Short answer: count everything in 18 mm modules, not in “ways”. A DIN-rail enclosure’s usable width is its number of 18 mm modules × 18 mm, and every device you fit — breaker, RCBO, RCD, isolator, surge protector, timer, contactor — eats an integer number of those modules. Add the incoming device, the RCD groups, the outgoing circuits, a 20–30% spare, and the width taken by terminal blocks and wiring ducts. Then check two things the module count does not tell you: the depth needed to bend the cables, and the temperature rise inside the closed box. Most undersized boards are undersized on the second and third counts, not the first.
Sizing a board looks like arithmetic and is really four decisions made in order: what has to be isolated, how circuits are grouped behind residual current devices, how much space the terminations need, and how much heat the enclosure has to get rid of. Get the order wrong and you end up with a board that is full before the spare capacity is added, or one wide enough but too shallow for a 25 mm² incoming cable.
We write as an independent trading company — we are not a manufacturer, and we are not an appointed distributor of any brand — so this is planning method, not a catalogue page.
Module, way, and why the two are not the same
The 18 mm module is the unit of the modular system: a single-pole MCB is one module wide, and the DIN rail that carries it is the 35 mm profile. Some accessories and half-width devices use a 9 mm step, which is why enclosure literature sometimes counts in 9 mm units and sometimes in 18 mm; the DB2 terminal distribution boxes, for example, are described by their 9 mm modular system. Convert everything to one unit before you add, or the sum will be wrong by a factor of two.
A “way” in trade language usually means one outgoing circuit, and that is where the confusion starts. A single-pole MCB is one module and one way. An RCBO that protects one circuit is commonly two modules wide — so a 12-way board fitted entirely with RCBOs has room for six circuits, not twelve. A four-pole RCCB is four modules and protects a group of circuits rather than one. Counting ways instead of modules is the single most common reason a board arrives on site one size too small.
| Item | Planning width | Notes |
|---|---|---|
| MCB, 1P | 1 module (18 mm) | Add one module if a side-mounted auxiliary contact is specified |
| MCB, 1P+N | 2 modules; some compact types 1 | Confirm on the datasheet — the compact ones are a different catalogue number |
| MCB, 3P / 4P | 3 / 4 modules | Three-phase final circuits and sub-mains |
| RCBO, 1P+N | Commonly 2 modules | NB1L, NB2LE and NXBLE-63 families are all offered in 1P+N |
| RCCB, 2P / 4P | 2 / 4 modules | NL1-63 covers 2P and 4P, with 1P+N and 3P+N variants in the manual |
| Modular isolator | 1 module per pole | NH4-125 is 1P to 4P, 32–125 A |
| SPD (surge protection) | 2–4 modules typical | Depends on Type and pole count; check the device |
| DIN-rail energy meter | 2–4 modules typical | Direct-connected and CT-operated types differ |
| Timers, relays, small contactors | 1–3 modules each | NTE8 time relays and small CJX2 frames are modular |
| Terminal blocks | Not modular — reserve rail | Incoming, outgoing and earth/neutral terminations need real space |
The table is for planning only. A “1 module” entry can become two when the device takes a shunt trip, auxiliary contact or motor operator, and those are exactly the accessories people forget. Read the width off the datasheet for the exact code before ordering, and add the accessories at the same time.
Worked example: a ten-circuit single-phase board
Take a small commercial unit: ten final circuits, single phase, with an incoming isolator, split across two residual current groups, plus surge protection at the origin. The naive count is ten ways. The module count is different.
| Line item | Quantity | Modules each | Total |
|---|---|---|---|
| Incoming isolator, 2P | 1 | 2 | 2 |
| RCCB 2P, 30 mA, group A | 1 | 2 | 2 |
| RCCB 2P, 30 mA, group B | 1 | 2 | 2 |
| MCB 1P outgoing, group A | 5 | 1 | 5 |
| MCB 1P outgoing, group B | 5 | 1 | 5 |
| SPD, Type 2, 2P | 1 | 2 | 2 |
| Subtotal | 18 | ||
| Spare capacity at 25% | 4–5 | ||
| Planning total | 22–23 modules |
So a “ten-way” requirement is really a 24-module board, before terminal blocks and wiring space. If the same ten circuits were protected by RCBOs instead of two RCCB groups, the outgoing count becomes 20 modules and the total passes 30 — which is why the choice between group RCDs and per-circuit RCBOs is a board-sizing decision as much as a protection decision. The trade is real in both directions: RCBOs remove the single point of failure and the discrimination problem, and they cost width and money per circuit.
The spare capacity is not optional
Twenty to thirty percent spare is the working figure, and it does two jobs. The first is obvious: circuits get added, and a board with no spare ways means a second enclosure, a new feed and a shutdown. The second is thermal. Every device in a closed enclosure dissipates heat, and the internal temperature rises with the number of loaded devices per unit volume; a board that is full from day one has no room to spread devices apart, and every breaker in it derates a little more than the schedule assumed.
Spare ways are cheaper than a replacement board. On the schedule, express the spare as modules rather than ways so the buyer and the panel builder mean the same thing, and state whether the spare is blanked or fitted with a device — blanking plates are not free either.
Depth, bending radius and termination space
Module width is two dimensions of three. The depth has to take the device plus the cable bending radius behind it, and the radius of a 25 mm² or 35 mm² incoming conductor is not small. A shallow enclosure with a busbar and a full row of breakers leaves nowhere for the incoming cable to turn, and the installer solves it by bending tighter than the cable allows — a latent fault, not a neat job.
Then the terminations. Neutral and earth bars need to be sized for the number of circuits and for the largest conductor, not the smallest. Incoming and outgoing terminals need rail space or a separate block, and on a board with more than a handful of outgoing circuits the terminal blocks take a full row of their own. Glands and knockouts consume wall area, and on an outdoor installation the gland plate has to keep the IP rating intact — an IP65 enclosure with a poorly sealed cable entry is an IP20 enclosure with a label.
Heat, IP rating and where the board goes
Once the count is done, the thermal check follows. The internal temperature of a loaded enclosure is typically 10–15 K above the room, more when the enclosure is sealed, painted dark, mounted in sun, or packed with contactors and power supplies. Every breaker inside then carries less than its rated current, and the derating has to be applied to the device and to the cable it protects. A board sized on module count alone and installed in a hot plant room is a board that will trip in service and be blamed on the breaker.
The environment also decides the enclosure family. Indoor flush and surface boards in the DB2 terminal distribution line, and the NX2 consumer units from 3 to 36 ways in flame-retardant ABS or PC, cover the ordinary residential and light commercial cases. Outdoor and washdown positions need a proper IP65 enclosure rather than a plastic consumer unit with a lid, and the gland arrangement matters as much as the enclosure rating. The IP rating of the empty box tells you nothing about the assembled board.
| Decision | What it changes | Common mistake |
|---|---|---|
| Group RCDs vs per-circuit RCBO | Width per circuit, discrimination, downtime | Counting RCBO modules as one way each |
| Spare capacity 20–30% | Board size, heat per unit volume | Adding spare ways but no spare modules |
| Enclosure depth | Cable bending radius, busbar clearance | Choosing on width and assuming depth is standard |
| Terminal and bar space | Neutral/earth bar size, terminal blocks | Sizing bars for the smallest conductor |
| Internal temperature | Breaker derating, enclosure ventilation | Using site ambient instead of internal temperature |
| IP rating and location | Enclosure family, glands, material | Rating the empty box, not the built board |
110–127 V and 60 Hz boards
Lower voltage means higher current for the same load, and higher current means more heat in the same modules. A board planned for a 230 V installation will be thermally tighter at 120 V even though the module count is identical, because the breakers carry nearly twice the current for the same watts. If the board is specified by load list rather than by circuit count, do the arithmetic at the actual voltage and frequency before fixing the enclosure size — and allow more spare, because the empty ways on a 230 V board are often populated on a 120 V one as loads grow.
What to send with the enquiry
The outgoing circuit list with current ratings and whether each is single or three phase; the earthing arrangement and whether residual current protection is per group or per circuit; the incoming supply size and conductor type; the required breaking capacity; the installation location, indoor or outdoor, and the maximum ambient; the IP rating required; whether a surge protector, meter, timer or contactor has to fit inside; and the destination country, since the certificate requirement attaches to the assembled board’s components as well as to the breakers. From that list the module count, the enclosure family and the internal layout can be fixed together instead of one at a time.
FAQ
How many circuits fit in a 12-way enclosure?
Twelve single-pole MCBs, if nothing else is inside and the board is only outgoing circuits. Once you add an incoming isolator, residual current devices, a surge protector and terminal blocks, the usable circuit count drops — often to six or eight. Count modules, not ways, and add the accessories before choosing the size.
Is one module always 18 mm?
In the modular DIN system, yes: the single-pole width is 18 mm, and some devices and accessories use a 9 mm half-step. Enclosure literature sometimes counts in 9 mm units, so check which unit the number refers to before comparing two boards, and convert to one unit before adding.
How much spare capacity should a distribution board have?
Twenty to thirty percent of the fitted modules is the usual planning figure. It covers future circuits and it also gives the thermal headroom to spread loaded devices apart. Express the spare in modules and state whether it is blanked or fitted, so the panel builder and the buyer are counting the same thing.
Do I need a deeper enclosure for a large incoming cable?
Usually yes. The depth has to take the device plus the bending radius of the largest conductor behind it, and a 25 mm² or 35 mm² cable needs real space to turn without being over-bent. Check the depth against the incoming conductor, not against the breaker.
Does the enclosure IP rating cover the finished board?
No. The rating applies to the empty enclosure as tested, and the assembled board’s protection depends on the cable entries, glands, blanking plates and door seal. An IP65 box with an unsealed gland plate is not an IP65 board, and outdoor installations should be specified with the gland arrangement in view.
Related: How to size a distribution board · MCB derating in high ambient temperature · IP rating enclosure guide · Breaker and cable coordination · 30 mA vs 100 mA RCD · Boards and enclosures · MCB range overview · Request a quotation