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max demand and diversity


musicspark

Question

musicspark

Hello guys and gals

I am a little concerned about the max demand situation in a property I am working on. !00A main fuse, two 8.5 Kw showers. Three rings two lighting circuits, cooker, immersion heater and one 30A breaker supplying a Garage.

the client has mooted something about changing the cooker for a Double range at a later date. I am I right thinking that there might be a possibility of blowing the main fuse - especially if she goes and buys an induction hob

I doubt weather it's ever likely but seems like a lot of juice

is there anything that could go the other side of the CU to protect the suppliers 1361 fuse?

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18 answers to this question

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no expert and sure others will coment but, just because she has 3 rings and 2 lighting and the garage dose not always mean its pulling loads of current, normal house could have 1 ring pulling the same currrent as 3. and its not all on at the same time. how much power does the garage use

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Andy™

there could be problems if both shower are used at same time as cooker/immersion, although if it really is a 100A fuse (and not 60 or 80), then it may hold up long enough til a shower is finished. how big is the house? any other large fixed/expected loads? how exactly will moving something to the other side of the CU 'protect' the suppliers fuse?

next thing up would be 3 phase, which would probably be expensive unless there is already a 3 phase service head

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Andy™

main bathroom and en suite.

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could be or could have loads of kids. not sure really if u have 2 then u would think u would use both at same time at some point at lease.

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is it ok for an electrician to change the service fuse up from 60 80 etc, when upgrading tails or is it down to DNO?

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musicspark

Thanks Andy

two bathrooms.,,,I was more concerned That if she upgrades to a double range and possibly induction Hob that this could pull a lot of juice along with two showers ( they sometimes have lots friends staying)

What would the diversity be on a double range with an induction hob?

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Evans Electric

Service fuses will carry a surprising overload for a fair period. I had a job where I rewired a Victorian house into two flats, each with cooker/shower. Local network had no problem with the original 100 A fuse feeding both flats.

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Usualy the service cable was 0.6, having a rating of 98 amps, therefore fuses were 100 amp, to protecr service cable.

PS I'm quoting measurements when as an apprentice I was working on Area Board Networks. This was a long time ago back in late 60's

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Evans Electric

Good question , what I meant was ,network cut-out fuses will take overloads if they are built up slowly, but a dead short will rupture it immediatly. I know one or two industrial premises where the current creeps up over the limit at certain times of the day. By the time the fuses and tails have warmed up , the load drops again , been like it for years.

Its like blocks of flats, 500 amp fuses in switch room, 20 floors with 4 flats on each. 80 people come home and cook dinner, say, between 1700 and 1900hrs, worst case senario, but with diversity, time factors and HRC fuse characteristics , it works.

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the fault current would def trip it would it not, provided that the mcb dont trip first. but like u say if it warms up slowly it can overload for a bit. what is the diversity rule of thum i have read 0.4

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musicspark

OK I was enlightened but once more cofused

can someone explain the figures of .4 an .6 for the service cable ( rule of thum) ??????

sorry I've obviously missed out on my training

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green-hornet

All you have to do is read the brb and look at the test curves, or characteristic curves, to see that all will take an amount of overload for a given period of time before any disconnection.

Worth having a re-read for those in doubt.

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Evans Electric
is it ok for an electrician to change the service fuse up from 60 80 etc, when upgrading tails or is it down to DNO?

Not really , Sellers, I mean you'd have to cut the seal and its their equipment, what can I say , you'll have the EP round. :|

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Andy™
is it ok for an electrician to change the service fuse up from 60 80 etc, when upgrading tails or is it down to DNO?

you should not change the fuse because it may only be the rating it is for a reason. change it and you could cause a fire.

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