Key takeaways
- The 90° bend deduction on 2 mm CRCA at R2 is 3.48 mm with K = 0.44, so a 400 mm body with two 150 mm walls develops to 693 mm, not 700 mm.
- Blanking the full 693 × 593 mm development needs about 157 tonne-force at a 300 MPa shear strength, which puts a one-hit box die on the 250 T press.
- A 6 × 4 mm gasket channel closed by 70 µm of powder on both walls loses 0.14 mm of width, so mask the channel section and not just its base.
- Keep holes at least 12 mm from a bend line on 2 mm sheet (R + V/2 + t at V = 16 mm), or the cover screw holes drift oval and the seal gaps.
- A 400 × 300 × 150 mm box is about 8 kg complete, which is a rail load on a 19-inch frame and a bracket decision, not a wall-thickness decision.
Electrical junction box manufacturing is decided by numbers that never reach the customer's single-line diagram: the depth of the body, the width of the gasket channel, and the dry film thickness on the flange the gasket seats on. A 400 × 300 × 150 mm body in 2 mm CRCA is four bends, four corner welds, a gland plate and a masked earth stud. Any one of those steps can move the gasket face by more than the 0.5 mm a 6 mm sponge gasket will absorb, and that is where the rework starts.
The route splits early, on volume rather than on size. Below about 5,000 bodies a year we cut the development on the fibre laser and bend it on the press brake, because the drawing is usually still moving and a die change costs weeks. Above that volume, with a drawing frozen for two years or more, a progressive die beats the laser on piece price and forms the corner notches in one hit.
What follows is the route for that 400 × 300 × 150 mm box: the forming numbers we work to, the four places the box usually fails, and the fields that belong on the drawing so the second lot matches the first.
What decides whether a junction box is buildable?
The gasket face decides it, not the wall thickness. A sealed box is a flat surface loaded by a sponge gasket at 30–40% compression, with screws pulling the cover down at a fixed pitch. If the face is dished, the screws clamp local spots and the seal between them stays open. If the face is flat but twisted, one corner lifts as the opposite corner is tightened.
The wall itself is never the problem. A 2 mm CRCA cover on a 400 × 300 mm opening, pressed by a 6 mm EPDM sponge at 35% compression, sees a gasket line load of roughly 20–40 N per 100 mm of length. Spread across the plate that is a few megapascals against a yield above 200 MPa. Thickness is chosen for impact, thread depth, gland plate stiffness and weldability, not for pressure. Going to 3 mm "to be safe" adds half again to shell mass and makes every corner weld harder to keep flat.
Cable load is the load that matters, and it arrives through the gland plate. Twenty metres of 4 × 25 mm² armoured cable hanging from a bottom gland plate on four M6 screws puts a few hundred newtons into a 2 mm plate. That is a stiffener problem: add a 25 mm return, or move to a 3 mm gland plate where the cable count is high.
Junction box dimensions and the limits we hold
These are the working values for a welded 400 × 300 × 150 mm body in 2 mm CRCA. Anything outside the third column is a conversation before the drawing is released, not after the first article.
| Parameter | Typical drawing callout | Ganesh limit or note |
|---|---|---|
| Body material | 2.0 mm CRCA, IS 513 D/DD | 1.2–3.0 mm on this route; 0.8–40 mm across the shop |
| Inside bend radius | 2 mm | 2 mm at R/t = 1; under 1.5 mm the outer skin thins and powder pulls thin at the bend |
| Bend deduction, 90° | not usually stated | 3.48 mm at R2, K = 0.44, 2 mm CRCA |
| Hole-to-bend distance | not usually stated | 12 mm minimum at R + V/2 + t with a 16 mm V die |
| Corner joint | 3 mm fillet, continuous | Continuous on all four corners; stitch welds leak under a jet test |
| Gasket channel | 6 × 4 mm for 6 mm EPDM sponge | Flatness 0.3 mm over 400 mm, measured with the cover fitted |
| Cover screw pitch | 150 mm | 150 mm maximum with 6 mm sponge; 200 mm needs an 8 mm section |
| Threaded inserts | M5 rivet nut | Plugged before coating; 70 µm on an M5 × 0.8 thread stops the screw starting |
| Earth stud | M6, masked | Stud plus a 10 mm bare zone; bond target below 0.1 Ω to the customer specification |
| Powder dry film | 70 µm nominal | 60–80 µm general; 80–120 µm for coastal or plant-room service |
| General tolerance | ISO 2768-1 class m | ±0.5 mm typical on sheet metal, ±0.3 mm on formed hole positions by agreement |
| Body mass | not usually stated | 7.1 kg of 2 mm CRCA, about 8 kg with door, gland plate, rail and gasket |
Electrical junction box manufacturing: cutting and forming by volume
The development for this body is not 700 × 600 mm. Two bends come off each direction at 3.48 mm, so the blank is 693.0 × 593.0 mm before the corner notches are taken out. That 7 mm difference is the whole argument for cutting in the flat with bend compensation rather than cutting to outside dimensions and hoping.
On the laser route the blank is nested on a 2,500 × 1,250 mm sheet, cut in one pass on a 12 kW machine, and the four corner notches are cut at the same time as the gland cut-out, the cover-screw holes and the earth stud hole. Nothing is measured by hand afterwards.
On the die route the same development is blanked and notched in one stroke. The tonnage is not a guess. Blanking force is perimeter times thickness times shear strength: a 2,572 mm perimeter at 2 mm and a 300 MPa shear strength gives 1.54 MN, or about 157 tonne-force. Add stripping and a 20% margin and the die belongs on the 250 T press, not on a 160 T machine. The press tonnage calculator will run the same sum for a different box, and it is worth doing before a die is quoted.
Between those two routes sits the low-volume case that gets badly quoted: 300 boxes a year, three sizes, one drawing still changing. There the laser is correct and the die is a liability. Our press forming versus CNC bending comparison sets out where the crossover stops being about tooling and starts being about tolerance.
Bending a 150 mm deep body without a creased corner
A 150 mm wall needs a gooseneck punch or the part will hit the ram before the bend reaches 90°. On the 200 T, 5-axis brake with the 4,450 mm bed we set the bend sequence so the two short walls are formed first and the long walls last, which keeps the part clear of the tooling on every stroke.
Three numbers control the result:
- Bend deduction 3.48 mm. Set it per material lot. A K-factor error of 0.02 shifts each bend about 0.06 mm, so four bends move the cover face 0.25 mm. The bend allowance method explains where K comes from and why it is measured, not assumed.
- Springback of 1–1.5°. On 2 mm CRCA at R2 the ram overbends by that amount, and the control compensates. If the operator trims the angle by eye on the first part, every part after it carries the eye.
- Corner relief of 1.5t. Without a relief notch at the end of each bend line, the material tears at the corner and the tear sits exactly where the weld has to seal.
The cover-screw holes are the other forming risk. Laser cut in the flat and then formed, a hole closer than 12 mm to the bend line pulls into a teardrop, and a teardrop will not hold a rivet nut square. Our box bending guide works through the depth and flange combinations that a standard punch can reach.
Welding corners, gland plate and cover flange
Four continuous corner welds are what make a folded body sealed. The sequence matters as much as the size. We tack the body square in the fixture, weld diagonally opposite corners first, then the remaining two, and let the body cool clamped so the gasket face stays in the plane the fixture set.
A 3 mm fillet on 2 mm sheet is generous; a 2.5 mm leg is enough for a sealed joint and puts less heat into the corner. What matters for the seal is continuity: a 20 mm skip in a corner weld is a leak path that no gasket can close, and it will not show until the jet test.
The cover flange takes a different treatment. Where the cover lands, we do not weld at all if the design allows a folded return; where it must be welded, the weld goes on the inside and the flange is checked with a straightedge afterwards. Spatter on the gasket face is removed before blasting, because blasting drives it into the surface rather than off it.
Spot welding is the cheaper joint for the gland plate and internal brackets, at a 25 mm pitch on a 12 mm flange. The spot welding guide covers the pitch and electrode access rules; the short version is that a spot weld needs 12 mm of clear flange on each side of the tip.
Coating, masking and the earth path
Every box goes through the seven-tank pretreatment line before powder. For CRCA the sequence removes the drawing oil, builds a zinc phosphate layer and dries the part, and the powder then goes on at 70 µm nominal in the 18 ft × 14 ft booth. A box with a phosphate coating weight around 2 g/m² and 70 µm of polyester is a C3 system in ISO 12944 terms, which covers sheltered industrial service, not a coastal plant room.
Coating a box is mostly a masking decision, and the masking list is short:
- Gasket channel, full section, so the sponge seats on bare phosphate rather than on powder.
- Earth stud and a 10 mm zone, so the bond lands on metal. The welded enclosure guide covers how the bond path is kept through a coated enclosure.
- Threads and rivet nuts, plugged, because powder in a thread is a tap and a touch-up.
- Hinge knuckles and pin bores, taped, so the door swings without binding on film.
- Mating faces of the gland plate, where the customer specifies metal-to-metal contact for continuity.
Coverage inside the box is line-of-sight. A 150 mm deep body powders well; a 300 mm deep body with an internal divider does not, and the inside back will read 30–40 µm against 70 µm outside. If the customer needs internal coverage, the drawing should say so and the box should be designed with access, because no gun angle fixes a shadow.
Inspection points before and after coating
Five checks catch almost every box that would come back:
- Gasket face flatness, with the cover screwed down at the specified pitch, using a straightedge and a 0.3 mm feeler. Do it after welding, and again after coating with the channel masked.
- Corner weld continuity, by visual and a dye penetrant check on the first article of each lot where the customer asks for a sealed joint.
- Screw-hole position, checked on the first part with a pin gauge on the cover pattern rather than a tape across the face.
- Dry film thickness, five readings per face with a magnetic gauge, plus one on the gasket land if it is coated.
- Earth bond, with a low-resistance ohmmeter between the stud and a masked point on the body, against the customer's stated limit.
First article inspection also records the internal dimensions at the gland opening and the door gap, and PPAP documentation is available where the customer's quality system calls for it.
The Ganesh route for a 400 × 300 mm box
The body is cut on the 12 kW fibre laser with the 40 ft × 16 ft bed, which takes a 693 × 593 mm development and its notches without a second set-up, then formed on the 200 T, 5-axis brake with a stored programme per part number. Corners are welded at one of the 30+ manual stations or on a CLOOS cell where the volume justifies a fixture, and the body goes through shot blast where the drawing calls for it, seven-tank pretreatment, powder in the 18 ft × 14 ft booth and oven cure without leaving the site.
The same route builds the electrical enclosures that sit beside our server and network rack range, and the enclosure family is indexed under the data centre and enclosure hub. Boxes that mount on a 19-inch frame add their mass to the rail, so check the rail load on the rack load calculator before the mounting ears are drawn.
What to put on the junction box drawing
- Overall size, depth and the gasket face datum, so flatness is measured against the face that seals.
- Material grade and thickness, with the CRCA standard, not just "2 mm MS".
- Gasket type and section, with the channel dimensions and the compression the supplier expects.
- Sealing requirement, stated as IP66 to IEC 60529 or as "welded corners, gasket only", and who runs the test.
- Cable entry schedule: gland plate size, hole pattern, thread type and whether the plate is removable.
- Coating system, with dry film thickness range, colour, interior coverage and the full masking list.
- Earthing: stud size, position, bare zone and the bond resistance limit with the method.
- General tolerance class to ISO 2768-1, plus any feature that is tighter and the gauge that proves it.
When a fabricated box is the wrong route
Fabrication is the wrong answer when the enclosure is a catalogue size in high volume: a die-cast or moulded box will be cheaper and lighter than anything folded from 2 mm steel. It is also the wrong answer for a washdown area in a food plant, where 304 or 316 stainless is needed and the whole route changes, including the weld procedure and the coating decision, which usually disappears.
Very large boxes are a different problem again. Beyond about 2,000 mm in any direction the body needs segmenting, and the joints become the sealing risk rather than the corners. A 3 m enclosure is a welded frame with panels, not a folded box.
We do not run ingress testing, we do not cast or mould, and we do not certify an enclosure to a standard on the customer's behalf. What we do is hold the gasket face, the weld continuity and the coating thickness to the numbers on the drawing, and say so on the inspection record. Send the current revision with the gasket section and the masking list through the RFQ route and the route, the fixture and the inspection plan come back with the price.
Frequently asked questions
What thickness of CRCA should a 400 × 300 mm junction box use?
2 mm CRCA covers most industrial junction boxes at this size, with 1.6 mm acceptable for indoor cable-only boxes and 2.5–3 mm where the gland plate carries heavy armoured cable or the box is mounted on a vibrating frame. Wall thickness is set by impact, thread depth and weldability, not by the gasket load, which produces only a few MPa of plate stress.
Can a junction box be welded or folded and sealed with gasket only?
Folded corners with an injected sealant can work indoors, but the four vertical corners of a welded body are continuous 3 mm fillets for a reason: a stitch-welded corner opens a path that a jet test finds. If the drawing calls for IP66, specify continuous corner welds and a gasket channel whose flatness is measured, not assumed.
Why is the earth stud masked before powder coating?
Because cured powder is an insulator. The stud, the star washer face and a 10 mm zone around the bond stay bare metal so the enclosure can be bonded below 0.1 Ω. Masking is cheaper than scraping a cured film, and scraping damages the phosphate layer underneath.
Does Ganesh run the IP66 ingress test in-house?
No. We build the geometry, welds and gasket faces that make a seal possible, and we check flatness, weld continuity and dry film thickness, but the IP test itself to IEC 60529 is a type test that the OEM or an accredited test house runs on the finished design.
How many junction boxes justify a press tool?
Above roughly 5,000 bodies a year with a drawing frozen for at least two years, a progressive die usually beats laser cutting and press bending on piece price. Below that, tooling amortisation dominates and the laser route is cheaper because it also absorbs drawing changes at no tool cost.
Sources and further reading
- IEC 60529 — Degrees of protection provided by enclosures, defining the IP dust and water jet levels an enclosure drawing cites · reviewed 3 Nov 2026
- IS 513 — Cold reduced carbon steel sheets and strips, the CRCA grades and thickness tolerances behind a 2 mm body · reviewed 3 Nov 2026
- ISO 2768-1 — General tolerances for linear and angular dimensions where the drawing does not call out a specific tolerance · reviewed 3 Nov 2026
- ISO 9227 — Corrosion tests in artificial atmospheres, the neutral salt spray method used to compare coating systems on a test coupon · reviewed 3 Nov 2026
Have a junction box drawing ready?
Send the current revision with the body size, gasket type, cable entries, coating system and annual quantity. Our engineering team reviews the route, the fixture and the masking plan before quotation.
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