- May 20, 2011
- 479
- 171
- 118
- If you're a qualified, trainee, or retired electrician - Which country is it that your work will be / is / was aimed at?
- United Kingdom
- What type of forum member are you?
- Practising Electrician (Qualified - Domestic or Commercial etc)
- Business Name
- Circitas Ltd
Diversity.
How do you tend to justify domestic installations when you come across what seems to be really common 60A BS1361 supplier fuses, where it all looks too tight according to recommendations in OSG?
Say we find 30A ring upstairs and 30A ring downstairs for starters. (plus 2 x lights ccts, immersion and cooker - not hob). For the rings alone, clearly the customer won't ever be drawing close to 230*30 = 6.9KW at the same time in each. OSG recommends "100% of current demand of largest and 40% of others". I am struggling with "current demand" since the way I read it, seems to be either "whatever you decide" or else the rated breaker (in this case 30A). If I decided to use whatever seems reasonable, I might expect ~ 2..3 KW max for each ring. Using 100% plus 40% on this, and I end up with say 18A. for the up and downstairs rings combined. If instead I use the rated breakers figure I get 30*100% +30*40% = 42A just for the sockets which is crazy. - It's only a little house!
Using the lower figure (say ~18A) for the pair of rings makes diversity calcs (after adding in lighting, oven (no hob) immersion and shed) just about look credible with existing 60A fuse. Using the breaker ratings for the existing rings, and it just can't work out.
I will happily refuse to assume 100W for every luminaire which is just crazy, but how far do you take common sense for ring loadings in the face of OSG recommendations?
I guess I just want to be sure I can cover myself if I were quizzed during an annual inspection.
To put it into context, on this particular place, I am replacing an existing shed supply lash-up with 1 way RCD CU via Henleys. Needing to keep costs down for the customer and not touch existing wiring in the house which is a few years old but not dangerous. When certificating, I need to be able to justify the overall loading for the whole place. So no actual loadings will alter, and 60A fuse has sat there happily for years.
- I guess I could always fit 16A breakers instead of the 30's already fitted to the house rings, and also maybe re-fit 16 for the oven (now also 30A - but oven is only 2.5 KW, gas hob). This makes it look more credible, and on OSG method, diversity then looks just about do-able. - It doesn't of course change anything at all, - but does that make me go away looking cleaner?
How do you tend to justify domestic installations when you come across what seems to be really common 60A BS1361 supplier fuses, where it all looks too tight according to recommendations in OSG?
Say we find 30A ring upstairs and 30A ring downstairs for starters. (plus 2 x lights ccts, immersion and cooker - not hob). For the rings alone, clearly the customer won't ever be drawing close to 230*30 = 6.9KW at the same time in each. OSG recommends "100% of current demand of largest and 40% of others". I am struggling with "current demand" since the way I read it, seems to be either "whatever you decide" or else the rated breaker (in this case 30A). If I decided to use whatever seems reasonable, I might expect ~ 2..3 KW max for each ring. Using 100% plus 40% on this, and I end up with say 18A. for the up and downstairs rings combined. If instead I use the rated breakers figure I get 30*100% +30*40% = 42A just for the sockets which is crazy. - It's only a little house!
Using the lower figure (say ~18A) for the pair of rings makes diversity calcs (after adding in lighting, oven (no hob) immersion and shed) just about look credible with existing 60A fuse. Using the breaker ratings for the existing rings, and it just can't work out.
I will happily refuse to assume 100W for every luminaire which is just crazy, but how far do you take common sense for ring loadings in the face of OSG recommendations?
I guess I just want to be sure I can cover myself if I were quizzed during an annual inspection.
To put it into context, on this particular place, I am replacing an existing shed supply lash-up with 1 way RCD CU via Henleys. Needing to keep costs down for the customer and not touch existing wiring in the house which is a few years old but not dangerous. When certificating, I need to be able to justify the overall loading for the whole place. So no actual loadings will alter, and 60A fuse has sat there happily for years.
- I guess I could always fit 16A breakers instead of the 30's already fitted to the house rings, and also maybe re-fit 16 for the oven (now also 30A - but oven is only 2.5 KW, gas hob). This makes it look more credible, and on OSG method, diversity then looks just about do-able. - It doesn't of course change anything at all, - but does that make me go away looking cleaner?
