Cable Grouping Factor Confusion

Hi all

Looking at the calculations for cooker cable ratings last night and decided that I needed guidance on the selection of grouping factors. Considering cables which run as a loose "loom" between the floors of a 2 storey house, would these be considered as closely grouped? At points where they pass through joists they are touching, between joists, some are touching and some have a cable or two diameters between. If you believe that these do represent grouped cables, then surely almost all installations are grouped (at least at the point where the cables bunch to connect to the CU). Also I can reasonably argue that the downstairs power ciruit alongside the cooker cable is lightly loaded (or will be as I intend to break out the kitchen onto a separate ring). So according to the On-Site-Guide I can ignore this in the grouping equation. But what if a future occupant decides to run electric heaters in each room?

I currently have a 6mm cable feeding a double oven and large hob. I don't want to go to the hassle and expense of uprating this unnecessarily. Taking diversity etc into account, it appears that a 51A rated load equates to approx 22A for cable load sizing.

Any light that anyone can shed on this would be appreciated

TIA

Phil

Reply to
TheScullster
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Assuming T&E then unless the cables are clipped to form a single layer then, yes, the /bunched/ grouping factors will apply to a 'loom.'

Near the consumer unit you will usually need to marshal the cables into a single layer, or into separate bunches of two or three cables, to prevent the grouping factors becoming too onerous.

Definitely not. The standard socket circuits (per Appendix 8 of the OSG) already include an allowance for diversity (based on the floor area served) and you shouldn't derate further. (The diversity allowed between circuits in order to determine the maximum demand is another matter.) For a standard ring circuit take the design current to be 20 A for each leg or unfused spur when checking the cable size required.

That's why you can't...

That sounds about right (first 10 A, plus 30% of remainder, plus 5A for any socket on the CCU). That gives you the Ib for the circuit.

Work steadily through the procedure in Appendix 4 of BS 7671 to determine (or check) the cable sizes. Make sure you understand the difference between Ib and In. For a domestic installation it's safe to assume that circuits will not be subject to simultaneous overload, and that can help a lot, as can avoiding the use of re-wireable fuses. Make sure you're using Table 4D5A for T&E current ratings, not Table 4D2A or ratings on wholesalers' Web sites, most of which are wrong.

Reply to
Andy Wade

There's many lurking in this group who are much more learned than me on this subject, but in their absence, SFAIUI if you are following standard circuits of OSG then those circuits allow for up to 5 cables (eg 5 FTE cables - not 5 separate conductors) to be bunched. The underlying logic being along the lines that, eg, the full load rating eg of 2.5mmsq FTE cable is still over 20amps even after grouping derating.

generally as above, but there is an aspect which always seems to be ignored. Why doesn't a hole through a joist count as conduit, & thus requiring further derating?

That's a shady area I've never properly understood. I await with interest what others have to say on bunching of cables near the CU.

No, if this is a ring you need to assume each cable may carry the full

20A load specified for ring circuits in OSG. For other circuits you need to assume that the maximum amps implied by the protecting fuse/mcb will be carried.

So far as selecting specific cable goes, you should install cable (& route it) to handle the maximum fault free load which the protection mechanism (fuses, mcb etc) allow the cable to carry. Diversity does not affect cable selection at this level.

There may be a misunderstanding here. Diversity deals with the overall supply requirements of an installation (or section). Tthat is, it is a calculation of the maximum amps a circuit section [for instance one controlled by an mcb or, indeed, a whole CU) will draw. Ultimately the total of all diversities is used to determine how large a generator (yours or the power company's) has to be applied to service the installation.

(or will be as I intend to break out the

then the mcb will go pop ;-).

check the rating of the ovens & hob. Chances are 6mm is adequate.

HTH

Reply to
jim

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