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Space factor for single cables inside trunking

George Mailer

Can someone clarify the following:

With reference to the IET books (on site guide and guidance note 1) the mentioned space factor for installing cables inside trunking. If you following the guidance in the tables and making sure the combined cable factors are less than the trunking factor then all is good.

But it also mentions that for other sizes and types of cables in trunking then a 45% space factor is recommended but my understanding is this:

45% space factor means that the cables can take up 55% leaving 45% space, I cant find my newer guidance note 1 but an older 2004 version actually states that the cables so take up no more that 55% (which is very clear) but the paragraph below from the current on site guide leads you to think that the space factor is the space taken up by the cables and 45%

From page 150:

Other sizes and types of cable or trunking

For sizes and types of cable or trunking other than those given in Tables E5 and E6,
the number of cables installed should be such that the resulting space factor does not
exceed 45 % of the net internal cross-sectional area.
Space factor is the ratio (expressed as a percentage) of the sum of the overall cross sectional
areas of cables (including insulation and any sheath) to the internal cross sectional
area of the trunking or other cable enclosure in which they are installed. The
effective overall cross-sectional area of a non-circular cable is taken as that of a circle of
diameter equal to the major axis of the cable.

Can someone please clarify this.

Regards George

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    Such guidance, in years gone by, used to be part of the regulations themselves.

    Going back to the 15th Ed, appendix 12 covered this.

    It said:

    945dde91d5bd24ea34c2b8ac9c8afce8-huge-image.png

    and part 2 defined a space factor as:

    2441e7eab44526a44820c53adac7662b-huge-image.png

    so it seems to be that the original intention at least was that the cables take no more than 45% and at so least 55% must remain empty.

    (perhaps “space factor” wasn't the best choice of words - sounding more like the factor of empty space, rather than the factor of the cables, but that's committees for you)

        - Andy.

Reply
  • 0

    Such guidance, in years gone by, used to be part of the regulations themselves.

    Going back to the 15th Ed, appendix 12 covered this.

    It said:

    945dde91d5bd24ea34c2b8ac9c8afce8-huge-image.png

    and part 2 defined a space factor as:

    2441e7eab44526a44820c53adac7662b-huge-image.png

    so it seems to be that the original intention at least was that the cables take no more than 45% and at so least 55% must remain empty.

    (perhaps “space factor” wasn't the best choice of words - sounding more like the factor of empty space, rather than the factor of the cables, but that's committees for you)

        - Andy.

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