Voltage Drop Calculation Basis

Three-Phase % Voltage Drop = Calculated Load in Amps * Effective Z * (Wire Length / 1,000) * 100 / Line-to-Neutral Voltage

Single-Phase % Voltage Drop = Calculated Load in Amps * Effective Z * (Wire Length / 1,000) * 100 * 2 / Voltage

Effective Z = R cos(theta) + X sin (theta) (See NEC Table 9 Note 2)

R = Resistance set in the Wire Ampacities command.

X = Reactance set in the Wire Ampacities command.

theta = Power factor of the electrical system. This value is based upon the Voltage drop power factor option in the Project Options command.

Line-to-Neutral Voltage is used for three-phase calculations based upon NEC Table 9 Note 2: "Multiplying current by effective impedance gives a good approximation for line-to-neutral voltage drop."

Voltage is the line-to-neutral voltage for single-pole circuits and line-to-line voltage for two-pole circuits.

The feeder impedance values are based upon NEC Table 9. These values can be modified by the user using the Wire Ampacities command.

Feeder Values

Calculated Load in Amps is the Total Estimated Demand Current on the distribution equipment. This load is calculated by Revit based upon the loads connected to the distribution equipment and the demand factors.

Wire Length is the Feeder Length value for the panel. Set the feeder length using the Panel Edit command.

Branch Circuit Values

Calculated Load in Amps is the the Apparent Current on the circuit. This load is calculated by Revit based upon the devices connected to the circuit.

Wire Length is the Circuit Length value for the circuit. Set the circuit length using the Circuit Edit command. The voltage drop calculation assumes the entire circuit load is located this distance from the panel.

Transformer Voltage Drop Basis

The transformer impedance values are for dry-type indoor transformers. These values can be modified by the user using the Panel Edit command or Family Edit command.

The voltage drop through transformers can be included, ignored, or reset to 0%. Use the Transformer voltage drop calculation method option in the Project Options command to control how the voltage drop is calculated.


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