Circuit theory

[Martyc]

After spending 3 hours drawing wiring diagrams in my workbook (only started 2365 level 2 in sept) it made me think about 2 real life scenarios that have made me think (dangerous I know) that I'm sure both answers are simple and I'm over-thinking them. Here they are;

1) a friend is changing virtually all the light fittings in his house to light/ fan units, he asked me to do it as he knows I'm at college, I declined as don't want to start messing with people's houses yet. But with this situation is it satisfactory to have the fan supplied by the lighting circuit and would any modifications have to be made to accommodate the extra demand of the fans? My personal view is: depends on the number of lamps on the circuit, is there sufficient capacity to accommodate the extra demand (I.e if the whole house is on a single circuit for lighting, it may be too much and have to be split into upstairs/downstairs circuits to accommodate potential demand).

2) I have a galley style kitchen, at the end of which is a downstairs toilet and utility room. There is a switch for the kitchen light by the kitchen entrance, and a single pull cord switch by the utility room door for the utility room and toilet in both of which there is a batten type lamp holder. If I turn on the kitchen light I can't turn on the utility/toilet light, but I can if it is switched off. The house is pretty old, and so is the wiring, but I can't think of any reason why it would purposely be wired this way. Would there be any reason why it would be wired this way (only reason I can think of would be to stop all lamps being on at once to reduce maximum load on circuit but seems daft!), and also how would it be likely to have been done? I can only think of the kitchen switch being a 2 way switch hence lighting working the way it does, but I haven't investigated yet.

Sorry for the essay, and sounding like a complete flid, but if you don't ask, you don't know (I tried googling but didn't seem to answer much) and I'd like to know how far off the mark my train of thought is!

 

[Marvo]

is it satisfactory to have the fan supplied by the lighting circuit and would any modifications have to be made to accommodate the extra demand of the fans? My personal view is: depends on the number of lamps on the circuit, is there sufficient capacity to accommodate the extra demand (I.e if the whole house is on a single circuit for lighting, it may be too much and have to be split into upstairs/downstairs circuits to accommodate potential demand).

Usually the lighting circuit would accommodate the extra load of a bathroom fan without any upgrading or alterations . They're usually about 20-50 watts which is less power than a single old fashioned filament lamp. As far as I know of the UK regs (which isn't very much) you need to install a fused isolator to supply the fan and it may need to be triple pole depending on the fan. You can spur from the supply existing lighting circuit.

There is a switch for the kitchen light by the kitchen entrance, and a single pull cord switch by the utility room door for the utility room and toilet in both of which there is a batten type lamp holder. If I turn on the kitchen light I can't turn on the utility/toilet light, but I can if it is switched off. The house is pretty old, and so is the wiring, but I can't think of any reason why it would purposely be wired this way. Would there be any reason why it would be wired this way (only reason I can think of would be to stop all lamps being on at once to reduce maximum load on circuit but seems daft!),

It sounds like it may have been a later DIY addition where the installer picked up the supply for the utility room switch from the switched live and neutral in the kitchen light fitting. I doubt it was some kinda power saving technique, more likely they either didn't understand what they were doing or they thought it was too much work to run the supply correctly from elsewhere.

 

[Martyc]

Usually the lighting circuit would accommodate the extra load of a bathroom fan without any upgrading or alterations . They're usually about 20-50 watts which is less power than a single old fashioned filament lamp. As far as I know of the UK regs (which isn't very much) you need to install a fused isolator to supply the fan and it may need to be triple pole depending on the fan. You can spur from the supply existing lighting circuit.



It sounds like it may have been a later DIY addition where the installer picked up the supply for the utility room switch from the switched live and neutral in the kitchen light fitting. I doubt it was some kinda power saving technique, more likely they either didn't understand what they were doing or they thought it was too much work to run the supply correctly from elsewhere.

It's not a bathroom fan, he's fitting a ceiling fan/light unit in every room, so it'd be 4 fans and if its the type I'm thinking of most have more than one lamp on each unit, although in reality the likelihood of all fans being on at once is unlikely.

For the kitchen lighting Q, if the bathroom light switch was on the switched live wouldn't the bathroom lights work when the kitchen light is on and not when the kitchen light is off? I wouldn't mind that so much, but it's a pain in the --- turning off the kitchen light to use the downstairs toilet at night when you need to walk through the kitchen to get to it!

 

[Baker1988]

Hi i am currently at college in my second year on the 2357 so i am not fully quilified so dont take my word for it but i do have some experiance

1) The light/fan units are ok to go off the lighting circuit as they are basically lights but how many lights are they in the house? get the maximum demand of the circuit it should say the maximum watts that the light/fan will pull probily in watts so add all the light/fans every thing on the lighting circuit and divide that number by 230 and you will get the amps dont worry if it is more then the 6amp as you should really take into account diversity as all the light's/fan's wont be on there maximum pull at the same time so for diversity you use 66% of the total lighting amps so what ever the amps is multiplyed by 66 divided by 100 and that will be the total load hopefully it will be less then the 6amp overlad device you have in place.

2) The light switch seems like they have put the live from the l2 of the swich to the pull cord so when the light in the kitchen is on it is swiched to the l1 side of the switch which means it can not travel down the wire to the pull cord but when it is turned off it is on the l2 side of the swich so the electric can now travel down the live wire to the pull switch you will probily find that the wire is in a single wire from l2 to the pull switch so you need to put the wire from l2 into the common terminal and it should all work fine but this is a guess with out seeing it hope this helps.

make sure you DO NOT work live isolate circuit before commencing any of this your self

 

[Paul.M]

Baker1988 answers are very good. The 2nd question? It could also be a switched neutral but without looking at it I could'nt say but it is a possibility.

Martyc, whats your location? Maybe a member could come around and look at this for you and give you a bit of hands on fault finding experience. You've only just started college and your working things out for yourself already, good for you, thats a sign of an electrician.