Phase Sequence Check Calculator

A three-phase phase sequence check compares the angular relationship of Phase A, Phase B, and Phase C with the expected system rotation. The calculator produces four screening values:

• Relative B angle
• Relative C angle
• Observed sequence
• Phase spacing error

These results are useful when reviewing assumed phasor angles, commissioning documentation, power-system studies, control logic, motor-rotation planning, or a design worksheet that must remain consistent with the intended ABC or ACB phase order.

The result does not determine conductor ampacity, AWG or kcmil size, raceway fill, voltage drop, or overcurrent protection. It addresses the phasor relationship behind a three-phase system: whether the entered angles represent the expected rotational order and approximately maintain 120-degree spacing.

Phase-Angle Relationship

In a balanced three-phase system, the phases are nominally separated by 120 electrical degrees. With Phase A selected as the reference at 0 degrees, the conventional positive-sequence arrangement is:

Phase	Nominal relative angle	Common sequence designation
Phase A	0 degrees	A
Phase B	120 degrees	B
Phase C	240 degrees	C

That relationship produces an ABC sequence under the calculator’s angle convention. Reversing the relative order produces an ACB sequence.

Phase sequence affects equipment whose operation depends on rotational order. Three-phase motors can run in the opposite direction when any two phase conductors are interchanged. Phase-sensitive equipment, paralleling arrangements, transfer schemes, protective relays, and certain control systems may also require the documented phase order to match the installed system.

The calculator uses the entered Phase A angle as its reference, then evaluates the entered Phase B angle and Phase C angle relative to that reference.

Calculator Inputs

Input field	Entry	Electrical use
Phase A angle	Degrees	Establishes the reference phase angle
Phase B angle	Degrees	Provides the measured or assumed B-phase angle
Phase C angle	Degrees	Provides the measured or assumed C-phase angle
Expected sequence	ABC or ACB	Sets the intended project sequence for comparison
Maximum spacing error	Degrees	Sets the allowed deviation from nominal 120/240-degree spacing

The input angles may come from an engineering phasor diagram, a simulation, a protective-relay record, a power-quality data set, or another documented source. The calculator does not obtain angles from energized conductors.

Maximum spacing error is a comparison tolerance selected for the worksheet. It is not an NEC acceptance limit, equipment setting, utility requirement, or substitute for manufacturer instructions.

Relative Angles and Sequence

The worksheet normalizes the Phase B and Phase C entries relative to Phase A. Conceptually, it removes the Phase A reference angle and expresses the other phases within a 0-to-360-degree cycle:

text{Relative B angle} = (text{Phase B angle} - text{Phase A angle}) bmod 360^circ

text{Relative C angle} = (text{Phase C angle} - text{Phase A angle}) bmod 360^circ

The calculator then infers the observed order from the lower relative angle:

• A lower Relative B angle than Relative C angle indicates abc
• A lower Relative C angle than Relative B angle indicates acb

The displayed observed sequence is lowercase—abc or acb—while Expected sequence is the project selection used for the pass/fail comparison.

For an ABC expectation, the nominal targets are:

\(\displaystyle B = 120^\circ\)

\(\displaystyle C = 240^\circ\)

For an ACB expectation, the relative position of B and C is reversed:

\(\displaystyle C = 120^\circ\)

\(\displaystyle B = 240^\circ\)

Phase Spacing Error

Phase spacing error measures how far the normalized phase angles depart from the applicable nominal 120-degree and 240-degree targets. A balanced ideal phasor set has zero spacing error.

For an expected ABC sequence, the calculator compares:

\(\displaystyle \text{Relative B angle} \text{ to } 120^\circ = \text{Relative C angle} \text{ to } 240^\circ\)

The spacing error reflects the angular deviation used by the worksheet’s comparison logic. The Sequence comparison reports whether the observed order matches the selected Expected sequence and whether the spacing remains within the entered Maximum spacing error.

A result can show the correct phase order while still having an excessive spacing error. For example, relative angles of 105 degrees and 255 degrees preserve the B-before-C order associated with ABC, but each angle is 15 degrees away from its nominal target. Whether that passes depends on the entered tolerance.

Calculation Example

Enter the following values:

Field	Value
Phase A angle	0 degrees
Phase B angle	120 degrees
Phase C angle	240 degrees
Expected sequence	ABC
Maximum spacing error	15 degrees

The normalized results are:

text{Relative B angle} = (120^circ - 0^circ) bmod 360^circ = 120^circ

text{Relative C angle} = (240^circ - 0^circ) bmod 360^circ = 240^circ

Because the lower relative angle is B at 120 degrees, the calculator reports:

Result	Value
Relative B angle	120 degrees
Relative C angle	240 degrees
Observed sequence	abc
Phase spacing error	0 degrees
Sequence comparison	Matches entered expectation

The result is consistent with an ideal ABC phasor arrangement and is within the entered 15-degree tolerance.

Electrical Use and Limits

Phase-angle consistency review may be completed before activities such as motor-control documentation, equipment startup planning, phasor-based troubleshooting, relay-study review, or confirmation of drawings that identify A-B-C conductor order. It can also help identify an inconsistency between a design assumption and a set of entered phasor angles before that assumption affects motor rotation or phase-sensitive control logic.

A correct worksheet result does not establish that a field installation has correct phase rotation. It does not identify energized conductors, verify a panelboard, switchboard, feeder, branch circuit, transfer switch, motor starter, or equipment terminal marking. It also does not replace a properly rated instrumented phase-sequence or phase-rotation check performed under applicable electrical safety procedures.

Field verification must account for the actual conductor terminations, equipment labeling, system voltage, lockout/tagout requirements, arc-flash hazards, manufacturer instructions, project specifications, and AHJ requirements. If the verified field rotation conflicts with drawings or equipment requirements, resolve the discrepancy before placing rotation-sensitive equipment into service.