How to Use the Insulation Resistance Test Calculator (September 2026 basis)
Megohmmeter readings only mean something against a reference value, and in the United States that reference is not a legal table. The NEC does not publish a pass or fail insulation resistance figure for installed equipment. Instead IEEE 43 gives recommended minimums for rotating machine windings, NETA acceptance and maintenance specifications supply commissioning values, and NFPA 70B sets how often testing should happen.
This calculator applies IEEE 43 Table 3. Form-wound AC stators and DC armatures built after about 1970 should read at least 100 megohms, random-wound machines and form-wound windings rated below 1 kV at least 5 megohms, and older windings, all field windings and anything else at least 1 megohm per kilovolt of rating plus 1 megohm. That last case is the familiar kV + 1 rule, so a 480 V machine needs 1.48 megohms. The recommended DC test voltage follows IEEE 43 Table 1 for the rated voltage you enter.
Because insulation resistance roughly halves for every 10 degrees C rise, IEEE 43 defines its minimums at 40 degrees C. The verdict here therefore uses the corrected one-minute value, which is why a cold winding can read high yet still be judged against the same bar. Polarization index, the 10-minute reading divided by the 1-minute reading, separates a dirty or damp winding from a sound one, and the 30-second ratio known as DAR serves the same purpose in a shorter test. Keep trend records rather than single readings, and have a qualified engineer confirm any decision to repair or re-energize.
Frequently Asked Questions
Not in the United States. The NEC does not publish a pass or fail insulation resistance table for installed equipment. IEEE 43 gives recommended minimums for rotating machine windings, NETA acceptance and maintenance specifications give test values for commissioning, and NFPA 70B sets the inspection intervals.
Insulation resistance roughly halves for every 10 degrees C rise, so readings taken in a cold plant look far better than the same winding in summer. IEEE 43 defines its minimum values at 40 degrees C, which is why the verdict here uses the corrected one-minute value.
IEEE 43 treats 2.0 or higher as good for thermal class B and above, 1.0 to 2.0 as questionable and usually a sign of moisture or contamination, and under 1.0 as a reason to stop and investigate. Above roughly 5,000 megohms the ratio loses meaning.