Recommended transformer
150 VA
110 VA sealed × 1.25 = 137.5 VA; inrush 210 VA screened at 5×.
UL 508A §32.2.4 · NEC 430.72(C)
Calculator 15 / UL 508A §32
Control transformer
Size control power from sealed and inrush VA, with a conservative transformer selection and fuse guidance.
Control-circuit devices
Typical: 3–10×
Typical: 3–8×
Typical: 1–2×
Typical multipliers are guidance only; device nameplate and manufacturer transformer inrush capability govern.
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VoltForgeControl Transformer Report
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Results
Engineering disclaimer: This design aid is not a substitute for engineering judgment. Confirm final design with a licensed professional engineer and the authority having jurisdiction.
Sealed load / minimum
110 VA / 137.5 VA
Sealed VA total and 125% design margin.
UL 508A §32.2.4
Inrush check
PASS
210 VA inrush vs approximately 750 VA momentary screening capacity.
Manufacturer guidance · UL 508A §32
Fuse guidance
Primary 167% · secondary 125%
Apply allowances to actual transformer FLA and select listed standard fuses.
NEC 430.72(C)
How this was calculated
- Sum sealed VA: 110 VA.
- Apply 1.25 design margin: 110 × 1.25 = 137.5 VA; round up to 150 VA standard size.
- Sum inrush VA (210 VA) and screen against approximately 5× selected rating; manufacturer data governs.
- Fuse primary at up to 167% and secondary at up to 125% of actual FLA, subject to NEC 430.72(C) conditions.
Engineering disclaimer: Verify transformer nameplate FLA, inrush, fuse class, control-circuit class, and UL 508A construction requirements.