Varistor Working Voltage for Mexico: The 127 Vac Line Plus 10 Percent Clears Walsin’s 140 Vac Step by 0.3 V, and Only 43 of 379 Parts Reach 550 Vac

Varistor working voltage for Mexico is a ladder problem. We took four commonly quoted nominal voltages, added 10 percent, and found the first Walsin step above each. 127 Vac clears the 140 Vac step by 0.3 V, 0.2 percent. 480 Vac needs the 550 Vac step, which only 43 of 379 parts reach.

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Venatech Limited, an authorised Walsin distributor in Hong Kong, holds the metal oxide varistor parametric export from Walsin Technology (Walsin): 379 rows, 35 maximum working voltages. This post is written for buyers and engineers in Mexico who size a varistor against a nominal line voltage. It uses that export only, since we hold no varistor datasheet. Every part number below is real.

How much headroom does each common Mexican nominal voltage leave on the Walsin working-voltage ladder?

From 0.2 percent to 5.0 percent, and the smallest is at 127 Vac. We added 10 percent to each nominal and took the smallest Max Working Volt. step in the export that is at or above it. The 140 Vac step clears 127 Vac plus 10 percent by 0.3 V, the thinnest margin of the four, and the step below it, 130 Vac, falls short by 9.7 V.

Nominal (Vac)	Plus 10 percent	First Walsin step at or above	Headroom	Parts at that step	Parts at or above it	Match / mismatch
127	139.7	140	0.3 V, 0.2%	17	300	Match, barely. The 130 Vac step is 9.7 V short
220	242.0	250	8.0 V, 3.3%	17	201	Match. The 230 Vac step is 12.0 V short
277	304.7	320	15.3 V, 5.0%	17	150	Match, but the 300 Vac step misses by 4.7 V
480	528.0	550	22.0 V, 4.2%	13	43	Match. The 510 Vac step is 18.0 V short

We counted every row in the file against each step. We have not seen this counted anywhere. The ladder has 35 working voltages, and the margin it offers over a nominal-plus-10-percent line ranges from 0.3 V to 22.0 V. A 127 Vac line running 0.3 V above our assumption already reaches the 140 Vac rating of the step.

Which of the 379 parts sit at each of those steps?

17 parts at three of the steps and 13 at the fourth. We listed every row at 140, 250, 320 and 550 Vac. The three lower steps each offer 17 parts from 5 mm to 53 mm; the 550 Vac step offers 13, because the four smallest, all 5 mm and 7 mm discs, are not made at that voltage.

Step (Vac)	Varistor voltage	Parts	Disc range	Peak current range	Smallest and largest part	Match / mismatch
140	220 Vdc	17	5 mm to 53 mm	400 A to 80,000 A	SR221K05D, SR221K34RD	Match for a 127 Vac line with 0.3 V headroom
250	390 Vdc	17	5 mm to 53 mm	400 A to 80,000 A	SR391K05D, SR391K34RD	Match for 220 Vac
320	510 Vdc	17	5 mm to 53 mm	400 A to 80,000 A	SR511K05D, SR511K34RD	Match for 277 Vac
550	910 Vdc	13	10 mm to 53 mm	2,500 A to 80,000 A	SR911K10D, SR911K34RD	Mismatch against the other three: no disc below 10 mm

The peak current figure in the export has no waveform attached, so the column compares parts inside the file and does not tell you what a surge standard requires. Walsin prints only Peak Current. There is no joule column and no clamping-voltage column in the file.

Are the parts at one step different from the parts at another?

Almost never. We compared the size, series and peak current of every part at every step. 19 of the 35 steps carry exactly one of two templates: 13 steps hold an identical set of 17 parts, and 6 steps hold an identical set of 13. Those 19 steps cover 299 of the 379 parts.

Part removed at 460 Vac and above	Size	Series	Peak current	Present at 140, 250 and 320 Vac	Present at 550 Vac	Match / mismatch
SR391K05D	5 mm	Standard (D)	400 A	yes	no	Mismatch
SR391K05E	5 mm	High Energy (E)	800 A	yes	no	Mismatch
SR391K07D	7 mm	Standard (D)	1,200 A	yes	no	Mismatch
SR391K07E	7 mm	High Energy (E)	1,750 A	yes	no	Mismatch
the other 13 parts	10 mm to 53 mm	all four series	2,500 A to 80,000 A	yes	yes	Match on size, series and peak current at every one of 13

The two templates mean that once you know the footprint and the peak current you need, the voltage step moves only the varistor voltage, not the rest of the row. The first template holds the 130 to 420 Vac steps except 195 Vac, which carries 14 parts. See how the varistor series differ for what the D, E, V and Industrial names mean, and energy ratings for the Industrial 40,000 A and 80,000 A split.

What does the Walsin varistor export not publish for a Mexican line?

Four things an application on a 127 Vac or 480 Vac line needs. We opened the file and counted its columns: ten, of which seven hold values. The three empty columns, Packing, SPQ and MOQ, are blank on all 379 rows, 1,137 empty cells. Nothing in the file states a joule rating, a clamping voltage, a DC working voltage or a surge waveform.

The working voltage is printed only in Vac. For a DC bus the export gives you nothing to compare. For a line-to-line or line-to-neutral choice, the file cannot tell you which voltage the device sees, because that depends on where you connect it. Which of 127 or 220 Vac, or 277 or 480 Vac, applies to your varistor is a circuit question, not a catalogue one. Compare a European 230 Vac case to see how the same ladder reads for a different nominal.

How should a Mexican buyer cross a varistor to Walsin?

Match the working voltage first, then the peak current, then the footprint, and keep your incumbent part beside the Walsin row. A 127 Vac line with a 10 percent margin has exactly one step to land on, 140 Vac, and 17 parts to choose from.

Field	Walsin export, SR221K10D	Your incumbent part	Match / mismatch
Maximum working voltage	140 Vac	to be filled in	Match only if equal or higher than your line plus margin
Varistor voltage	220 Vdc, printed plus or minus 10 percent	to be filled in	Compare the tolerance reference, which the export does not state
Peak current	2,500 A, no waveform stated	to be filled in	Compare the waveform before the number
Disc size	10 mm	to be filled in	Compare the footprint
Joule rating, clamping voltage	not published	to be filled in	Mismatch by absence. Ask for the datasheet

We hold 28 Walsin datasheets, 451 pages, and none covers varistors, so confirm against both datasheets and qualify in your own circuit. What is in Hong Kong is on ready stock, 1,125 part numbers as at 23 September 2026, a dated snapshot and not a live feed. Cross a varistor with the free cross-reference tool, and see the range on the varistor product page.

The short version

Add 10 percent to a nominal and the Walsin ladder answers with a step: 140 Vac for 127, 250 for 220, 320 for 277, 550 for 480. The headroom runs from 0.3 V to 22.0 V. 17 parts sit at each of the three lower steps and 13 at 550 Vac, where nothing below 10 mm exists. 19 of the 35 steps are copies of two templates, covering 299 parts. The file has no joule, clamping or DC working voltage, and three of its ten columns are empty on 379 rows. We will not tell you a step is right for your line. We will show you the arithmetic. Qualification stays with you.

Send us the nominal voltage and the margin your design uses and we will show you the step it lands on. A question back: if your 127 Vac line sits 0.3 V from the step, is that a margin or a coincidence?

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Frequently asked questions

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