How to Megger Test a Motor or Feeder

1. Pick the right test voltage

Test voltage follows the rated voltage of what you're testing. Standard practice per NETA ATS Table 100.1: 250V and below gets 500V DC; 251V through 600V — 480V motors and feeders, the bulk of commercial work — gets 1,000V DC. Medium-voltage gear steps up to 2,500V or 5,000V DC, but that's specialty testing.

For motor windings, IEEE 43 accepts 500V DC on machines rated under 1,000V, so either 500V or 1,000V is defensible on a 480V motor — just use the same voltage every time on the same machine so your trend data means something. Never test above the insulation rating: a 120V control circuit doesn't get 1,000 volts.

2. De-energize, lock out, verify dead

Open the disconnecting means, apply your lock and tag, and verify absence of voltage phase-to-phase and phase-to-ground on every conductor you're about to touch, using a meter proven on a known live source before and after. That's the electrically safe work condition of NFPA 70E Article 120 — non-negotiable when you're about to handle bare conductor ends.

On a feeder, control both ends. If the far end lands in a panel someone else can access, lock it out or station somebody there — test voltage appears at every point on the conductor.

3. Isolate the circuit — get the electronics off

A megger will destroy solid-state equipment. Before testing, disconnect anything electronic on the conductors under test: VFDs, soft starters, surge protective devices, power monitors, control transformers, pilot lights, capacitors. On a drive-fed motor, lift the motor leads at the drive's output terminals — never megger through a VFD in either direction.

For a feeder, isolate at both ends and lift load-side connections so you're testing conductors only — anything left connected either drags the reading down or gets damaged. For a motor, disconnect at the peckerhead or the starter's load terminals, and note whether the motor leads are in the reading so the next test matches.

4. Test a feeder: phase-to-phase and phase-to-ground

With conductors isolated at both ends, test every combination. Three-phase feeder with neutral: A-B, B-C, C-A, then A-G, B-G, C-G, and N-G with the neutral lifted from the bar (a bonded neutral reads zero to ground by design). Apply the voltage for one minute and record at the one-minute mark — resistance climbs as the DC charge stabilizes, and consistent timing is what makes readings comparable.

Healthy new conductors in dry conduit read hundreds of megohms to gigohms. Single-digit megohms on a new pull means damage — insulation skinned at a coupling, a nick at a bushing, or water in the raceway. One low phase points at physical damage; all three uniformly low points at moisture or contamination.

5. Test a motor: windings to ground

On a standard three-phase motor the windings are internally connected (wye or delta), so phase-to-phase readings just measure copper continuity — the meaningful test is winding-to-ground. Connect one lead to a motor lead and the other to the frame at a clean, paint-free spot, and test for one minute. Because the windings are tied together, one reading tests all of them; on a 9- or 12-lead machine with the jumpers pulled, test each winding group to ground and to the other groups.

While you're in the peckerhead, look. Darkened insulation, a varnish smell, brittle leads, or moisture all corroborate a low reading. A marginal motor that sat in a wet room may be moisture-soaked and recoverable with a bake-out; burned insulation is done.

6. Interpret the numbers — and correct for temperature

The old rule of thumb is 1 megohm plus 1 megohm per kV of rated voltage — roughly a 1.5 megohm floor for a 480V system. Treat that as a do-not-energize line, not a passing grade. IEEE 43 sets 5 megohms (corrected to 40°C) as the minimum for typical random-wound motors under 1,000V, and NETA ATS Table 100.1 calls for 100 megohms on 600V-class cable and equipment. A new installation barely above the rule-of-thumb floor has a problem even though it technically passed.

Temperature matters: insulation resistance roughly halves for every 10°C rise, so a motor pulled off line hot reads a fraction of what it reads cold the next morning. Record the temperature with every reading and correct to the 40°C base per IEEE 43 before comparing against minimums or last year's baseline — otherwise your trend line is measuring the weather.

7. Discharge, record, restore

The DC test charges the circuit like a capacitor, and long feeders and big windings store real energy. After each reading, discharge through the megger's discharge function or a grounding jumper, leave the ground applied at least four times as long as the test voltage was, and verify zero volts before touching conductors.

Record everything on the spot: equipment ID, date, test voltage, each reading, temperature, and what was connected. A single reading tells you pass or fail today; a series on the same motor tells you it's lost half its insulation resistance in two years and belongs on the replacement budget. Then re-land conductors per your markings, torque terminations per the equipment labeling (NEC 110.14(D)), reconnect the electronics, and clear LOTO per procedure.

What is a good megger reading for a 480V motor?

IEEE 43 sets a minimum of 5 megohms corrected to 40°C for typical random-wound motors under 1,000 volts, and a healthy motor usually reads far higher. Anything near the old 1-megohm-per-kV rule of thumb is a do-not-energize condition, not a pass.

What test voltage should I use on a 480V system?

1,000V DC is standard for conductors and equipment rated 251 to 600 volts per NETA ATS Table 100.1, and IEEE 43 also accepts 500V DC for motor windings under 1,000 volts. Use the same test voltage every time on the same equipment so readings stay comparable.

Will a megger damage a VFD or other electronics?

Yes. The DC test voltage destroys drive output transistors, surge protective devices, and other solid-state components. Disconnect all electronics before testing, and never megger through a VFD in either direction.

Does the NEC require megger testing?

The NEC does not mandate a specific test, but NEC 110.7 requires completed wiring to be free from short circuits and ground faults, and the megger test is the accepted way to demonstrate that. Many project specifications require documented readings before energization.

How does temperature affect insulation resistance readings?

Insulation resistance roughly halves for every 10°C increase, so a hot winding reads dramatically lower than the same winding cold. Record the temperature with every reading and correct to the 40°C base per IEEE 43 before comparing readings.

Why do I have to discharge the circuit after a megger test?

The DC test voltage charges conductors and windings like a capacitor, and that stored energy can deliver a serious shock. Ground the circuit for at least four times the test duration and verify zero volts before touching conductors.