Can an MOV AC MCOV Be Calculated Directly From Its DC Rating?

No. Dividing a metal oxide varistor’s maximum DC operating voltage by √2 does not create an allowable AC rating. The calculation converts the peak of an ideal sine wave to its root-mean-square (RMS) value; it does not reproduce the manufacturer’s qualification of the part under continuous AC stress. Maximum AC voltage, VAC, and maximum DC voltage, VDC, are separate maximum continuous operating voltage (MCOV) limits and should be read from the exact ordering code’s product table.

The shortcut looks plausible because AC peak voltage and DC voltage use the same unit and often have similar magnitudes. An MOV, however, is not evaluated only at one instantaneous voltage. Leakage over the waveform, time spent near the nonlinear region, dielectric loss, self-heating, ambient temperature, and long-term stability all contribute to a continuous-voltage limit. √2 is useful inside a waveform calculation, but not as a substitute for a rating.

Four voltage quantities must remain separate

VAC is normally the continuous sinusoidal RMS limit and VDC is the continuous DC limit. Varistor voltage, such as V1mA, is a reference point measured at a stated DC test current; it describes the voltage-current characteristic and production band. Clamping voltage is measured at a specified surge current and waveform. None of those fields can be swapped simply because each is expressed in volts.

Temporary overvoltage (TOV) is another distinct condition. A neutral fault, ground fault, load rejection, or control failure may apply an elevated voltage for milliseconds, seconds, or longer—far beyond a microsecond surge but not as an intended indefinite operating state. Survival must be checked against the exact series’ TOV data and protection architecture. A high single-pulse current rating does not establish TOV capability.

What the square-root-of-two calculation actually establishes

For an ideal sine wave, V_peak=√2×V_RMS. A nominal 230 V RMS source therefore has an ideal peak near 325 V. At a 253 V RMS high-line value, the ideal peak is about 358 V. Those figures help define the electrical node, but they omit harmonics, clipping, common-mode offset, switching spikes, TOV, measurement uncertainty, and the candidate MOV’s thermal conditions.

TDK’s SIOV selection guide makes an important distinction. Product tables state the permissible AC and DC operating voltages. For nonsinusoidal AC, the guide directs the designer to compare the applied peak or amplitude with the permissible DC operating voltage. That instruction still depends on the published VDC field; it does not authorize deriving VDC from VAC or vice versa. RMS heating and abnormal-voltage exposure remain separate checks.

A bounded product-table example shows why the ratio is not a rule

The following two rows come from the Bourns MOV-20DxxxK datasheet and are used only as a calculation example. The table lists maximum continuous AC and DC voltage at its stated 25 °C conditions. These values belong to the named series and ordering codes; they are not generic values for every 20 mm MOV.

A calculation can therefore look conservative in one row and nearly exact in another. Other families may use different voltage steps, materials, temperature ranges, or agency conditions. A defensible design record stores the manufacturer values, datasheet revision, and complete ordering code rather than a ratio copied from a neighboring part.

AC and DC create different continuous operating cases

Under DC, the MOV remains at one polarity and operating point. Steady leakage and temperature establish continuous dissipation, while ripple, charger overshoot, regenerative energy, light-load rise, and control saturation may raise the real bus above its nominal setpoint. The relevant value is the highest sustained voltage at the MOV terminals, not merely the controller’s nominal output.

Under AC, the varistor traverses positive and negative voltage every cycle. Even if an ideal peak is below a particular DC number, repeated leakage near the nonlinear region and thermal cycling can differ from a DC exposure. Frequency, harmonic content, and clipped waveforms change how long each cycle spends near high field. The fact that an MOV is bidirectional does not make its AC and DC continuous ratings interchangeable.

Phase-controlled outputs, inverter pulse-width modulation, rectifier inputs, and waveforms with DC offset are not defined adequately by one RMS number. Reporting only peak voltage can hide thermal loading, while reporting only RMS can hide a damaging amplitude. The engineering record should retain waveform, peak, RMS, frequency, DC component, measurement bandwidth, and exposure duration.

A rectified bus is more than VAC multiplied by √2

A bridge rectifier and bulk capacitor may charge near the AC peak at light load, but the highest sustained DC bus also depends on high-line tolerance, diode drop, capacitance, discharge load, precharge behavior, power-factor correction, and regenerative energy. An MOV installed after the bridge sees that DC-node envelope and must be checked against VDC. A part installed before the bridge sees the AC node and must satisfy VAC plus the relevant abnormal-voltage conditions.

For a 230 V system evaluated at +10% high line, 358 V is only the beginning of the bus-voltage analysis, not an MOV recommendation. The designer must add credible control overshoot and uncertainty, then confirm that the selected MCOV does not push varistor voltage and clamping voltage beyond the downstream withstand budget. The device must avoid excessive conduction in normal service while still providing useful limitation during a surge.

Build the decision from operating conditions, not a ratio

A reviewable selection and validation workflow

1. Lock the exact series and ordering code, then copy its published VAC, VDC, temperature conditions, and derating notes from the current datasheet.
2. Calculate or measure the MOV terminal envelope in normal service: sustained RMS, peak, DC offset, ripple, frequency, and waveform, including source and instrument tolerances.
3. Use the appropriate manufacturer field. Check sinusoidal AC against VAC and DC against VDC. For nonsinusoidal AC, follow the manufacturer’s amplitude guidance while also evaluating RMS thermal loading.
4. At the highest relevant ambient and case temperature, evaluate leakage, steady dissipation, temperature rise, and any material-specific derating. A brief room-temperature reading is not a life test.
5. Create separate TOV and microsecond-surge cases with voltage, current, waveform, source impedance, pulse count, interval, and initial temperature defined.
6. Combine worst-case MOV clamping, interconnect voltage, and the downstream circuit’s minimum withstand in one protection-voltage budget.

Frequently asked questions

If VAC × √2 is below VDC, is the MOV acceptable?

That comparison is insufficient. It only compares an ideal sine peak with one DC number. It does not establish compliance with the exact part’s VAC rating, leakage, temperature rise, life conditions, or abnormal-voltage requirements.

Does a higher VDC than VAC mean DC is easier for the MOV?

No. The two values are maximum continuous limits under different applied conditions, not a ranking of stress severity. Their ratio cannot be transferred to another family.

Is more MCOV margin always safer?

It provides more separation from normal continuous voltage, but it commonly raises the varistor and clamping levels. The downstream circuit may then see a higher residual voltage. Both non-conduction in normal service and effective surge limitation must pass.

What if the datasheet provides no TOV curve?

Do not substitute the surge-current or joule rating. Ask the manufacturer for applicable evidence, select a family with defined TOV behavior, or bound the risk through controlled system tests and thermal protection. Without evidence, long-duration withstand should not be claimed.

Practical conclusion

AC and DC MCOV are separate manufacturer limits. √2 belongs in the waveform analysis, not in a rule that invents a component rating. Determine the real envelope at the installed node, use the exact product-table field, and then treat leakage, heating, TOV, surge, clamping, and thermal protection as independent gates. That approach produces a selection record another engineer can reproduce.

Continue with MOV MCOV Selection, MOV Key Parameters, MOV Leakage-Current Test Conditions, and How to Select an MOV.

References

• TDK Electronics, SIOV General Technical Information
• TDK Electronics, SIOV Selection Procedure
• Bourns, MOV-20DxxxK Series Datasheet