45 AMP MCB'S

[kai]

Why were 45amp MCB's withdrawn by the majority of consumer unit manufacturers - despite the 10.5.kw rating being the number one shower rating in the UK?
Were they deliberately withdrawing themselves from an enormous market.
Sometimes the stupid things that people do are just unbelievble.

 

[JohnW2]

Why were 45amp MCB's withdrawn by the majority of consumer unit manufacturers - despite the 10.5.kw rating being the number one shower rating in the UK?

It might be more appropriate to ask why 45A ones were ever made. AIUI, the standard ratings were not determined by the power requirements of loads but, rather, on the basis of the Renard R10 series of preferred numbers - 6A (6.3 rounded), 10A, 16A, 20A, 25A, 32A (31.5A rounded), 40A, 50A, 63A & 80A (and others) are all R10 numbers, but 45A is not.

Kind Regards, John
Edit: Missing 32A included!

 

[kai]

I think that the renard system was designed just to look pretty, and cannot meet the practical needs of the electrician and installer at hand.
Yet a 32amp breaker is widely available. Personally I prefer the traditional fuse ratings of 05 , 15, 20, 30, 45, 60, 80 and 100amp for domestic use. it made more sense as it was designed by engineers and not Bean Counters as was the renard series of breakers. A stupid fault of the renard system, means you cannot have two 32 amp ring final circuits in a standard 63amp two space consumer unit any more, in the old days it just worked fine, as the old units were designed of sound engineering principles, and not vanity items by bean counters.

 

[JohnW2]

I think that the renard system was designed just to look pretty, and cannot meet the practical needs of the electrician and installer at hand.

I'm not sure the Mr Renard would be too impressed by the idea that his numbers were designed to 'look pretty' - as you undoubtedly know, the actual idea is that each step in a Renard series is approximately the same ratio.

Yet a 32amp breaker is widely available.

Whoops - unintended omission on my part - 32A (31.5 rounded) is part of the R10 series! I will correct my post.

A stupid fault of the renard system, means you cannot have two 32 amp ring final circuits in a standard 63amp two space consumer unit any more, in the old days it just worked fine ...

Diversity! If you added up the ratings of all the MCBs in the average modern CU, you would usually end up with a number considerably in excess of the rating of the RCD and/or main switch feeding them.

Kind Regards, John

 

[Iggifer]

A stupid fault of the renard system, means you cannot have two 32 amp ring final circuits in a standard 63amp two space consumer unit any more, in the old days it just worked fine ...

Diversity! If you added up the ratings of all the MCBs in the average modern CU, you would usually end up with a number considerably in excess of the rating of the RCD and/or main switch feeding them.

Quite correct John. If there was no such thing as Diversity we'd all have to have 100A 3P+N supplies!

 

[Taylortwocities]

Hasn't kai fallen into the beginner's trap of forgetting what the MCB is actually for!
It is there to protect the cable, not the shower itself.

So what is wrong with a 50amp MCB if the cable is big enough?

 

[JohnW2]

Hasn't kai fallen into the beginner's trap of forgetting what the MCB is actually for! It is there to protect the cable, not the shower itself. So what is wrong with a 50amp MCB if the cable is big enough?

Well, as I implied, he also seems to have failed to recognise the concept of diversity, and is therefore seemingly concerned that a 50A MCB will 'use up' 50A's worth of a perceived 'total permissible MCBs rating' of 63A or 80A!

Kind Regards, John

 

[kai]

The 45amp breaker is a very handy size, MK still make them, and GET, MEM & Wylex used to make them (with a green lever).
It is just really surprising that they were all pulled from the market, as they matched the standard UK shower rating perfectly.
You cannot have diversity on a shower circuit as far as I am aware, yes the breaker does protect the cable.
If I were a Board Manufacturer, I would have B45s in my range, and no B40 and B50's as they do not Directly correspond to any current known UK applications.

 

[winston1]

I think that the renard system was designed just to look pretty, and cannot meet the practical needs of the electrician and installer at hand.
Yet a 32amp breaker is widely available. Personally I prefer the traditional fuse ratings of 05 , 15, 20, 30, 45, 60, 80 and 100amp for domestic use. it made more sense as it was designed by engineers and not Bean Counters as was the renard series of breakers. A stupid fault of the renard system, means you cannot have two 32 amp ring final circuits in a standard 63amp two space consumer unit any more, in the old days it just worked fine, as the old units were designed of sound engineering principles, and not vanity items by bean counters.

Perhaps they should not have rounded 31.5 to 32. Two 31.5A MCBs would work.

 

[JohnW2]

The 45amp breaker is a very handy size, MK still make them, and GET, MEM & Wylex used to make them (with a green lever). It is just really surprising that they were all pulled from the market, as they matched the standard UK shower rating perfectly.

Yes, but as TTC told you, provided the breaker rating is at least high enough for the shower, that's fine - it's there to protect the cable, not the shower.

You cannot have diversity on a shower circuit as far as I am aware ...

Truew, but you can apply diversity to w whole installation, or part of the installation - as I said/implied, if that weren't the case, you'd probably need a 150A-200A Main Switch and RCDs in the average CU - just try adding up the MCB ratings in the average CU!

If I were a Board Manufacturer, I would have B45s in my range, and no B40 and B50's as they do not Directly correspond to any current known UK applications.

Again, breaker ratings are nothing to do with 'current UK applications' but, rather, to the cable (which, in turn, is required to be adequate for the load). For example, if the cable is up to it, there would be absolutely no problem (electrically) running a 10.5kW shower off, say, a 63A MCB.

Kind Regards, John

 

[JohnW2]

Perhaps they should not have rounded 31.5 to 32. Two 31.5A MCBs would work.

I presume that they did not want 'non-whole-number' ratings. In any event, if one understands the concept of diversity, one does need that for it to 'work'. A 63A RCD will very commonly be feeding MCBs whose ratings add up to far more than 63A.

Kind Regards, John

 

[kai]

Provided that the Kitchen Ring and Electric Shower are fed off separate RCD's in the multi-rcd board.

 

[JohnW2]

Provided that the Kitchen Ring and Electric Shower are fed off separate RCD's in the multi-rcd board.

You recently posted this is another thread ...

Although it's not a dramatic example, you have MCBs with ratings totalling 86A fed from an 80A RCD. If you take the other side of the CUI into account, you have MCBs with ratings totalling 146A fed from what I presume is a 100A Main Switch. That's all fine, and quite 'normal'.

Kind Regards, John

 

[Iggifer]

Yup. Stick a clamp meter round the tails in 'normal use' (without an electric shower going or the oven on) and you'll find it's less than 10A. Whenever I've done it, it's more like 5 or 6A

 

[ericmark]

I will admit to have MCB’s rated 13, 16, 20, 25, 32, 45, 55 would match ref method 100 however not all cables are thermal plastic or installed ref 100 and there are other items with a current rating other than the cable ceiling roses often 5 or 6 amp. With domestic the shower is about the only item rated over 32A even when the cooked is rated over 32A with diversity we still can use a 32A MCB.

The heat inside a distribution board where all the MCB’s are near their limit is rather high they all produce heat and that heat has to go some where. With the older and larger Loadmaster MCB’s the range went up to 70A as a MCB with a 100A isolator but there was more area to dissipate the heat.

It is reported that there have been many consumer unit fires so I can see how manufacturers would want to limit the sizes of the MCB’s fitted. Although the consumer unit has a limit of 100A the distribution board which uses the same MCB may be rated at 300 amp and the MCB’s are mounted above each other so the top ones will get very hot.

I would at 45A be considering moulded breakers rather than an MCB, clearly not an option in domestic Eaton on their spec do list both 40 and 45 amp MCB’s for showers. Hager state 6A ~ 63A MCB and 6A ~ 50A RCBO but there is a scale of values for the rated diversity factor and adjacent circuit breakers with a 10 way board this can be 0.5 however I would not expect a shower to be used for more than ½ an hour.

In real terms with domestic we have it easy, we in real terms just through together the wiring with very little design, once one gets into industrial units volt drop and diversity can’t be ignored. And if you run a supply in 4mm cable and then have to rip it out and replace because the volt drop is over spec your not flavour of the month so you have to design you simply can’t simply look as it and say that should do.

So in real terms with domestic we have it easy, convention is we use 6A for lights, 16A for immersion heater, 32A for final rings and cooker, and 40A for shower OK that is only 9.2 kW but over that we need to ensure the supply is large enough we still have houses with 60A supplies. Also the jump 6mm @ 32A, 10mm @ 40A and 16mm @ 50A means the cable and the MCB would both need changing to move to next size MCB.

So in the main the shower size is selected to match the cable and MCB, rather than the cable and MCB selected to match the shower. This is why my shower comes direct from the hot water supply so I have the full output of the gas boiler if required I think 22 kW and I can be in and out of shower in minutes the idea of having a hand held sprinkler to wash off the soap just does not appeal to me. In my house unless I change the consumer unit maximum would be 32A and I don’t fancy a 7.5 kW shower.