Voltage Drop Calculation Basis

$$Three-Phase\>\%\>Voltage\>Drop = {{Load\>in\>Amps \times Effective\>Z \times {Wire\>Length \over 1,000}} \over Line-to-Neutral\>Voltage} \times 100$$ $$Single-Phase\>\%\>Voltage\>Drop = {{Load\>in\>Amps \times Effective\>Z \times {Wire\>Length \over 1,000} \times 2} \over Voltage} \times 100$$ $$Effective\>Z = {{\Big(Resistance \times Power\>Factor\Big) + \Big(Reactance \times \sin\big(\arccos(Power\>Factor)\big)\Big)} \over Parallel\>Runs}$$

(Visit NEC Table 9 Note 2)

The default feeder and branch circuit $Resistance$ and $Reactance$ values are based upon NEC Table 9 and can be modified using the Wire Ampacities command.

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

$Power\>Factor$ is based upon the Voltage drop power factor ▾ option in the Project Options command.

When applicable, the $Effective\>Z$ is divided by the number of $Parallel\>Runs$ for the feeder or branch circuit.

$Line-to-Neutral\>Voltage$ 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.

Feeder Values

$Load\>in\>Amps$ is the Total Estimated Demand Current on the distribution equipment, often referred to as the calculated load. Revit determines this load based upon the loads connected to the distribution equipment and the demand factors.

$Wire\>Length$ is the Length ▾ value for the feeder to the panel in the Panel Edit command, converted to feet if needed.

Branch Circuit Values

$Load\>in\>Amps$ is the Apparent Current on the circuit, often referred to as the connected load. Revit determines this load based upon the devices connected to the circuit.

$Wire\>Length$ is the Actual Length value for the circuit in the Circuit Edit command, converted to feet if needed. The voltage drop calculation assumes the entire circuit load is located this distance from the panel.

Transformer Voltage Drop

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.