Size a control transformer on inrush first, then check sealed VA. Add the inrush VA of every coil that pulls in at the same time, combine it with the sealed VA, and pick the smallest transformer whose 90% secondary voltage inrush rating is at or above that number. Then make sure the transformer VA is at or above the total sealed VA. Sizing on sealed VA alone is the most common mistake. The panel works on the bench and then contactors chatter at pull-in.
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The four steps (Micron method)
| Step | Do this |
|---|---|
| 1 | Application inrush VA = √(inrush VA² + sealed VA²). Inrush VA is the sum of every coil, relay, timer, solenoid and light that can energize at once. Sealed VA is the sum of everything running at the same time |
| 2 | Pick the column. Load can stand a 10% sag at start up: use 90% secondary voltage. Only 5%: use 95% |
| 3 | Read down that column to the first value at or above your application inrush VA |
| 4 | Read left to the transformer VA. Final check: transformer VA must be at or above total sealed VA |
Source: Micron Industries control transformer selection guide (catalog CCT-16A). Micron notes NEMA requires magnetic devices to operate at 85% of rated voltage, so the 90% column is the one most often used. Get inrush and sealed VA from each component maker.
Micron regulation chart: max inrush VA at 20% power factor (NEMA)
| Transformer VA | 95% sec. voltage | 90% sec. voltage | 85% sec. voltage | 480 to 120 V part |
|---|---|---|---|---|
| 50 | 170 | 200 | 240 | B050BTZ13JK |
| 75 | 310 | 410 | 540 | B075BTZ13JK |
| 100 | 370 | 540 | 730 | B100BTZ13JK |
| 150 | 780 | 930 | 1,150 | B150BTZ13JKF |
| 200 | 810 | 1,150 | 1,450 | B200BTZ13JKF |
| 250 | 1,400 | 1,900 | 2,300 | B250BTZ13JKF |
| 300 | 1,900 | 2,700 | 3,850 | B300BTZ13JKF |
| 350 | 3,100 | 3,650 | 4,800 | B350BTZ13JKF |
| 500 | 4,000 | 5,300 | 7,000 | B500BTZ13JKF |
| 750 | 8,300 | 11,000 | 14,000 | B750BTZ13JKF |
| 1,000 | 15,000 | 21,000 | 27,000 | B1K0BTZ13JKF |
| 1,500 | 10,500 | 15,000 | 20,500 | Ask for part |
| 2,000 | 17,000 | 25,500 | 34,000 | B2K0BTZ13JKH |
| 3,000 | 24,000 | 36,000 | 47,500 | B3K0BTZ13JXH |
| 5,000 | 55,000 | 92,500 | 115,000 | B5K0BTZ13JXH |
Source: Micron regulation data chart, NEMA values. 50 to 100 VA are 105°C insulation, 150 to 1,000 VA are 130°C, 1,500 to 5,000 VA are 180°C. Micron also lists a 1,000 VA 180°C unit at 13,000 VA inrush (90% column). Part numbers are Micron ImperviTRAN Group A: 220x440 / 230x460 / 240x480 V primary, 110/115/120 V secondary.
Worked example: 480 V panel, 120 V control
| Step | Numbers | Result |
|---|---|---|
| Inrush | 6 contactor coils pull in together at 300 VA inrush each | 1,800 VA |
| Sealed | Coils, relays, pilot lights and a small power supply running | 200 VA |
| Application inrush | √(1,800² + 200²) | 1,811 VA |
| 90% column | 200 VA unit is good for 1,150 VA. 250 VA unit is good for 1,900 VA | Pick 250 VA |
| Sealed check | 250 VA is at or above 200 VA sealed | Pass. Micron B250BTZ13JKF |
Example coil values only. Use the inrush and sealed VA from your contactor and relay datasheets. Sized on sealed VA alone, this panel gets a 200 VA unit, and the secondary drops more than 10% when all six coils close.
kVA sizes
For bigger control loads, see the Micron pages by size: 1 kVA, 1.5 kVA, 2 kVA, 3 kVA, 5 kVA. All sizes and voltages are on Micron control transformers by kVA and in the Micron collection.
Common questions
Why not just add up the inrush VA?
Micron uses the industry formula: the square root of inrush squared plus sealed squared. It accounts for the sealed load already on the transformer when the coils pull in.
When should I use the 95% column?
When the load can only stand a 5% sag at pull in. An older Micron catalog also says to use the 95% column when the primary voltage swings 5 to 10% from nominal.
Where do I find inrush and sealed VA?
On the contactor, relay or solenoid datasheet. Micron says to get both values from the component maker. Count only the coils that can close at the same moment.
Does the insulation class matter?
Yes. In the Micron chart a 1,000 VA unit with 130°C insulation is rated 21,000 VA inrush at 90%, and the 180°C unit is rated 13,000 VA. Check the chart row for the exact part.
What about the fuses?
Micron ImperviTRAN part numbers with an R in the suffix come with a Class CC primary fuse block installed. Size the fuses to NEC 450.3 and the transformer full load current.
Get it sized and quoted
Send the primary and secondary voltage, the coil list with inrush and sealed VA, and how many close at once. We send back the Micron part with lead time.
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Related: Micron by kVA · Micron control transformers · Micron collection · Line reactor vs load reactor · Heat load calculator · All cross references
Chart values and selection method are from Micron Industries published catalog data. NEC is a registered trademark of the National Fire Protection Association. Check the current manufacturer datasheet before you specify.