Condensation in a motor terminal box: seal, drain and heat it

Short answer: the litre of water in a motor terminal box came in through a breach — an open cable entry, a gland missing its sealing ring, or a cover gasket that stopped sealing. Condensation alone banks millilitres, not litres; the trapped air of a 20 x 16 x 10 in (508 x 406 x 254 mm) box holds about 1.4 mL of water even at 85% RH. Seal every entry, keep the gasketed cover under even compression, and hold the box 5-10 C (9-18 F) above ambient whenever the motor stands idle. Leave the drain out on a gasketed outdoor box: an open hole ends the seal you just fixed. At a coastal 30 C (86 F) and 85% RH the dew point sits near 27 C (81 F). A box that tracks the night temperature wets itself every night.

What the water tells you

The symptom points at the entry, so read the box before you wipe it dry.

• A litre you can pour out: an entry above the waterline is open. A tight box cannot bank that much from the air.
• A dried tide mark at one gland, or under the gland plate: that entry is the path, and the rest of the seal may be sound.
• Fog on the underside of the cover that clears by mid-morning: the box is tight but cold, cycling below the dew point overnight.
• White or green bloom on the terminals, or cable tails that feel sticky: water has sat on the copper; the sticky-insulation check separates water from flux residue.
• Carbon tracks across the terminal board: a wet, salted film has already flashed over once. Treat the next idle period as the deadline.

Wet-then-dry cycling is what grows tracking paths at 6.6 kV. Finding which entry fed it comes next.

Where the water gets in

Rank the paths by what each one can deliver, and the litre starts to make sense.

The litre itself almost always rides the first three rows. A conduit run from outdoors is a rain collector with a drain hole aimed at your terminals; a gasket leak weeps; breathing alone caps out at millilitres, though it keeps the box saturated between storms. Medium-voltage feeders often arrive as single-core cable in long runs, which makes the collector effect worse, not better.

How to find the breach

Lock the starter out and verify absence of voltage before the cover comes off; a 6.6 kV box holds stored charge, and this is a lockout job.

• Trace the stains. Water leaves a dried line from the hole it used; start under each gland before you blame the gasket.
• Audit every entry. Each gland matches the cable's outer sheath with its sealing ring seated, every spare opening is plugged metal-to-metal, and the conduit seals at the box wall where it leaves for weather.
• Walk the conduit. A run from a roof or trench holds water at each low point, and any point above your entry drains into the box; backflowing conduit runs work the levels out in detail.
• Test the gasket with paper. A thin strip at each bolt should pinch tight; where it slides out, compression is short.
• Read the insulation warm. Measure IR at 1 minute and 10 minutes for the polarization index, PI = IR10/IR1, then read it against the IEEE 43 acceptance floor set out in the FAQ below; a PI of 1.5 on a warm machine means it is still drying out.
• Log one night. A logger on the wall and in the air shows the surface crossing the dew point in the small hours; that window is your condensation clock, and where to measure ambient temperature sets the probe positions.

Heater watts for a 20 x 16 x 10 in box held above the dew point

The inputs, and where each one comes from:

• Box: 20 x 16 x 10 in (508 x 406 x 254 mm), stainless walls — a typical mid-size terminal box, assumed for this example; swap in your drawing's numbers.
• Wall heat transfer k = 3.7 W/m²K, the stainless value this site's heat guides use.
• Site: 30 C (86 F) days at 85% RH and a night low of 24 C (75 F), a coastal case like the ones behind this failure. The Magnus relation puts the dew point at 27 C (81 F); the full dew-point math sits in the condensation guide.
• Target: hold the interior 5 C above ambient. The walls must clear the 27 C dew point, and the 24 C night is 3 C short of it, so 5 C carries margin.

The rule: the continuous loss through the walls is Q = k · A · ΔT.

The arithmetic:

A = 2(0.508 × 0.406) + 2(0.508 × 0.254) + 2(0.406 × 0.254) = 0.413 + 0.258 + 0.206 = 0.88 m²

Q = 3.7 W/m²K × 0.88 m² × 5 K = 16 W

Sixteen watts holds the box 5 K above a still night. Fit a 25 W anti-condensation heater, the usual next step above 16 W. An exposed windy wall roughly doubles the loss to 32 W, so 50 W there. Extra metal adds loss in proportion, 18.5 W per m², so a 0.3 m² window costs about 6 W. The sizing holds only while the box stays tight — a heater fighting an open gland dries the weather, not the box.

The fix: seal first, heat second, drain last

Start with the entries. Fit glands that match each cable's outer sheath, with the sealing ring seated and the clamp carrying the cable's weight — a 6.6 kV single-core feeder is heavy and stiff. Seal the conduit to the box wall with compound where the run leaves for weather, and plug every spare opening metal-to-metal. Where the choice is a locking connector against plain conduit, the motor-cable entry trade-off sets out both sides.

Then the cover. Re-gasket rather than re-torque a hardened gasket, and check compression with the paper strip at every bolt. The corners go first: raised lip against flat face explains why a flat gasket on a lipless flange weeps there.

Then the heat. Wire the motor's own space heaters through the starter's auxiliary contact so they switch on the moment the motor stops. EASA, the motor-service trade association, publishes space-heating guidance on exactly this practice: hold the windings a few degrees above ambient while the machine idles. Take the heater watts from the motor datasheet; for the box alone, the worked example's heater carries it, and the fuller method sits in the anti-condensation heater article.

The drain comes last, if at all. An open drain sets the assembly rating to the drain's rating: a 6 mm field hole makes a holed box, and NEMA 250 and UL 50E write drain openings into Types 3R and 3RX designs, sized 3-6 mm (1/8-1/4 in), not into Type 4X. NEC 314.15 permits drainage openings of 3 mm (1/8 in) to 6 mm (1/4 in) in boxes listed for damp or wet locations, pointed clear of live parts — the escape for a sheltered design that truly collects. On an outdoor gasketed box the hole imports the humid air the heater then pays to dry. Seal, heat, and leave the wall solid.

Keeping it dry year after year

• Quarterly, measure the heater current: a healthy 25 W element at 230 V draws about 0.11 A, and zero means a failed element, thermostat or supply.
• Quarterly, trend IR and PI against the 2.0 PI floor and the 100 MΩ baseline; a falling trend finds wet windings before the first trip.
• Annually, walk the entries and gasket: plugs seated, sealing rings tight, gasket imprint continuous, cover screws re-torqued.
• After storm season, open the box at the next planned outage and look for standing water; a film is a warning, a pool is a path.

Keep the log. Each reading extends the trend, and the trend tells you which autumn to re-gasket before the first flashover, not after.

Re-specifying the box, and the voltage question

Re-specify when the checks turn up structural trouble: tracking on the terminal board, corrosion under the gland plate, a flange that will not hold gasket compression, or a second litre after the entries are sealed.

Standard low-voltage terminal boxes ship at IP55 in the motor guides — dust-protected and jet-resistant under IEC 60529 (a 6.3 mm nozzle at 12.5 L/min), one step short of hose-directed water. A coastal box that takes rain and salt wants the class above. Order a gasketed 316L box — brine and salt air are 316L work — with the gland plate drilled to the real cable schedule and mounting holes for the heater in the cutout drawing; the replacement terminal box guide lists the dimensions to send, and the custom enclosure builder takes material, size and cutouts from one page. We build to that drawing and deliver in 10 business days from your approval of it, rated NEMA 4X / IP66 by the manufacturer; not UL listed. Keep the enclosure type on the drawing, because NEC 110.28 and its Table 110.28 are what a US inspector reads the location against.

As for stepping 6.6 kV down to 440 V: the voltage follows the plant distribution and the machine's size, and it did not cause this failure. Both ratings flash over in a wet box, so keep the machine and fix the box; the numbers behind that call sit in the FAQ below.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

Questions