Editorial Technical Reference

Power distribution unit (PDU) for a control cabinet (main breaker, busbars and outgoing breakers, and the diversity factor they are rated with)

How much current the outgoing circuits of a cabinet power distribution unit can carry together · rated diversity factor from IEC 61439, the 0.9 to 0.6 table used for switchboard busbars, the 0.8 to 0.5 table for distribution boards, continuous loads over 30 minutes, and the assembly maker's own figure

A power distribution unit takes one supply through a main breaker onto busbars and splits it into outgoing circuits. Outgoing breakers mounted side by side heat each other, and the IET article notes that "it wouldn’t be economical for a manufacturer to rate the full assembly with all outgoing circuits rated at 100 %". The rated diversity factor states how far the circuits can be loaded together, and the buyer has to know which figure applies before sizing the load.

Power distribution units inside control cabinets and small switchboards: incomer, busbars and outgoing protective devices, rated for simultaneous loading of the outgoing circuits. Busbar systems and distribution blocks are covered on the Control-cabinet power distribution unit (busbar system and power distribution blocks) page; distribution boards and panelboards on the Power distribution panel (distribution board, panelboard) page; busbar copper on the Copper busbar page; motor control centres on the Motor control center (MCC) page; smart home panels on the Smart home circuit breaker panel: load-managing residential panel for service-upgrade avoidance page.

CNFX editorial research · Sources checked October 13, 2026 · 2 sources

What the diversity factor is Which table applies

Open grey control cabinet with a three-pole main breaker fed by red, yellow and blue marked cables, copper busbars behind a clear protective cover, a column of outgoing breakers with colour-marked cables, and an earth bar at the bottom — Cabinet distribution unit with main breaker, covered busbars and outgoing breakers

Start with your task

What the diversity factor isDefinition and loads. Which table appliesSwitchboard or DBO.

What the rated diversity factor is and which loads it applies to

Joe Cannon, writing in the IET's Wiring Matters (Issue 96, July 2023), quotes BS EN 61439-1:2011's definition of the rated diversity factor: "Per unit value of the rated current, assigned by the assembly manufacturer, to which outgoing circuits of an assembly can be continuously and simultaneously loaded taking into account the mutual thermal influences." A factor of 0.5 on a 32 A circuit, for example, means 16 A. [1]

The article notes that continuous loads are not defined in the BS EN 61439 series, but that the standard warns about loads energised for more than 30 minutes; some manufacturers treat more than 30 minutes as continuous, others more than 1 hour. Electric vehicle charging points and heat pumps are given as sustained loads where the factor matters. [1]

It also quotes BS EN 61439-1: "The rated diversity factor multiplied by the rated current of the circuits shall be equal to or higher than the assumed loading of the outgoing circuits", and notes that a circuit's rating inside an assembly may be lower than the device rating under its own product standard. [1]

List which outgoing circuits carry continuous loads (more than 30 minutes on) and which run together, and ask the assembly maker for the rated diversity factor it assigns and how it was verified.

Two tables: switchboard busbars and distribution boards

Schneider Electric's Prisma G catalogue (2015) takes the diversity factor into account when sizing busbars and attributes this table to "Standard IEC 61439-1 and 2 §4.7": 0.9 for 2 and 3 circuits, 0.8 for 4 and 5, 0.7 for "6 and 9" (6 to 9) and 0.6 for 10 and more. [2]

The IET article gives a lower table for distribution boards operated by ordinary persons, from BS EN 61439-3:2012 Table 101, and calls it the assumed loading factor, "the maximum permitted rated diversity factor to be applied by the user". In the consumer unit instructions it surveyed, British General, Contactum, Hager and Lewden use Table 101, while Fusebox, Live and MCG recommend a 60% factor where breakers are loaded continuously. [1]

Outgoing circuits	IEC 61439-1/-2 table cited by Schneider	BS EN 61439-3 Table 101 cited by IET
2 and 3	0.9	0.8
4 and 5	0.8	0.7
6 to 9	0.7	0.6
10 and more	0.6	0.5

State whether the unit is built as a switchboard assembly (IEC 61439-2) or a distribution board for ordinary persons (IEC 61439-3), and use the maker's assigned factor rather than either table where the maker gives one.

Cabinet PDU details

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Supply: [voltage; incomer rating; fault level]
Outgoing: [number; device ratings]
Loads: [continuous; intermittent; simultaneous]
Rating: [RDF assigned; verification]
Standard: [IEC 61439-2; -3]
Ambient: [temperature; IP of enclosure]
Busbar: [current; material]

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Cabinet power distribution unit sources

Manufacturer profiles associated with Power Distribution Unit (PDU).

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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 is a rated diversity factor?

The per-unit share of rated current that outgoing circuits can carry continuously and simultaneously, as assigned by the assembly manufacturer under BS EN 61439-1. [1]

What factor applies to 10 outgoing circuits?

0.6 in the IEC 61439-1/-2 table Schneider cites for busbars; 0.5 in BS EN 61439-3 Table 101 for distribution boards. [2] [1]

Which loads count as continuous?

The standard does not define them; the IET article notes the 30-minute caution and makers using 30 minutes or 1 hour. [1]

Who sets the factor?

The assembly manufacturer, who agrees it with the designer or states it in installation instructions. [1]

Sources and what still needs a direct check

Sources checked 2026-10-13. The IET publishes Wiring Matters (Issue 96, July 2023, article by Joe Cannon); Schneider Electric's Prisma G catalogue 2015 is read from an online flip-book copy.

Still to check yourself

List continuous circuits.
Count simultaneous loads.
Confirm the standard part.
Get the maker's RDF.
Check ambient and IP.
Verify the legal supplier and factory.

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