holes in joist

[chrisreid186]

hi guys

looking through electricians guide to building regs (small green book) pg 31 and
having trouble understanding :

' holes on centre line in a zone between 0.25 and 0.4 x span'

in regards to running cables in joists

could someone please explain in layman's terms

thanks in advance

 

[andyb]

hi guys

looking through electricians guide to building regs (small green book) pg 31 and
having trouble understanding :

' holes on centre line in a zone between 0.25 and 0.4 x span'

in regards to running cables in joists

could someone please explain in layman's terms

thanks in advance

I'm not sure laymen should be drilling holes in joists.

 

[Ampere]

Remember to keep holes in the same joist spaced by three times the diameter of hole

 

[telectrix]

all it's saying is don't drill joists in the centre ( 2/5 of the span), or close to the ends(1/4 span)

 

[edrobbo8]

Sorry to dig up an old thread, is there any advice on best practice when there are existing notches in a joist (for example when carrying out a re-wire)?

The house I am currently doing is notched and drilled very randomly, and would like to make the cable layout a lot more simple and holes at equal distance between 0.25 and 0.4 of the span and at the same height in the joist! As the old holes are here there and everywhere, I don't want to weaken the joists by drilling more holes.....any thoughts?

 

[telectrix]

use old holes where it suits and drill new ones to line up where you want. if it's an old house, the joists will be of proper seasoned timber and not kiln dried rubbish, selected to be the weakest, thinnest the builder could get away with , like new builds, so a few more holes ain't going to bring the house down.

 

[edrobbo8]

Thanks for that Telectrix, I thought that would be the ideal way, but better to get a 2nd opinion.

It definitely is old, Georgian - early 1800s! And some of the wiring looks like it might of complied to 1st edition.......
,

 

[telectrix]

but deffo drill joists, not notch. notches are for lazy plumbers.

 

[Superspark23]

My college tutor told us we should only put 1 cable in each hole. That seems absuloute rubbish to me. His reason was grouping factors.

 

[nickblake]

your tutor should know better , thats why we include grouping factors in the calculations so that cable sizes can be cose to accomadate cable grouping , wonder how he would do trunking with singles ??

 

[telectrix]

LOL, imagine 100 lengths of trunking (or conduit) with one cable in each. the mind boggles. what a plonker! and how about 15 adjacent holes in a joist. mr. suit building inspector would just love that.

 

[edrobbo8]

but deffo drill joists, not notch. notches are for lazy plumbers.

Yes and having to protect the cable as well on notches, much better to drill!

 

[PC Electrics]

My college tutor told us we should only put 1 cable in each hole. That seems absuloute rubbish to me. His reason was grouping factors.

I cannot find anything written to support this but I was taught that in the domestic environment, it was considered acceptable to group up to five cables together without taking grouping factors into account. I believe it's to do with the very high diversity within domestic which is unlikely to be present in comm / ind.