"Fuse box" sits in an awkward place in the OEM vocabulary. It's everywhere on the harness, but the term covers everything from a twelve-circuit cabin panel to a sixty-five-circuit central block with a relay pack in the same housing. If you're writing a fuse-box specification for a new commercial-vehicle program, the decisions that matter are not "which fuse box do I buy" — they're "what does this fuse box need to do, where does it sit, and what does it talk to."
This page is how to choose a published step, the fuse format on each rail, and the housing-plus-gauge mix. Which enclosure type you need at all — fuse box, relay box or junction — is the enclosure-type comparison. Vibration and sealing on tracked machines are the excavator PDB notes.
1. Pure fuse box, or central distribution box?
The single biggest decision is upstream of any spec sheet. A "fuse box" can mean two very different things:
| Decision factor | Pure fuse box | Central distribution box (PDB) |
|---|---|---|
| What's inside | Bus bar, fuses and terminal blocks. | Fuses and relays in one coordinated layout — and on the CAN-enabled platforms such as the NBX‑952, body-control logic in the same housing. |
| The job it does | Overcurrent protection only. | Protection + relay switching + harness consolidation. |
| Best when | Relay switching already lives elsewhere — body control via BCM, or a separate relay box like NBX‑2404. | The program wants to consolidate body-harness wiring and reduce the number of separate enclosures. |
| Youlai examples | NBX‑955 (12-way); NBX‑981 (15-way) is fuse-first but carries seven relay positions in the same housing, and NBX‑2301 (17-way) takes optional relay sockets per programme | NBX‑957 body PDB; NBX‑958 / NBX‑970 / NBX‑953 on the body ladder; NBX‑972 when density forces a vented dry-zone box |
There is a third path the two columns do not show. Where a cab needs protection plus a handful of switched loads but nothing like a full-size central box, the compact NBX‑950 puts both in one cabin-sized housing — its reference build is 16 mini-blade fuses and five relays across twelve cab functions — which is the alternative to a fuse box plus a separate relay box beside it.
2. Circuit count: the catalogue moves in steps, not increments
Circuit count is usually the first number a program puts on the requirement. On the IP54 numbered ladder below, the gaps are uneven: three circuits separate the two smallest boxes, fifteen separate the last cabin-sized step from the first body-sized one, and eighteen separate the 47-way from the high-density unit above it. So the question is not "how much headroom do I add", it is "which step do I land on, and how far past it am I".
| Published circuits | Model | What the published build carries | Enclosure class |
|---|---|---|---|
| 12 | NBX‑955 | ATO/ATC positions on two rails (ON and ACC), no relay positions — switching lives elsewhere. | IP54 |
| 15 | NBX‑981 | Bussmann 227 ATC, Bussmann BK ATC and Littelfuse 287 ATO, 1 A–30 A; the reference build carries seven ISO mini-relay positions alongside the fifteen fuses. | IP54 |
| 17 | NBX‑2301 | ATO/ATC positions, optional relay sockets per programme. | IP54 |
| 32 | NBX‑958 | Multi-fuse plus multi-relay, configured against the harness drawing. | IP54 |
| Not a step (36 + 25 reference) |
NBX‑957 | 36 ATO/ATC across 15 body functions with 25 ISO mini relays in the same housing, configured to the harness. | IP54 (IP65 variant) |
| 42 | NBX‑970 | Multi-fuse plus multi-relay, Tyco / Delphi / Bussmann module options. | IP54 |
| 47 | NBX‑953 | 47 ATO/ATC and 4 MAXI blade fuses, 23 ISO mini relays, 7 cassette diodes. | IP54 |
| 65 | NBX‑972 | 65 fused positions in mixed formats — 60 mini-blade, 4 J-case main feeds and 1 ATO, spare positions among them reserved for late additions — plus 25 ISO relays; the harness lands on 9 sealed connectors. | IP4X, vented |
Two things follow from that column of numbers. The first is that landing between steps on this ladder costs real money: a harness with 19 protected branches that stays on the IP54 rungs buys a 32-way box and pays for positions it will never populate, which is the point at which it is worth asking whether two smaller boxes sitting close to their loads beat one half-empty central box. That is an architecture question rather than a catalogue one, and it belongs in the enclosure-type comparison.
The second is that density is not a free axis. Past roughly thirty circuits the box is as much a relay pack as a fuse box — the 47-way carries 23 relays, the 65-way carries 25 — and that changes both the fuse formats it needs (below) and the sealing it can accept. On this numbered ladder the 65-way is the only step that publishes no water digit: the NBX‑972 is a vented IP4X enclosure so the relay pack can cool, and it is specified for cabin, under-dash and dry zones only. An open-style block such as NBX‑966 is a different case again — the rating lives in the installation, not on the part. If a requirement asks for 60-plus circuits and immersion sealing in one housing, those two lines are pulling against each other and the program should hear that before the drawing is frozen, not after.
Off this ladder sit two mid-density siblings that are why “past thirty circuits you cannot seal” is the wrong reading: the passive NBX‑954 publishes about 31 circuits (26 ATO plus 5 spare) and 12 ISO relays in an IP65 enclosure, and the CAN-enabled NBX‑952 is the logic-carrying sibling on that same mid-density sealed platform. Construction-machinery boxes specified by vibration and sealing rather than by a numbered step — the NBX‑961 — are on the excavator PDB notes.
3. Fuse standard: branch blade, mini-blade, box feed and battery module
A box rarely settles on one fuse format. What decides the mix is not preference but where in the current path the position sits. The published builds show four jobs, not one catalogue preference:
- Branch positions — ATO/ATC blade. The default on the IP54 ladder and the reason a driver can service the box at all: field-replaceable, colour-coded, stocked globally. Most boxes leave the accepted fuse families to quotation against the harness drawing; the NBX‑981 is the step that publishes them (see the table above), which matters when a service network already stocks one brand and does not want a second.
- Dense branch positions — mini-blade. Same job, less panel area, and it is what lets a high circuit count fit a mountable footprint at all: the 65-circuit NBX‑972 packs 60 mini-blade (APM/APS-style) positions into a body of roughly 285 × 173 mm.
- Main feeds inside the box — MAXI blade or J-case. The rails that feed the branches are fused too, still as a larger plug-in format, not a stud. The NBX‑953 uses MAXI blades for those rails; the NBX‑972 uses J-case.
- Battery main feed — a bolt-down module upstream. That position is not specified inside the body fuse box. It is what the NBX‑980 MEGA / MIDI module is for. Teams that try to put the battery feed inside the body box usually discover the terminal format, not the current rating, is what stops them.
The rating map itself is programme data, and it is worth seeing what "per programme" actually looks like before the first quotation round. The NBX‑955 publishes an illustrative map: two rails, an ON rail with a 30 A reference main feed and an ACC rail at 40 A, each carrying six branch positions in a 5 A to 15 A band for body and accessory loads. Your own map will differ in every number, but the shape — rails first, branches under them, ratings set by the wire rather than the load's appetite — is what the control plan will document position by position.
4. Connector pathway: harness side comes first
The connector decision is harness-driven, not box-driven. Some boxes publish one family as the standard build; others are quoted against a short list the programme already stocks. The NBX‑955 ships against YAZAKI 7283-series housings, the NBX‑953 against DJ7-series blade connectors, the NBX‑2301 against KET, Tyco-AMP or THB options, and the sealed NBX‑971 against Deutsch harness-side housings as the standard build, with Bussmann, Tyco Superseal or Delphi GT where a programme already stocks them. Confirming that family comes before specifying the box.
What catches programs out is one level below the housing. The NBX‑955 mating reference does not stop at a housing part number — it maps each terminal part number to a wire cross-section band, so a 0.35–0.50 mm² branch and a 1.5–2.5 mm² branch take different terminals inside the same housing. Quoting the housing while leaving the gauge mix open is how a connector line survives an entire RFQ round and still fails at first-article build. Send the gauge mix with the drawing. Sealing behaviour at that interface, and why an IP-rated box does not stay IP-rated behind an unsealed connector, is on the commercial-vehicle connector guide.
5. IP rating: the zone sets a floor, density can veto it
Which class belongs on which mounting zone is on the IP ratings guide — cab interior through wheel-arch is a position question, not a vehicle-type question. How a published rating is designed and validated is on the IP65 / IP67 protection page.
The step you land on can veto that floor. Past the 47-way rung the only published option on this ladder vents, for the thermal reason set out under circuit count — so a requirement for both that density and chassis spray or immersion is two boxes, not one rating. Where the sealing is the fixed requirement, a purpose-designed box such as the NBX‑971 reaches IP67 through a gasketed split-line and a pressure-equalising vent membrane. Adding silicone to a vented or body-cavity enclosure does not produce that rating: those boxes were not built or validated as that assembly.
6. Working temperature, environmental and vibration class
Most numbered boxes on the IP54 ladder publish a −40 to +85 °C working window. Read the window on the model you actually land on: NBX‑972 publishes −30 to +85 °C operating (−40 to +90 °C storage), and so do the mid-density sealed siblings NBX‑954 and NBX‑952. A requirement that pairs those models with a cold-climate floor has to be confirmed against that model, not the series. Wider envelopes are quoted per programme when a project asks for them.
Vibration class is selected for the segment. The NBX‑972 publishes a 10–500 Hz sine-sweep result for heavy-truck duty; a tracked machine is a different spectrum and is on the excavator PDB notes. Screening is done in our in-house environmental laboratory with EMC pre-compliance equipment.
7. Documentation: what this specification has to write down
What turns the decisions above into something you can quote against is a control plan that records, for this box: the published step you landed on and how many unused positions that implies; the housing family and the terminal-to-gauge mix, not the housing alone; the density and IP pair, including any dry-zone limit; the working-temperature window on that model; and the fuse-format mix by rail. Mechanical and electrical drawings, BOM and FMEA travel with that plan. The quality-system and PPAP wrapper is on the IATF 16949 page. Regional marks (e-Mark / ECE, SASO, FCC / DOT) are available upon project requirement — not blanket-claimed across the catalogue.
8. The shortest possible decision tree
For most commercial-vehicle programs, the shortest path looks like this:
-
Does your BCM (or another smart module) already handle relay switching?
YesPure fuse box NoCentral distribution box - Count the protected branches, find the published step above that count, and check how far past the step you land before paying for positions you will not populate.
- Confirm the connector standard from the harness drawing — housing family and the terminal gauge mix, not the housing alone.
- Take the IP class from the mounting zone, then check it against the density you just chose — a vented high-density step cannot follow a chassis or immersion zone into one housing.
- Confirm the working-temperature window on that specific model, the vibration class and the fuse-format mix by rail.
- Get the supplier to put all of the above in writing — the control-plan fields above — before the sample order.
For drawings, sample requests or an RFQ review against your harness, please use the contact page or message +86 134 6767 4786 on WhatsApp. Typical reply within one business day.