what is the best way to understand which MCB to use?

[ravb]

Hi

what is the bet way to work out which msc to use? for example i know lighting is 6amp but how to work out radials or for ovens etc

thanks

 

[1john]

It has to satisfy the design current of the circuit, also the cable has to have a higher current carrying capacity than the over protective device after derating factors have been applied.

 

[PrenticeBoyofDerry]

You have three main type of socket circuits in domestic property
A1, Ring 30/32A protective device, 2.5mm twin&earth cable, Max floor area 100m2
A2, Radial 30/32A protective device, 4mm twin&earth cable, Max floor area 75m2
A3, Radial 20A protective device, 2.5mm twin&earth cable, Max floor area 50m2.
These are recognized standard socket circuits.

The basic calculation for the protective device size is.
power divided by voltage equals current.
Your protective device size can then be determined by using the one that is equal to or slightly above the current.

So for example you have a cooker that has 7 kilowatts output of power and a nominal voltage of 230 Volts, the design current will be 30.44A.
7000 Watts / 230 Volts = 30.44 Amps.
Your nearest protective device size is going to be 32 Amps.
The next stage is to find out what size cable is suitable to carry that current safely.

 

[ravb]

thank you so much. that is very clear.

 

[ban-all-sheds]

Re the last part of the above, the current that a cable can safely carry is affected by how it is installed.

And the stage after that is to check that voltage drop and the R1+R2 value for the cable won't be too high.

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  

And if you've got rewireable fuses rather than MCBs you have to derate all the cables by 0.725 because of their lack of sensitivity.

ravb - why do you want to know?

 

[ravb]

Thanks for the reply,

I am a security engineer and also i do do electrical too, but not as my main trade. but i have studied electrical at college and it was reallyt hard to understand especially when your doing in the evenings. so i wanted to find out the easiest way to determine which breakers to use.

i know about the R1 and R2 rules, and also checking for correct polarity before connecting to the board.

Its soemtimes better to learn by asking direct questions.

 

[Adam_151]

[/size][*] http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf

Looks like the original site is back up again

http://kevinboone.net/cableselection_web.html

Goodbye wayback machine

 

[londonboy]

I am a security engineer and also i do do electrical too,.....
.

I hope your not one of the many that shoves 0.5mm flex into the closest fuse/circuit breaker

 

[tonyelectric]

Hi ravb, there has been some sound advice given by previous posters - as others have said apart from calculating the right breaker for the load you need to know what cable to use, this can be a bit more tricky as long lengths of cable can't carry the same load as shorter ones of the same size due to the resistance of the cable, an effect known as voltage drop. The temperature that the cable is going to work at and how it is laid.

I would suggest you to get a copy of the 17th edition if for nothing else than the excellent tables in the back that will give you an instant answer to all these considerations (or photocopy the tables for twin and earth (which I guess applies the most here) from someone elses book!!!)

 

[PrenticeBoyofDerry]

or a least expensive purchase, would be the on site guide.
http://www.amazon.co.uk/IEE--site-G...8546/ref=sr_1_1?ie=UTF8&qid=1291898228&sr=8-1

 

[ravb]

I am a security engineer and also i do do electrical too,.....
.

I hope your not one of the many that shoves 0.5mm flex into the closest fuse/circuit breaker

NO, NEVER. we only use, 1.5 flex. from spur to panel. 3amp fuse.

 

[ravb]

thank you all, for the reply. i have the 16th edition, but 17th is new i know.

rav