Insulation Resistance Calculator

This calculator converts a field insulation resistance (IR) reading into a temperature-corrected value and compares it against a user-entered screening reference, producing a documentation-ready worksheet figure. It does not issue a pass/fail acceptance result, does not confirm safety clearance, and does not determine code compliance — those determinations require a qualified person applying the governing test standard, the equipment manufacturer’s data, and the site-specific acceptance criteria set by the engineer of record or the Authority Having Jurisdiction (AHJ).

Insulation resistance testing itself is not defined by the National Electrical Code (NEC). The NEC governs installation requirements (conductor sizing, ampacity, overcurrent protection, grounding, etc.), not maintenance or acceptance testing procedures. IR test methodology and minimum-value guidance instead come from:

• IEEE 43 (Recommended Practice for Testing Insulation Resistance of Rotating Machinery) — temperature correction to 40°C, polarization index (PI), and minimum IR guidance for motors and generators.
• ANSI/NETA ATS / MTS — acceptance and maintenance test values for transformers, cables, switchgear, and other apparatus, including minimum DC test voltage by equipment voltage class.
• NFPA 70B (Standard for Electrical Equipment Maintenance) — maintenance testing intervals and documentation practices.

Where any NEC article number or requirement is referenced elsewhere in your workflow (e.g., grounding, disconnecting means, or overcurrent protection tied to the circuit under test), confirm whether your jurisdiction has adopted NEC 2023 or NEC 2026 — NFPA published NEC 2026 on August 20, 2025 with a September 9, 2025 effective date, but as of mid-2026 most states continue to enforce NEC 2023, with only a small number of jurisdictions (e.g., Massachusetts, Texas) having adopted the 2026 edition. Verify the edition your AHJ enforces before citing a specific article number.

Key Terms

• Insulation resistance (IR): The resistance measured between a conductor and ground (or between conductors) with a DC megohmmeter, expressed in megohms (MΩ).
• Temperature correction factor \(K_T\): A multiplier applied to a measured IR value to normalize it to a standard reference temperature, conventionally 40°C per IEEE 43. IR decreases roughly 50% for every 10°C rise above 40°C and doubles for every 10°C drop below it: K_T = 0.5^((40−T)/10).
• Screening reference: A user-entered comparison threshold (in MΩ) used for internal quality screening on this worksheet. It is not a code-mandated minimum; common historical references include the “one-megohm rule” (1 MΩ minimum, or 1 MΩ per kV of rated voltage) from pre-1970 IEEE 43 guidance, but current acceptance minimums should be taken from the applicable NETA table or manufacturer specification.
• Test duration: The elapsed time from voltage application to the recorded reading. A 60-second reading is the standard “spot” or “short-time” reading; a 10-minute reading paired with the 1-minute reading produces the Polarization Index (PI = R₁₀/R₁).

Formulas

1. Corrected resistance = Measured resistance × Temperature correction factor
2. Screening reference used = entered screening reference value (no calculation, passed through as recorded)
3. Screening margin = Corrected resistance − Screening reference
4. Screening ratio = Corrected resistance ÷ Screening reference
5. Screening status = comparison of corrected resistance against the screening reference, recorded for worksheet context only — it is not a code pass/fail determination.

Worked Example

Using the calculator’s default input set:

Step-by-step:

• Corrected resistance = 80 MΩ × 1.25 = 100 MΩ
• Screening margin = 100 MΩ − 1 MΩ = 99 MΩ
• Screening ratio = 100 MΩ ÷ 1 MΩ = 100
• At or above entered screening reference: Yes (1) — 100 MΩ exceeds the 1 MΩ reference by a factor of 100

This output documents that the corrected reading clears the entered screening reference by a wide margin under the stated test voltage and duration. It does not, by itself, indicate the winding or cable insulation is acceptable under IEEE 43 or NETA criteria — those require comparing the corrected value against the specific minimum for the equipment’s voltage class and construction, and, where applicable, calculating PI or DAR from ratio readings.