Wireless power inside a stainless steel enclosure

A pair of 20-30 mm coils, co-axial inside a stainless tube with only a few millimetres of clearance, will transfer power through the metal wall's near field as long as you accept some loss in that wall. The flux does not need a hole to pass; it induces circulating eddy currents in the stainless, and those currents turn part of your transmitter power into heat. Whether the link still works depends on frequency, coil size, gap and how much steel sits in the flux path. This article walks through what to expect and how to design the geometry so the link survives.

What happens when coils sit near a stainless wall

An inductive link works by near-field magnetic coupling. The transmitter coil drives an alternating flux; the receiver coil sits inside that flux and picks up a voltage. Anything conductive inside the flux path is a shorted secondary turn. A stainless wall a few millimetres from the coil is exactly that: eddy currents flow in it, their own field opposes your coil's field, and the energy they dissipate never reaches the receiver.

Three numbers decide how bad it is.

• Frequency. Eddy-current loss roughly scales with frequency squared for a given geometry. A 100-200 kHz Qi-style link loses far more to a nearby wall than a 20-50 kHz link does. If you control the design frequency, lower is kinder to the metal.
• Conductivity of the wall. Stainless is about 1.4 MS/m against copper's 58 MS/m, so per unit area it is a weak mirror of the flux. That is why links survive stainless where they would die in an aluminum or steel tube of the same size.
• Distance and coverage. Loss grows fast as the wall closes in on the coil and as more wall area falls inside the coil's footprint. A wall that only clips the edge of the flux costs little; a wall wrapped tightly around the coil costs a lot.

For your geometry, 20 mm coils with a 10 mm gap inside roughly a 40 mm bore leaves maybe 10 mm of stainless around each coil's edge. That is a workable but lossy arrangement: expect a noticeable efficiency penalty and some wall warming at more than a few watts. The 30 mm coils in the same bore leave almost no clearance, and the flux will couple into the tube wall along its whole circumference. That version will run hot unless you shield it.

Why the grade of stainless matters less than you would think

Both 304 and 316L are austenitic and effectively non-magnetic when annealed, so their relative permeability sits close to 1. Neither concentrates flux the way carbon steel does, which is the worst case: a ferromagnetic wall both shields the coil and heats strongly. If your project specifies carbon or galvanized steel around the link, rethink the layout or move to stainless or plastic at that spot.

The differences between the austenitic grades are minor for this problem. Cold work can make 304 mildly magnetic at bends and welds, adding a little loss; 316L stays quieter. Their electrical conductivities are close, so eddy currents behave much the same. Choose the grade for the environment, not for the link: 304 for most washdown and outdoor sites, 316L for salt, brine and chlorides, as set out in 304 vs 316L stainless steel for enclosures and in more depth in the stainless steel guide.

Ferrite backing: the fix that usually settles it

A thin ferrite sheet between coil and wall redirects the flux into the receiver instead of into the metal. It is the standard fix for coils that must sit close to conductive structure.

• Flexible ferrite sheets around 0.5-1 mm thick, cut to slightly larger than the coil, are common below a few hundred kilohertz. They are easy to trim to a round former inside a tube.
• The sheet must cover the whole coil footprint. Flux that spills past the edge finds the wall anyway, so err on the generous side.
• Do not let the ferrite touch the wall directly with nothing between; a small air gap of 0.5-1 mm keeps the flux loop sane and prevents the sheet acting as a loss concentrator.
• Check the sheet's saturation flux density against your coil current. Under-sized ferrite saturates, loses its shielding effect, and the wall heats again.

With ferrite in place, most low-wattage links inside stainless behave close to their free-space efficiency. Remaining losses usually come from geometry, not from the wall.

Geometry choices that buy back efficiency

If you control the mechanical design, spend your millimetres where they help most.

• Favor diameter over gap. Coupling between co-axial coils improves with coil diameter and suffers from misalignment more than from a modest gap. Larger coils with a clean 10 mm gap beat small coils squeezed against a wall.
• Keep the wall out of the coil's plane. Flux is densest in the plane of the coil. If the tube bore must stay tight, shift the coils so their plane sits toward the bore's centre, not hard against the wall.
• Break the circumferential path. Eddy currents love a continuous ring of metal around the coil. A slot, a plastic section, or a window in the wall behind the link interrupts that ring and cuts loss sharply. This is the same physics that drives non-magnetic gland plates on high-current panels; the gland-plate article preventing eddy current heating in high-current gland plates covers that variant in detail.
• Trim the metal you do not need. Where the enclosure is custom, metal that serves no structural purpose near the link can simply be left out or cut away on the drawing.

If you are also enclosing a radio with its antenna, the same metal-wall thinking applies on the radiating side; the wireless radio board enclosure article treats that case. The near-field physics behind low-frequency links follows the same eddy-current rules described above, so the bench test below covers both questions.

How to check it before you commit

You can validate the whole arrangement with a bench loop and a thermometer.

1. Drive the transmitter at your working frequency and current with the receiver at its planned gap, first in free air. Record the received power or load voltage. This is your reference.
2. Slide the parts into the tube. Measure again. The ratio tells you the wall's penalty directly.
3. Feel or measure the wall temperature after five minutes at full power. Warm is expected; too hot to hold means the loss is real watts, not rounding error.
4. Add the ferrite sheet and repeat both measurements. Most of the gap to free-air performance should close.
5. If the numbers are still short, change geometry: bigger coils, more gap, or a slot in the wall.

This takes an afternoon and settles the frequency and ferrite questions before anything is ordered.

What to put on the order

When the link works on the bench, the enclosure order should carry the geometry, not just the outside dimensions.

• Material: 304 for most sites, 316L for salt, brine or chemical exposure. Both come in our stainless wall-mount lines, sized from 6 x 4 x 4 in up to 60 x 36 x 16 in (H x W x D), and every dimension between them can be custom.
• A drawing that marks the coil positions, the ferrite sheets and any slot or window behind the link, so the openings land exactly where the flux needs relief. Our cutout designer and the custom page take these directly; the cutout drawing checklist in stainless steel enclosure with gland plate: when to specify one shows how much detail a drawing needs for clean quoting.
• Wall thickness stated only where structure requires it; thinner metal behind the link is a feature, not a compromise.
• Any penetrations for the power feed and load wiring specified as gland cutouts so the rating you order is the rating you keep. Wall-mount bodies are rated NEMA 4X-type / IP66 by the manufacturer; not UL listed.

We build each enclosure from one piece to your dimensions and cutouts, delivered in 10 business days from your approval of the drawing. Send the coil layout with the drawing and the wall around your link will be working for you, not against you.

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Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.

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