Replacing an Explosion Proof Junction Box: What to Record Before You Order

What you can see is not yet a specification. An enclosure that looks identical in a photo can still differ in cable entries, fixing centres, terminals or material — and those differences decide whether the replacement works on site. Record the unit in front of you, and the quotation can be checked field by field instead of guessed from a picture.

A photograph is not a specification

Most replacement enquiries begin with a photo of the old unit — a good start, but only a start. A photo flattens depth, hides the mounting side, and rarely shows whether an entry is M20 × 1.5, G 1/2″ or another thread. Two boxes can share the same outline and still not be interchangeable.

Three patterns account for most failed matches:

• The photo matches, the entries do not. Count, face, thread or spacing of the cable entries differ — and the gland that fits the old box will not enter the new one.
• The outline matches, the fixing points do not. Bracket positions or hole centres differ by a few millimetres, and the unit no longer bolts to the existing frame.
• The outside matches, the inside does not. Terminal count, rail length or earthing arrangement cannot carry the wiring the job needs.

Treat the photo as the index to the real record. The record closes when the images, the measurements and the nameplate describe one and the same unit.

The record set that answers most questions:

Measure the footprint and the fixing points

Start with three overall numbers: height, width and depth — and say what the depth includes. A cover, cable bosses or a mounting plate change the real figure, so record the measuring points as well as the numbers.

Then record how the unit is held in place:

• fixing hole centres, horizontally and vertically — centre to centre, not edge to edge;
• hole diameter or thread, and the fastener actually used;
• how it mounts: bolt-on feet, wall brackets, frame or pole adapters;
• which face sits against the structure, and any stand-off;
• the clearance needed to open the cover with the box in place.

Two notes from practice. First, size naming is not a measuring standard: makers label similar dimensions differently, so measure your unit instead of inferring from the old model name. Second, trust the measurement, not the print: cast or stamped figures can differ from a worn, repaired or modified enclosure. A tape measure visible in the photo is a useful scale check.

Map every cable entry: face, position, thread and cable range

Entries are where “interchangeable” quietly breaks. For every entry, record:

• which face it sits on, and where — offsets from the edges, or a simple sketch;
• the thread form — M20 × 1.5, G 1/2″ and other forms look alike, so read the marking or gauge the thread rather than guessing;
• the cable outer diameter the entry currently serves, and the gland fitted;
• fittings such as plugs, blanking elements, or a breather drain at the low point.

Count positions, not just holes: note which entries are plugged and which are empty — the next modification usually needs one more way than the old box had.

If the new unit must keep the same gland positions, say so in the request: gland direction and cable bend radius decide whether an entry on a side face actually works on site.

Record the internals: terminals, rails and earthing

With the supply isolated and the cover open, record:

• terminal blocks: type family, conductor size, count;
• mounting rail: type, length, and how many rails;
• earthing: stud size and positions, internal and external bonding points;
• the internal plate or bracket, and any component added after the original build.

A straight-on photo of the wiring, or a quick hand sketch, saves a follow-up visit. For retrofit work, count spare ways too — the next change usually needs room the old layout did not leave. If there is no space, better to find that in the review than on site.

Photograph the nameplate and collect what you already have

The nameplate is the unit’s identity record. Photograph it straight-on, close and evenly lit — no glare, no angle. Record the text exactly as printed, including the model code and the marking string; the request should carry the original text, not an interpretation of it.

The photo set that answers most questions:

1. the complete unit in place — position, surroundings and cable runs;
2. the nameplate, square and legible;
3. every entry face, including plugged entries;
4. the interior with the cover open;
5. the mounting side or bracket;
6. any drawing or wiring diagram pages you hold.

Collect what exists: the original drawing or data sheet, a spare-parts list from an earlier order, and the project wording for the item — tag number, and phrases like MAKE … OR EQUAL if your specification uses them. Where a document is missing, mark it missing: a stated gap can be resolved in review; a guessed value gets built wrongly.

When the original make is no longer available

Replacement projects often start exactly when a make is discontinued, lead times stretch or budgets change. The record set matters more here, not less: with the original unit unobtainable, your measurements are the only baseline.

If the specification says “or equal”, copy that wording into the enquiry exactly as it appears. The phrase is a constraint, not a blank cheque — what counts as equal is decided against the project’s requirements. What your record set does is give every reviewer the same fields to compare — footprint, entries, internals, material, marking — and give you answers in writing on which fields match and which differ.

Send the same set to each supplier you approach, so the offers stand on one baseline. To compare the answers, use the same columns for every quotation — see Explosion Proof Junction Box Price: What Actually Drives the Cost.