Polarity Test Calculator

A polarity test calculator evaluates a signed DC or control-voltage measurement and produces four usable values:

• Voltage magnitude: the absolute voltage present, without regard to sign.
• Magnitude margin: the amount by which measured magnitude exceeds or falls below the required minimum.
• Actual polarity: positive or negative, based on the sign of the measurement.
• Polarity and magnitude comparison: whether the measured sign and magnitude satisfy the entered requirements.

This calculation is used when commissioning or troubleshooting low-voltage DC control circuits, control power supplies, signaling circuits, battery-backed systems, relay coils, and equipment terminals that require a known positive or negative reference. It helps distinguish a correct 24 VDC reading from a reversed -24 VDC reading, even though both readings have the same voltage magnitude.

This worksheet is limited to signed DC/control-voltage polarity math, not for receptacle or branch-circuit wiring approval.

Signed Control-Voltage Measurement

The Signed measured voltage input is the meter reading expressed with its sign.

A reading of 24 V means the test lead arrangement indicates a positive 24-volt potential at the point being tested relative to the selected reference. A reading of -24 V indicates the same 24-volt magnitude but opposite polarity relative to that reference.

The sign is not a separate voltage value. It identifies the direction of potential relative to the reference point and the meter-lead orientation used for the measurement.

Signed measured voltage	Voltage magnitude	Actual polarity
24 V	24 V	positive
-24 V	24 V	negative
12 V	12 V	positive
-9 V	9 V	negative
0 V	0 V	neither positive nor negative

The Expected polarity field defines the required sign for the measurement. Select Positive when the point under test should be positive relative to the chosen reference. A negative measurement against that expectation indicates reversed polarity, reversed test leads, an incorrect reference point, or another condition requiring investigation.

The Minimum expected magnitude field sets the minimum acceptable absolute voltage. It does not change whether the measured polarity is positive or negative.

Calculation Method

The calculator uses the sign and absolute value of the entered DC/control-voltage measurement.

\(\displaystyle \text{Voltage Magnitude} = |V_{\text{measured}}|\)

\(\displaystyle \text{Magnitude Margin} = |V_{\text{measured}}| - V_{\text{minimum}}\)

Where:

• V_{text{measured}} is the Signed measured voltage
• |V_{text{measured}}| is the absolute value of that measurement
• V_{text{minimum}} is the Minimum expected magnitude

Polarity status is determined by comparing the sign of the signed measurement with the selected Expected polarity.

• A positive signed value matches an expected positive polarity.
• A negative signed value matches an expected negative polarity.
• A sign opposite the expected selection produces a polarity mismatch.
• A zero-volt result has no positive or negative polarity to match.

Magnitude comparison is determined separately:

\(\displaystyle |V_{\text{measured}}| \geq V_{\text{minimum}}\)

If the measured magnitude is equal to or greater than the minimum expected magnitude, the result is At or above entered minimum. If it is lower, the reading does not meet the entered magnitude threshold.

Calculation Example

A nominal 24 VDC control circuit is measured at a terminal relative to its designated DC common reference.

Input	Entered value
Signed measured voltage	24 V
Expected polarity	Positive
Minimum expected magnitude	12 V

The calculator evaluates the magnitude:

\(\displaystyle |24\text{ V}| = 24\text{ V}\)

It then calculates the margin above the required minimum:

\(\displaystyle 24\text{ V} - 12\text{ V} = 12\text{ V}\)

The resulting values are:

Result	Value
Voltage magnitude	24 V
Magnitude margin	12 V
Actual polarity	positive
Polarity comparison	Matches entered expected polarity
Magnitude comparison	At or above entered minimum

The circuit has positive polarity relative to the selected reference point, and its 24 V magnitude exceeds the entered 12 V minimum by 12 V.

If the same circuit produced a reading of -24 V with Expected polarity set to Positive, the voltage magnitude would still be 24 V and the magnitude margin would still be 12 V. The polarity status would change to a mismatch because the measured sign is negative.

Field Verification Boundary

This polarity worksheet compares signed voltage and voltage magnitude only. It does not establish that conductors are landed on the correct terminals, that a control circuit is safe to probe, that a DC source is correctly bonded or grounded, or that an installation complies with applicable code, equipment instructions, or AHJ requirements.

A polarity result depends on the reference point and test-lead placement. Confirm the meter is set for DC voltage, identify the intended circuit reference, verify lead condition and meter operation, and follow the equipment manufacturer’s test procedure. Where the polarity affects controls, electronic inputs, relay logic, battery systems, or communication equipment, verify the terminal designations and circuit documentation rather than relying on voltage sign alone.