Cable and Breaker Sizes

[Jupiter01]

I have been doing some reading on this topic and would welcome your comment/validation of the scenarios below please:

1. Oven
Design current for this oven is 13 amp

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Therefore closest breaker size but higher than 13 amps is 16 amp. I will be using a 16 amp MCB which is within the protection range of the RCD

Minimum cable size for this initially appears to be 2.5mm T&E based on reference method 103. However, when I look at the voltage drop calculations and tables, I think this requires a minimum of 4mm T&E. This seems to be adequate up to a length of 66 metres. I have a 12 metre distance.
That said, I think the cable rating needs to be higher than the breaker rating? So, perhaps it needs to be 6mm T&E?


2. Lighting
Design current for the following is 0.634783 amps
24 4 Watt downlights
2 10 Watt bulbs
3 10 Watt external lights

Therefore closest breaker size but higher than 0.634783 amps is 3 amp. I think that's the lowest rating available? This is within the protection range of the RCD

Minimum cable size for this appears to be 1mm T&E based on reference method 103. I couldn't find anything in the voltage drop table for a 3 amp protective device. It starts at 6 amp. Why is that?

Some specific questions please:
- I think my lighting scenario has quite a lot of lighting and yet it's not even an amp. Why are almost all lighting circuits protected at 6amps?
- In scenario #2 if someone fitted a light which suddenly tipped the total current to 5amp, the 1mm cable is till good up to 8 amp but presumably the breaker will trip as soon as it exceeds 3 amps?
- Should cable rating always be higher than the fuse rating?
- I can understand how the MCB trips when the current is exceeded but what is the role of the MCB if a nail is hammered into the cable?

I would welcome @Taylortwocities comments on this one.

 

[ericmark]

It seems 2.5 mm would do.

3.7 volt drop is well within limits.

 

[ericmark]

Worked at 1 mm at 3 amp is that what you wanted?

 

[winston1]

I have been doing some reading on this topic and would welcome your comment/validation of the scenarios below please:

1. Oven
Design current for this oven is 13 amp

Bosch HBS534BS0B Black Built-in Single Multifunction Oven | DIY at B&Q

Bosch HBS534BS0B Black Built-in Single Multifunction Oven - B&Q for all your home and garden supplies and advice on all the latest DIY trends

www.diy.com

Therefore closest breaker size but higher than 13 amps is 16 amp. I will be using a 16 amp MCB which is within the protection range of the RCD

Minimum cable size for this initially appears to be 2.5mm T&E based on reference method 103. However, when I look at the voltage drop calculations and tables, I think this requires a minimum of 4mm T&E. This seems to be adequate up to a length of 66 metres. I have a 12 metre distance.
That said, I think the cable rating needs to be higher than the breaker rating? So, perhaps it needs to be 6mm T&E?

2.5mm twin and earth is rated up to 27 amps.
For ovens we use diversity. Take the first 10 amps plus 30% of the rest. Comes to 10.9 amps. Heck you could use 1.0mm2 cable which is rated up to 16 amps. (but I wouldn't).

2. Lighting
Design current for the following is 0.634783 amps
24 4 Watt downlights
2 10 Watt bulbs
3 10 Watt external lights

Therefore closest breaker size but higher than 0.634783 amps is 3 amp. I think that's the lowest rating available? This is within the protection range of the RCD

Minimum cable size for this appears to be 1mm T&E based on reference method 103. I couldn't find anything in the voltage drop table for a 3 amp protective device. It starts at 6 amp. Why is that?

- I think my lighting scenario has quite a lot of lighting and yet it's not even an amp. Why are almost all lighting circuits protected at 6amps?

UK lighting circuits have traditionally been on 5a fuses or 6a MCBs. Goes back to the days of real light bulbs. 10 fittings of 100w (= 1kW) each on a circuit.

In Europe lighting circuits are often on a 16a breaker.

- In scenario #2 if someone fitted a light which suddenly tipped the total current to 5amp, the 1mm cable is till good up to 8 amp but presumably the breaker will trip as soon as it exceeds 3 amps?
- Should cable rating always be higher than the fuse rating?
- I can understand how the MCB trips when the current is exceeded but what is the role of the MCB if a nail is hammered into the cable?

1mm cable is rated up to 16amps. Breakers don't trip the moment the current exceeds their rating.
The fuse or breaker is to protect the cable (not what is on the end of it) so of course the cable needs to be rated higher than the protective device.

If a nail is hammered into the cabe an excessive current can flow. Surely you know what the MCB will do.

 

[Jupiter01]

2.5mm twin and earth is rated up to 27 amps.
For ovens we use diversity. Take the first 10 amps plus 30% of the rest. Comes to 10.9 amps. Heck you could use 1.0mm2 cable which is rated up to 16 amps. (but I wouldn't).

But 2.5mm T&E drops to 13.5 amps under reference method 103. Is this a factor in the above?

In Europe lighting circuits are often on a 16a breaker.

Presumably, their cable must be higher rating than 16 amps as it needs to carry the over current to the breaker before it trips?

If a nail is hammered into the cabe an excessive current can flow

Sorry but can you elaborate on this. I understand how a load current can be pulling excessive current and how the MCB responds to this. I don't understand how the nail causes excessive current.

 

[Harry Bloomfield]

This is within the protection range of the RCD

You mentioned that twice, which suggests a misunderstanding....

The purpose of the MCB is to trip before the cable rating is exceeded, the purpose of the RCD is mainly to protect you from suffering a shock, secondly to trip on a type of fault which an MCB would not detect. 6amp MCB's are standard for lighting circuits, because they originally supplied more power hungry tungsten, halogen and fluorescent lights. Usually 1.5mm would be used, not for current rating but simply because it was more robust than 1mm. 1mm is certainly more than adequate for LED lighting in most cases.

 

[Jupiter01]

It seems 2.5 mm would do.
View attachment 271041
3.7 volt drop is well within limits.

But in here, the only entry for method 103 with a 16 Amp breaker is for a 4mm cable.

Additionally, 2.5mm cable has a rating of 13.5 amps under reference method 103 and the breaker is rated at 16 amp. Does this not require the cable to be higher rated than the breaker?

Thanks for your help.

 

[Jupiter01]

You mentioned that twice, which suggests a misunderstanding....

The purpose of the MCB is to trip before the cable rating is exceeded, the purpose of the RCD is mainly to protect you from suffering a shock, secondly to trip on a type of fault which an MCB would not detect. 6amp MCB's are standard for lighting circuits, because they originally supplied more power hungry tungsten, halogen and fluorescent lights. Usually 1.5mm would be used, not for current rating but simply because it was more robust than 1mm. 1mm is certainly more than adequate for LED lighting in most cases.

I thought there was also a rating on the RCD which requires the sum of the MCB ratings to be within this? I was referring to that.

 

[ericmark]

2.5mm twin and earth is rated up to 27 amps.

Since when? Remember @Jupiter01 has stated reference method 103, 27 amps is reference method C.

In Europe lighting circuits are often on a 16a breaker.

May be, but I personally have had a BA22d fitting weld the contracts to the bulb so whole fitting needed changing, Ikea bulb it seems don't have built in fuses, and the British ceiling rose is rated at 5/6 amp, so in the UK in the main we are limited to a 6 amp MCB for lights as some thing some where is likely to be only rated at 6 amp.

In the main a BA22d bulb holder is rated 2 amp, and the bulb should have a built in fuse, however now bulbs not made in Britain I would not rely on CE (Chinese export) bulbs to have the built in fuse.

For ovens we use diversity

This to some extent is true, size of largest element is however the start point, not looked as to what size the largest element is, however many ovens use time sharing to ensure it does not use over 13 amp, specially with functions like closed door grilling, my stand alone cooker which has a 32 amp supply can use 10466 - 12455 W rated 220 - 240V - 50Hz which is well over 32 amp, the 32 amp supply has already taken diversity into account, you can't do it twice.

 

[Harry Bloomfield]

I thought there was also a rating on the RCD which requires the sum of the MCB ratings to be within this? I was referring to that.

I was answering the OP's apparent misunderstanding. The RCD has to be rated for the total load it needs to support and the loading it needs to be able to reliably break.

 

[ericmark]

Additionally, 2.5mm cable has a rating of 13.5 amps under reference method 103 and the breaker is rated at 16 amp. Does this not require the cable to be higher rated than the breaker?

The rules allow for the limit to be either end, as long as no branches, so the 13 amp fuse in the plug or FCU is going to ensure no more that 13 amp for an extended time. Seems odd to be reference method 103.

Design current and maximum current are not the same.

 

[Jupiter01]

The rules allow for the limit to be either end, as long as no branches, so the 13 amp fuse in the plug or FCU is going to ensure no more that 13 amp for an extended time. Seems odd to be reference method 103.

Design current and maximum current are not the same.

aha, I can see how a FCU at the oven changes the equation. For my understanding, if there were no FCU, would the cable then need to be upgraded as else the breaker cannot protect the cable in the above situation?

 

[ericmark]

I am not sure on that, if appliance rated 13 amp then only under fault conditions can this be exceeded, but tried to see what elements fitted, with my stand alone cooker I am sure it can exceed 32 amp, however been on a 32 amp MCB/RCBO for years, in two homes, likely now 20 year old, and the cooker supply has never tripped. Even if in theory it can draw over double the MCB/RCBO rating.

But if you have time sharing for functions like closed door grilling, then it is possible this goes wrong so two elements or more on together, so could exceed 13 amp but not be a short circuit, with a short circuit clearly the magnetic part of the MCB/RCBO will operate so disconnection time is short.

We as the installer or user have no idea of the internal wiring of the oven, so no idea if under fault conditions two 2 kW elements could switch on together, however unlike my stand alone cooker the manufacturer rates it at 13 amp, where as with my cooker not rated 32 amp, it is just recommended that a 32 amp over load device is used.

So I would say your allowed to believe what the manufacturer says, and if they say 13 amp you can believe this to be true.

Where the problem lies is when the oven is swapped for a new one, theory is the new installer is given the installation or minor works which will say 13 amp, and when have you ever been given the installation certificate for a domestic, commercial yes, domestic very rare.

But should you even try to protect against those who follow getting it wrong? Like the ring final which is no longer a ring final but two radials, its not the installers fault if some one plays after it was installed.

However we have the division between installation and appliance, the installation with the exception of the lights does not cover current using equipment, the appliance is inspected and tested independent of the installation, two separate certificates. So personally I would want a FCU so even if some one in the future fits a 16 amp oven it is protected to 13 amp.


Not really required, it is a watch my back exercise, in my own home would not worry about the FCU, in some ones else's home, we look as what some jobs worth may put on an EICR, I was worried about some one picking up I had used too much cable and exceeded the volt drop, which is why I wrote the program to work out volt drop, however when I had written the program I realised the loop impedance meter is not accurate enough to prove one has made an error unless rather blatant. And once you leave you have no idea what others have done, so no real way to prove your fault.

It is like the really bad EICR, we know the inspector has skipped things, but unless he admits it, very hard to prove, court cases seem to have only found guilty where the inspector has admitted he got it wrong.

 

[winston1]

But 2.5mm T&E drops to 13.5 amps under reference method 103. Is this a factor in the above?


Presumably, their cable must be higher rating than 16 amps as it needs to carry the over current to the breaker before it trips?

They us 1.5mm2.

Sorry but can you elaborate on this. I understand how a load current can be pulling excessive current and how the MCB responds to this. I don't understand how the nail causes excessive current.

A nail across live and neutral or live and earth will draw excessive current, a short circuit.

 

[EFLImpudence]

The overload - until the breaker trips - is allowed for in the cable ratings.