AC Solenoid Coil Inrush vs Holding Current: A Sizing Guide
Many engineers are caught out when a bank of AC solenoid valves trips a power supply, chatters a relay or blows a fuse the moment it is energised — even though the coils' running load looks modest on paper. The reason is that an AC solenoid coil does not draw a single, steady current. It draws a high inrush (pull-in) current for a fraction of a second as it opens, then settles to a much lower holding (sealed) current once the armature has closed. Size your transformer, switching contacts and protection for the holding figure alone and you will run into nuisance trips, contact welding and premature coil failure. This guide explains where the two currents come from, how to read them on a coil label, and how to size a reliable installation.
Why an AC coil draws two different currents
The current an AC coil draws is governed largely by its inductive reactance, and that reactance depends on the size of the air gap in the magnetic circuit. When the valve is de-energised, the armature (plunger) is held back by its spring and the magnetic gap is at its largest. A large gap means low inductance, low reactance and therefore a high current — the inrush. As the coil pulls the armature home and the gap closes, inductance rises sharply, reactance climbs and the current falls to its steady holding value. In other words, the coil is at its thirstiest for the brief instant it is doing the most mechanical work, and it eases off once the valve is seated.
This behaviour is specific to alternating current. It is also why AC coils are rated in volt-amperes (VA), the apparent power, rather than in watts alone: the inductive load has a power factor well below unity, so the VA figure — not the wattage — is what your supply must actually deliver.
Inrush vs holding: what the figures mean
A good coil datasheet quotes both values, typically as an inrush VA and a holding VA. The inrush is always the larger of the two. The exact ratio depends on the design, but for many mains-voltage AC coils the inrush is roughly two to six times the holding value; always take the actual numbers from the manufacturer's data rather than assuming. The table below summarises the two operating points.
| Characteristic | Inrush (pull-in) | Holding (sealed) |
|---|---|---|
| When it occurs | Instant of energisation, armature open | Continuous, armature closed and seated |
| Magnetic air gap | Large | Minimal |
| Relative current / VA | High (the larger figure on the label) | Low (steady running load) |
| Duration | Momentary (a fraction of a second) | For as long as the coil is energised |
| What it stresses | Contacts, transformer, fuses, PLC outputs | Long-term heat rise and power budget |
Why it matters when you specify a system
Three parts of an installation must be rated for the inrush, not just the holding load. First, the power supply or transformer: if several valves can energise at the same instant, a worst-case design sums their inrush VA, because a transformer that sags under the combined surge may not develop enough voltage to pull the armatures in at all. Where valves switch at different times, you can often size for the total holding load plus an allowance for one or two coils pulling in together — but you need to know the switching sequence to justify that.
Second, the switching device — a relay contact, contactor or PLC digital output — has to make the inrush without welding or degrading. Semiconductor (triac or solid-state) outputs in particular have a clearly stated surge rating that must exceed the coil's inrush; where it does not, an interposing relay rated for the load is the safe answer. Third, fuses and circuit breakers should be time-delay (anti-surge) types so the momentary inrush does not cause nuisance tripping, while still protecting the holding circuit.
AC versus DC coils: a key difference
DC coils behave quite differently. A DC coil's current is set essentially by Ohm's law — the supply voltage divided by the coil resistance — so it is broadly constant and there is no large, sustained inrush of the AC kind. That makes DC-powered systems more predictable to size and generally kinder to switching contacts, which is one reason battery, solar and PLC-driven installations often favour them. The trade-off is that a DC coil has no natural current drop after pull-in, so it relies on the coil design (and sometimes electronics) to manage heat. If you are still weighing up the supply type, our guide to AC vs DC solenoid valve coils sets out the practical differences, and the solenoid coils selection guide lists the voltage and connection options we stock.
When inrush becomes a fault
The high inrush current is only meant to last for the moment of pull-in. If the armature never seats — because the valve is fitted the wrong way up, the plunger is jammed with debris, the differential pressure is above the valve's rated maximum, or the coil voltage is too low — the coil stays stuck near its inrush current instead of dropping to the holding value. The result is a coil that runs hot and can burn out, often accompanied by an audible hum or buzz from the laminations. If you are chasing a coil that buzzes or one that gets unusually hot, an armature that is failing to pull fully home is a common root cause, and it is worth checking orientation, pressure differential and supply voltage before condemning the coil.
A practical sizing checklist
Before you finalise a design, confirm a few things from the coil data and your control scheme: the rated coil voltage and frequency match your supply; the inrush VA and holding VA are both known; the transformer or power supply can deliver the worst-case simultaneous inrush; the switching contacts or PLC outputs are rated for that inrush; and the protective devices are time-delay types. For multi-coil systems, staggering energisation by even a short delay in the control logic can dramatically reduce the peak the supply ever sees. If the totals are marginal, DC coils or latching (bistable) valves — which draw only a brief pulse to change state — can transform the power budget.
If you would like help matching a coil voltage, VA rating and valve function to your application, our Valve Search Wizard will narrow the options for you, and the technical team is always happy to sanity-check an inrush or transformer calculation before you order.