When to use: Use this when selecting a control (machine-tool) transformer that powers a control circuit of relays, contactors, PLC supplies, pilot lights, and solenoid valves. A control transformer must carry the continuous sealed VA of everything held energized, and simultaneously supply the inrush VA of the largest device that picks up — without the secondary voltage dipping so far that other coils drop out. Size it for both the sealed load and the inrush voltage dip.
A control transformer steps down line voltage to a safe control voltage (typically 120 V) to power relays, contactors, solenoids, PLC supplies, and pilot lights. Engineers size it to carry both the continuous sealed load and the worst-case inrush of the largest device picking up simultaneously, without causing enough voltage dip to drop out other energized coils.
The required VA is the larger of two calculations: the continuous (sealed) load, which is everything held energized at the same time, and the inrush requirement, which ensures the voltage dip when the largest device energizes does not exceed an allowable threshold (typically 5–10%). The inrush formula is VA_inrush_required = (V_A_inrush × PF_inrush) / (dip% / 100). If the transformer is too small, the voltage dip during inrush will cause other relay coils to drop out, causing nuisance faults.
AC coil inrush current is typically 6–10× the sealed current, and inrush power factor is approximately 0.2–0.4 compared to 0.7–0.9 sealed. Solenoid valves and contactors have the highest inrush-to-sealed ratios. The standard convention is to size for sealed VA and inrush dip separately, then select the next standard NEMA transformer size (50, 75, 100, 150, 200, 250, 300, 350, 500, 750, 1000 VA) that satisfies both.
NEC Article 430.72 governs control circuit conductors and overcurrent protection. The transformer primary fuse is sized per NEC Table 430.72(B), which limits the fuse to 500% of the transformer's secondary current rating for short-circuit protection of the control wiring. NEMA ICS 2 and UL 508A govern industrial control panel construction and require the control transformer to have adequate VA for all energized loads. Machine safety standards such as NFPA 79 require that the control voltage be derived from a transformer with one secondary terminal grounded for ground-fault detection.
The critical design constraint is voltage dip during inrush: if the transformer impedance is too high relative to its rating, the voltage dip during the largest inrush event will drop other energized coils below their hold-in voltage (typically 85% of rated). For a drop-out-free design, allowable dip is 5–10% maximum. Engineers should also add 20–25% spare VA capacity above the calculated requirement to accommodate future additions and thermal derating. Select control transformers with 600 V or higher primary insulation for industrial use, and specify a primary-to-secondary isolation barrier for machine control transformers per UL 508A.
Enter the total sealed VA of relays, PLC power supplies, pilot lights, and other continuously energized devices. Enter the sealed VA of solenoid valves held energized. Enter the inrush VA of the single largest device that picks up (not the sum of all devices; only one device energizes at a time in the worst case). Set the inrush power factor (0.2–0.4 for AC coils) and allowable voltage dip percentage. The calculator outputs the required VA from both the sealed and inrush criteria, selects the next standard transformer size, and shows the predicted inrush dip with that transformer.
During inrush, the transformer secondary voltage drops because the inrush current is limited by the transformer impedance. If this dip exceeds 10–15%, other coils already energized may drop out, causing nuisance faults or unsafe de-energization of safety circuits. The inrush sizing criterion ensures the transformer is large enough that voltage stays above the hold-in threshold for all energized coils during the worst inrush event.
AC solenoid and relay coils have an inrush power factor of approximately 0.2–0.4, compared to a sealed power factor of 0.7–0.9. The low inrush PF means the apparent VA during inrush is much higher than the real power, which is why inrush VA is the dominant sizing criterion for most control transformers with large solenoid loads.
Standard NEMA control transformer sizes are 50, 75, 100, 150, 200, 250, 300, 350, 500, 750, 1000, 1500, 2000, 3000, and 5000 VA. Always select the next standard size at or above the calculated requirement. For redundant or critical control systems, consider selecting one size above the minimum to provide additional inrush margin.
Identify the single largest coil in the circuit — typically a main contactor or large solenoid valve — and obtain its inrush VA from the manufacturer's data sheet or the NEMA coil data table. If data is unavailable, use the rule of thumb that AC coil inrush is 6–10× the sealed VA. Only one device energizes in the worst case, so do not sum all inrush VAs.
120 V AC is the most common control voltage in North American industrial panels and is recommended by NFPA 79 for machine tools. It provides adequate safety margin over touch voltage, is widely available from 480V/120V or 240V/120V control transformers, and is compatible with the widest range of coil voltages. 24 V DC is common for PLC I/O and small SCADA panels, but requires a separate DC power supply rather than a control transformer.
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