Control-cabinet power distribution unit (busbar system and power distribution blocks)
Inside the control cabinet · Busbar systems, connection adaptors and power distribution blocks that split one feed into branch circuits
A control-cabinet power distribution unit is the assembly inside an industrial control panel that carries the incoming supply from the main switch to the branch breakers, fuses and starters. It is built either as a busbar system, copper bars on insulating supports with touch-protection covers and plug-on adaptors, or as power distribution blocks that split one conductor into two or more. Ask three things first: which current each part is rated for, how the unit affects the panel's short-circuit current rating, and how many branch outputs and conductor sizes it must take.
Power distribution units mounted inside control cabinets and industrial control panels: busbar systems with supports, covers, connection and component adaptors and bus-mounted fuse bases, and open or finger-safe power distribution blocks. Complete low-voltage distribution boards, panelboards and consumer units are covered on the Power distribution panel (distribution board, panelboard) page. Rack-mount PDUs for IT racks, vehicle power distribution units and printed circuit boards are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 3 sources
Build the distribution on a 60 mm busbar system and choose adaptors by current
Rittal's RiLine60 catalogue shows how a busbar-based unit is put together: 3- and 4-pole busbar supports at 60 mm bar centre distance for flat copper bars from 15 × 5 mm to 30 × 10 mm, base trays and cover sections that give touch protection, busbar connection adaptors to bring the feed in, OM adaptors and circuit-breaker component adaptors to mount switchgear, and bus-mounted NH fuse bases and fuse-switch disconnectors. [1]
Its 3-pole busbar connection adaptors are listed at 60 A, 125 A, 250 A and 600 A at 600 V AC, taking round conductors from 6–16 mm² (60 A) up to 95–300 mm² (600 A), with 800 A and 1600 A adaptors for laminated copper bars. OM adaptors for mounting switchgear run from 25 A to 65 A at 600 V AC, with some at 690 V. Rittal lists the system for feeder circuits under UL 508A, file E191125, notes that a base tray is compulsory for UL applications, and marks some parts as without UL approval. PLS busbar connectors are tightened to 10–15 N·m for PLS 800 and 15–20 N·m for PLS 1600. [1]
Editorial note: because some parts in the same system carry no UL approval, ask the panel builder to list the UL status of every busbar part used if the panel will be UL 508A labelled. [1]
02
Keep the panel's short-circuit current rating when adding the distribution unit
Schneider Electric's North American motor control data bulletin summarises the code basis: control panels must be marked with their SCCR using an approved method such as UL 508A (NEC 409.110); the available short-circuit current at the installation must be documented, and the panel must not be installed where it exceeds the panel's SCCR (NEC 409.22); interrupting devices need sufficient interrupting rating (NEC 110.9, OSHA 1910.303(b)(4)); and components not meant to interrupt a short circuit must be paired with a protective device (NEC 110.10, OSHA 1910.303(b)(5)). [2]
UL 508A Supplement SB uses a weak-link approach: the panel SCCR follows from four steps, finding each component's SCCR, checking whether a current-limiting device can raise a branch component's rating, finding the interrupting rating of all breakers and fuses, and taking the lowest. Schneider names the power distribution block among devices not designed to interrupt a short circuit, which must be applied with the right fuse or breaker to reach its rating, and warns against using the main protective device's interrupting rating as the panel SCCR. [2]
Editorial note: a distribution block or busbar adaptor with a lower SCCR than the rest of the panel sets the panel rating, so ask the supplier for the SCCR of each distribution part and the protective device it was tested with. [2]
03
Split one feeder into branch circuits with power distribution blocks
Where a busbar is not used, power distribution blocks split the primary conductor. Mersen lists its MPDB series as UL 1953 power distribution blocks, UL and CSA approved, meeting spacing requirements for feeder and branch circuits under UL 508A and the National Electrical Code, with single or dual primary inputs, up to 30 secondary outputs, and specialty blocks with up to 7 primary inputs, in open-style and finger-safe (IP20) versions. [3]
The series comes in miniature, intermediate and large sizes in aluminium and copper. Listed examples include an open-style tin-plated copper 1-pole block rated 1520 A at 1000 V with four 500 kcmil to #4 openings each side, and copper stud-to-stud blocks rated 400 A at 1000 V. Across the range Mersen's filters show ratings from 85 A to 2660 A for copper and 65 A to 1360 A for aluminium. [3]
Editorial note: compare these ratings with Rittal's 600 V adaptor ratings only after confirming the standard each is listed to (UL 1953 for the blocks, UL 508A feeder use for the busbar parts) and the conductor material. [3][1]
04
Ratings and details to put on the order
Item
Published example
State for your panel
Rated current
Adaptors 60–1600 A (Rittal); blocks 65–2660 A (Mersen) [1][3]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What bar spacing does a 60 mm busbar system use?
Rittal's RiLine60 uses a 60 mm bar centre distance for flat copper bars from 15 × 5 mm to 30 × 10 mm, in 3- and 4-pole versions. [1]
Does a power distribution block affect the panel SCCR?
Yes. Schneider explains that UL 508A Supplement SB takes the lowest component rating, and that distribution blocks must be paired with a fuse or breaker to achieve their rating. [2]
How many outputs can a power distribution block have?
Mersen lists MPDB blocks with up to 30 secondary outputs, and specialty blocks with up to 7 primary inputs. [3]
Sources and what still needs a direct check
Sources checked 2026-10-12. Rittal and Mersen describe their own busbar systems and distribution blocks; Schneider Electric's data bulletin summarises NEC, OSHA and UL 508A requirements on short-circuit current ratings.
