent for feeder circuits at the outlet furthest from the source. That is, the circuit load determines the overcurrent protection (circuit rating) required, which when specified, then determines the wire sizes required for hot conductors, neutral conductors and ground conductors.
Revit calculates the load for a circuit as the sum of the loads for the individual devices connected in the circuit and adjusted for ambient temperature ( ). Revit does not automatically specify the circuit rating. After the circuit is created, you specify the circuit rating, in accordance with applicable electrical codes, based on the apparent current and the type of load for the circuit. The circuit instance Rating parameter specifies the size of the circuit protection required. The circuit rating is used to determine the wire sizing.
In the Electrical Setting dialog, you select wire types from the table to specify the wire types that can be used within a project. Multiple wire types can be specified for a project (in the Electrical Settings dialog). When more than one wire type is specified for a project, the type can be selected on the circuit’s Properties palette. The first entry specified in the wiring types table is the default selection. This should be the wire type used for the majority of the wiring in a project.
The wire size of hot conductors is initially determined by the circuit current rating, the wire type, temperature ratings (60 degrees C, 75 degrees C, and 90 degrees C), correction factor, and then adjusted to maintain a voltage drop within 2% for feeders and 3% for branch circuits. Based on these factors, Revit specifies hot wire sizes according to the .
The lists ground conductors sizes according to the current rating for the circuit. Where conductors are run in parallel in multiple raceways or cables as permitted by applicable electrical codes, each parallel equipment grounding conductor shall be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the raceway or cable.
When sized according to the hot conductors, a neutral multiplier is used to calculate the actual size of the neutral conductor. The calculation uses the of the conductors rather than the ampacity. It is intended to handle the current increase that results from harmonic loads. Harmonic loads are caused by switching the power supplies found in many types of electronic equipment. These switching power supplies create harmonic distortion in the current waveform and cause current to flow at a higher value than would be expected in an electrical system. Wire type must also permit the user to set a maximum size at which point the wire size no longer increases but conductors are paralleled to serve large loads.