Okay, let's break down how to choose the correct voltage for a voltage drop calculation when dealing with 120/208V or 277/480V systems. The choice depends on the load you are supplying and the wiring configuration.

Understanding the System Voltages 120/208V System

This is a 3-phase, 4-wire wye system.

208V is the phase-to-phase voltage (voltage between any two of the three phases). 120V is the phase-to-neutral voltage (voltage between any phase and the neutral conductor). 277/480V System

This is also a 3-phase, 4-wire wye system.

480V is the phase-to-phase voltage. 277V is the phase-to-neutral voltage.

Key Rule: Match the Voltage to the Load's Voltage

The most important principle is to use the voltage that directly supplies the load.

Here's a breakdown of when to use each voltage



Using 120V or 277V (Phase-to-Neutral)

If your load is a

single-phase load connected between one phase and neutral, you use the phase-to-neutral voltage (120V or 277V). Examples

Lighting circuits with 277V ballasts (in a 277/480V system) Standard 120V receptacles and lights (in a 120/208V system). Single-phase motors rated for 120V or 277V (connected phase to neutral).

Using 208V or 480V (Phase-to-Phase)

If your load is a

single-phase load connected between two phases you use the phase-to-phase voltage (208V or 480V) Examples

Electric heaters or other loads specifically designed to run on 208V, connected between two phases of a 120/208V system. Electric heaters or other loads specifically designed to run on 480V, connected between two phases of a 277/480V system. If your load is a

three-phase load (like a 3-phase motor), you use the phase-to-phase voltage (208V or 480V). Three-phase motors are almost always connected to all three phases.

Summary Table| System Voltage | Load Type | Voltage to Use

:-------------- | :------------------------------------ | :-------------

120/208V | Single-phase (phase-to-neutral) | 120V

120/208V | Single-phase (phase-to-phase) | 208V

120/208V | Three-phase | 208V

277/480V | Single-phase (phase-to-neutral) | 277V

277/480V | Single-phase (phase-to-phase) | 480V || 277/480V | Three-phase | 480V |Example Scenarios1. 277/480V System, Lighting Circuit

You're calculating the voltage drop for a circuit supplying 277V lighting fixtures (each fixture is connected between one phase and neutral). You use

277V in your calculation.2. 120/208V System, Receptacle Circuit

You're calculating the voltage drop for a circuit supplying 120V receptacles (connected phase-to-neutral). You use

120V in your calculation.3. 277/480V System, 3-Phase Motor

You're calculating the voltage drop for a circuit supplying a 3-phase motor. You use

480V in your calculation.4. 120/208V System, 208V Heater

You're calculating the voltage drop for a circuit supplying a single-phase 208V heater, wired between two phases. You use

208V in your calculation.

Formula Considerations

The voltage drop formula you use might also slightly influence your choice (although the underlying principle remains the same). Some formulas are designed for phase-to-neutral voltage, while others are for phase-to-phase. Always check the documentation for the voltage drop formula you are using.

Important Considerations Accurate Load Information

Knowing the exact voltage requirements of the load is crucial. Check the nameplate of the equipment.

Wiring Configuration

Double-check how the load is wired to the system (phase-to-neutral, phase-to-phase, or three-phase).

Neutral Conductor: Voltage drop on the neutral conductor can* be significant in unbalanced systems (where the loads on each phase are not equal). However, voltage drop calculations typically focus on the phase conductors.By following these guidelines, you'll be able to confidently select the correct voltage for your voltage drop calculations in 120/208V and 277/480V systems. Remember to always consult the applicable electrical codes and standards for your location. Flag for review