Circuit Identification Calculator

The Circuit Identification Screen Calculator compares three entered line-to-line voltage readings—A-B, B-C, and C-A—and produces an average line voltage plus the largest voltage deviation in volts and percent. The result provides a quick consistency screen for a nominal three-phase line-to-line system, such as a 480 V feeder, motor branch circuit, panelboard supply, switchboard section, or distribution circuit.

Its output is useful during voltage-drop review, load troubleshooting, equipment startup review, and preliminary circuit documentation. A balanced set of line-to-line readings may support further investigation of a three-phase supply condition; an unbalanced set can indicate that additional testing is needed before relying on a circuit for motor loads, sensitive equipment, or load-transfer work.

The worksheet does not identify energized conductors, confirm phase labels, establish conductor function, or certify a circuit.

Related field checks: Voltage Unbalance Calculator, Continuity Test Calculator, and Phase Sequence Check Calculator.

Line-to-Line Voltage Review

A three-phase line-to-line system is evaluated by measuring voltage between each pair of phase conductors:

Calculator input	Electrical reading
Nominal line voltage (V)	Expected line-to-line system voltage used as a reference
Measured A-B voltage (V)	Voltage measured between phase A and phase B
Measured B-C voltage (V)	Voltage measured between phase B and phase C
Measured C-A voltage (V)	Voltage measured between phase C and phase A
Voltage tolerance (%)	Maximum acceptable percentage deviation entered for the comparison

For a nominal 480 V system, the expected readings are generally near 480 V line to line. Actual field voltage can vary with utility supply conditions, transformer regulation, feeder loading, conductor impedance, voltage drop, load balance, and measurement conditions.

The calculator treats the three readings as comparison values. It does not determine whether the conductors are correctly labeled A, B, and C, whether phase rotation is correct, or whether a conductor is safely de-energized.

Average Voltage and Deviation

The calculator first determines the arithmetic average of the three measured line-to-line voltages:

\(\displaystyle \text{Average line voltage} = \frac{V_{AB}+V_{BC}+V_{CA}}{3}\)

It then determines the largest absolute deviation of an individual measurement from the nominal line voltage:

\(\displaystyle \text{Maximum deviation (V)} = \max\left( |V_{AB}-V_{\text{nom}}|, |V_{BC}-V_{\text{nom}}|, |V_{CA}-V_{\text{nom}}| \right)\)

The percentage result is:

\(\displaystyle \text{Maximum deviation (\%)} = \frac{\text{Maximum deviation (V)}}{V_{\text{nom}}} \times 100\)

The calculator compares that maximum deviation percentage with the entered Voltage tolerance (%). If the largest deviation does not exceed the entered value, the Voltage comparison displays Within entered tolerance.

The nominal line voltage is the comparison reference for the deviation percentage. The average is reported for context, while the maximum deviation is measured from nominal line voltage so the result follows the entered tolerance basis.

Calculation Example

For a 480 V three-phase line-to-line screen, enter:

Input	Value
Nominal line voltage (V)	480 V
Measured A-B voltage (V)	480 V
Measured B-C voltage (V)	480 V
Measured C-A voltage (V)	480 V
Voltage tolerance (%)	2%

The average line voltage is:

\(\displaystyle \frac{480+480+480}{3}=480\text{ V}\)

Each entered reading equals the average, so:

\(\displaystyle \text{Maximum deviation}=0\text{ V}\)

\(\displaystyle \text{Maximum deviation}=0\%\)

The resulting outputs are:

Result	Value
Average line voltage	480 V
Maximum deviation	0 V
Maximum deviation	0%
Voltage comparison	Within entered tolerance
Voltage screen model	Three phase line to line screen

A 0% deviation means the three entered line-to-line readings are identical. It does not establish that the source is properly phased, that the system is suitable for a specific motor, or that the measured conductors correspond to the circuit labels in the field.

Application to Motor and Feeder Review

Three-phase voltage consistency is commonly reviewed before diagnosing motor performance, evaluating apparent voltage drop, or investigating uneven loading on a feeder. A noticeable difference between A-B, B-C, and C-A readings can justify further testing of the supply, transformer secondary, overcurrent device, terminations, raceway conductors, disconnect, contactor, or connected loads.

For a motor circuit, voltage readings alone do not establish motor operating conditions. Motor current, load condition, connection configuration, phase rotation, winding condition, terminal connections, and manufacturer voltage limits require separate verification. Likewise, a feeder voltage review does not replace ampacity calculations, conductor AWG or kcmil selection, terminal rating evaluation, insulation temperature rating review, or voltage-drop design.

Where voltage drop is being investigated, compare readings at appropriate points in the circuit—such as supply terminals and load terminals—under a known load condition. A single set of voltage readings without load context may not reveal the actual voltage-drop behavior of a branch circuit or feeder.

Field Verification Limits

Use a properly rated test instrument and follow the employer’s electrical safety procedures before measuring any energized equipment. Select measurement points deliberately, confirm the meter category and voltage rating, and account for accessible energized parts, available fault current, arc-flash boundaries, and required PPE.

The calculator’s Three phase line to line screen result is limited to the three entered voltage values and the tolerance selected by the user. It does not:

• Identify which conductor is energized.
• Verify conductor phase labels or phase sequence.
• Prove a circuit is de-energized or safe to work on.
• Determine whether a branch circuit or feeder meets ampacity, overcurrent protection, raceway fill, or voltage-drop requirements.
• Replace equipment commissioning procedures, manufacturer instructions, or AHJ requirements.
• Diagnose a voltage imbalance cause from readings alone.

Electrical installation, testing, and acceptance decisions require field verification beyond this arithmetic screen.