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

TheScullster wrote:

Considering cables which run as a loose "loom" between the floors of a 2
storey house, would these be considered as closely grouped?

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.'

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).

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.

Also I can reasonably argue that the downstairs power ciruit alongside the
cooker cable is lightly loaded

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.

But what if a future occupant decides to run electric heaters in each
room?

That's why you can't...

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.

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.

Any light that anyone can shed on this would be appreciated

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.

--
Andy

TheScullster wrote:
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?

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.


At points where they pass through joists they are touching, between joists,
some are touching and some have a cable or two diameters between.

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?

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).

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.


Also I can reasonably argue that the downstairs power ciruit alongside the
cooker cable is lightly loaded

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
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?

then the mcb will go pop ;-).


I currently have a 6mm cable feeding a double oven and large hob.

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

HTH