Going to install a shower light extractor unit over the shower in the bedroom do i take the feed from the ceiling lighting circuit of 5a. The shower light extractor unit says i have to reduce it down to 3a do i do this with a 13a spur and put a 3a fuse in it
In theory what you propose doing seems OK but in practice you have a problem in many fans require two separate supplies. Some form of isolation is also required. There has been a report in a trade magazine of a house fire caused by single rather than double fusing to extractor fans one supply did not come through FSU. It may be possible to fuse down whole lighting supply in the area to 3 amp. I am assuming it has duel supply one switched the other which maintains to fan running for set time after the light is switched off. Before the house fire report I would have just connected to 5/6 amp lighting circuit it does seem the lighting circuit where the fire occurred was 16 amp which is unusual as most ceiling roses are rated 5/6 amp. Under the new regulations, it would of course, be protected with an RCD so less likely to cause a problem. I used one with a humidity detector so single supply. Also of course, you need to ensure all sockets in the bedroom are no closer than 3.5 meters to shower tray and all sockets have earth leakage protection. Not sure on Part P. I would think any rooms containing a bath or shower becomes a bathroom so would require Part P but I will leave it to you to work out there are links on the projects page. Also do note how the lights are wired before you disconnect anything the amount of posts asking how to wire lights is unbelievable.
If i take feed off lighting circuit to 3a spur then live to pull switch then to junction box then two feeds one to light transformer and one to extractor fan. Then neutral from tranformer and fan back to junction box and then to spur is that better thanks
That sounds OK. If it were in my house where everything has earth leakage and lights are on 6 amp mcb I would not bother with fused spur, but as yet very few houses have RCD protection on lights, so fused spur is best option as you suggest.