Size of suppliers fuse vs RCD

[Knobhead]

Hi Duncan. You state mini consumer unit. Is there more than one? If so are the supply tails split to this consumer unit or is it fed from the main consumer unit?

Does it matter?

The OP won't understand the question because it's on page 6 of his five page £3500 booklet.

 

[DuncanS]

Hi Duncan. You state mini consumer unit. Is there more than one? If so are the supply tails split to this consumer unit or is it fed from the main consumer unit?

Yeah the tails split to the mini consumer unit...

 

[DuncanS]

(Just dealing with the reality of the way the world is rather than the way we think it ought to be for a second, the guy is here and is asking the question. Does it make more sense to rip the guy to shreds or to respond in a civilised way?)

Duncan, does the 100A rating of the main switch/RCD mean "I can routinely carry up to 100A load current" or "I will detect an overcurrent and disconnect if I carry above 100A"? Answer almost certainly the first but it's down to you to check that. It is quite a basic distinction that you should be aware of to practice competently. Good luck with the assessment.

It can routinely carry 100a, which was my concern in the first place, because if some spark has put this in some time ago and, there I go drawing 100a through it I'm gonna blow the main fuse, which would be a bugger as the DNO get funny about people just replacing them, no?
I appreciate those of you with sensible replies, for the trolls, let me remind you that once you were new to this and for me this is the first install I've done unsupervised so please do excuse my nerves, I'd rather come on here and double check my worries than leave something unsafe...

 

[NickD]

Yeah the tails split to the mini consumer unit...

Like Tony I am stumped as to the relevance of this.

 

[NickD]

It can routinely carry 100a, which was my concern in the first place, because if some spark has put this in some time ago and, there I go drawing 100a through it I'm gonna blow the main fuse, which would be a bugger as the DNO get funny about people just replacing them, no?

But it is rated for up to 100A which is fine; the DNO fuse is lower and so protects it. It is not the job or concern of the main switch to limit the current to something that will prevent the DNO fuse from blowing. It is the collective job of the installation design and the overcurrent devices which are downstream from the main switch, allowing for diversity because bearing in mind on virtually any installation if all the protective devices were carrying all of their rated maximum current at once it would blow the DNO fuse. My install here has 3 x 30A, 6 x 15A and 1 x 5A fusebox fuses for example, theoretical maximum draw 185A, on a 60A DNO fuse.

 

[Geoffsd]

because if some spark has put this in some time ago and, there I go drawing 100a through it I'm gonna blow the main fuse, which would be a bugger

How are you going to do that?


The 100A is the Current Carrying Capacity - the same as that for a conductor.

 

[DuncanS]

But it is rated for up to 100A which is fine; the DNO fuse is lower and so protects it. It is not the job or concern of the main switch to limit the current to something that will prevent the DNO fuse from blowing. It is the collective job of the installation design and the overcurrent devices which are downstream from the main switch, allowing for diversity because bearing in mind on virtually any installation if all the protective devices were carrying all of their rated maximum current at once it would blow the DNO fuse. My install here has 3 x 30A, 6 x 15A and 1 x 5A fusebox fuses for example, theoretical maximum draw 185A, on a 60A DNO fuse.

Thanks, as you can tell I didn't install this unit just adding another two circuits... I'm probably over thinking things and just being paranoid but.... If this unit is protected by an rcd, does that rcd effectively become it's main switch? Or should it have a separate one as the mini consumer unit is not fed by the main consumer unit, like I say I didn't install it but I wouldn't like to assume some one else did a good job when it's me being assessed... In school we always would start with a dp switch and work along from there...

 

[GLENNSPARK]

They can fight for his crown all they like mate, they'll lose.

oh aye..?

and what crown`s that then?

no-one comes near me in here for spite...

 

[GLENNSPARK]

It can routinely carry 100a, which was my concern in the first place, because if some spark has put this in some time ago and, there I go drawing 100a through it I'm gonna blow the main fuse, which would be a bugger as the DNO get funny about people just replacing them, no?
I appreciate those of you with sensible replies, for the trolls, let me remind you that once you were new to this and for me this is the first install I've done unsupervised so please do excuse my nerves, I'd rather come on here and double check my worries than leave something unsafe...

just caus a fuse`s I N rating is ...say in this case 100A...doesn`t mean that it will let go at 100A

further more i`d love to see a domestic installation that got anyware near 80A..never mind 100A....

theres such a thing as diversity as well...

 

[trev]

oh aye..?

and what crown`s that then?

no-one comes near me in here for spite...

Biff's funny with it though mate
Not that you don't have your moments mind

 

[mechelec]

oh aye..?

and what crown`s that then?

no-one comes near me in here for spite...

its not so much a crown more of a purple helmet.

 

[DuncanS]

I appreciate diversity but not so much with a shower, they draw full load from the start, at least that's what my osg tells me...

 

[trev]

I appreciate diversity but not so much with a shower, they draw full load from the start, at least that's what my osg tells me...

OSG is right. That's why you cannot apply diversity to an instantaneous water heater.

 

[DuncanS]

Yup it's the flow of water that causes temperature changes I believe, rather than a rheostat..

 

[trev]

Correct mate, that's why I laugh at people who buy megawatt showers thinking it's somehow better than say a 7.6Kw job. Your skin can only take so much heat comfortably.
Madness I tell thee, madness

 

[Meggerman]

It can routinely carry 100a, which was my concern in the first place, because if some spark has put this in some time ago and, there I go drawing 100a through it I'm gonna blow the main fuse, which would be a bugger as the DNO get funny about people just replacing them, no?
I appreciate those of you with sensible replies, for the trolls, let me remind you that once you were new to this and for me this is the first install I've done unsupervised so please do excuse my nerves, I'd rather come on here and double check my worries than leave something unsafe...

My question regarding split supply was just to confirm that this mini consumer unit you have added circuits to was not a sub main fed from main DB, which you have confirmed it is not. The important distinction you need to understand here is that the current rating of your RCD Main Switch will not limit the current you can draw through it, the installation it supplies decides that. The current rating of the RCD (in your case 100A) is the maximum it is designed to handle.

 

[mechelec]

Correct mate, that's why I laugh at people who buy megawatt showers thinking it's somehow better than say a 7.6Kw job. Your skin can only take so much heat comfortably.
Madness I tell thee, madness

But will a higher wattage shower not give the same temperature as a lower wattage one only with a proportionally higher water flow ?

 

[Meggerman]

Correct mate, that's why I laugh at people who buy megawatt showers thinking it's somehow better than say a 7.6Kw job. Your skin can only take so much heat comfortably.
Madness I tell thee, madness

In deep winter with almost freezing water supply, I'd take the 10.8Kw Mira Sport every time

 

[trev]

But will a higher wattage shower not give the same temperature as a lower wattage one only with a proportionally higher water flow ?

Perhaps, but it's still governed by the pressure of the incoming water supply

 

[Meggerman]

But will a higher wattage shower not give the same temperature as a lower wattage one only with a proportionally higher water flow ?

The more powerful the shower, the higher the volume of water it can heat as it passes though