Bigger MCB/CircuitBreaker

[LeeBuckman]

Hi. Can anyone help me please. I have a separate circuit for my garage which has on it 1 x double socket and some garage lighting also. i.e when it trips out, both the lights and power sockets go off.

I have a 3HP (2.2KW) air compressor that runs 95% of the time untroubled. However, when it gets low on air and the Motor kicks back in the surge of power sometimes (20% of the time maybe) causes the MCB to trip out on my consumer unit.

The MCB is a B16 (I assume it means 16A). Is it ok and SAFE to swop out the 16A MCB for a 20A MCB, or even a 32A MCB.


I need to check but i think the wire is 2.5mm T & E.

Please advise.

Regards

Lee

 

[PrenticeBoyofDerry]

When your compressor starts to surge, there will be extra demand on the circuit which is causing your MCB to trip.
If the circuit is a radial, you will be limited to the size of MCB you can protect the circuit with.
2.5mm t&e can carry a maximum of 27A, providing there are no de-rating factors such as thermal insulation along the route of the cable and voltage drop this will depend on the length of the cable being used.
So if there are no de-rating factors go for a 20A or even 25A MCB but not a 32A.
If the circuit is a ring final circuit then 32A may be possible.

 

[JohnW2]

So if there are no de-rating factors go for a 20A or even 25A MCB but not a 32A.

If the circuit will support it (Zs-wise), which it probably would, might not changing to a Type C (or even Type D) MCB be another possible solution? I guess it partially depends upon how long the 'surges' last.

Kind Regards, John.

 

[ban-all-sheds]

• 
  
  
  
• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm (all subsections)
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• //www.diynot.com/wiki/electrics:mcb2
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm
  
  
• //www.diynot.com/wiki/electrics:part-p
  

 

[LeeBuckman]

When your compressor starts to surge, there will be extra demand on the circuit which is causing your MCB to trip.
If the circuit is a radial, you will be limited to the size of MCB you can protect the circuit with.
2.5mm t&e can carry a maximum of 27A, providing there are no de-rating factors such as thermal insulation along the route of the cable and voltage drop this will depend on the length of the cable being used.
So if there are no de-rating factors go for a 20A or even 25A MCB but not a 32A.
If the circuit is a ring final circuit then 32A may be possible.

Thanks for the replies guys. Please bear in mind i know very little on this matter.

What do you mean by radial, consumer unit to the sockets and right back to the consumer unit again ?

What is meant by thermal insulation ? - The cable i believe is just in its sleeve and in a plastic covering in the gareg and routed through the kitchen into the garage, about 6-8m in length.

Sorry for so many questions but if you could explain a little further that would help me i guess.

Thanks Again

Lee

 

[ban-all-sheds]

What do you mean by radial, consumer unit to the sockets and right back to the consumer unit again ?

No - that's a ring.

A radial does not return to the CU.

http://www.tlc-direct.co.uk/Book/6.3.2.htm

http://www.tlc-direct.co.uk/Book/6.3.3.htm

What is meant by thermal insulation ?

Kingspan, Celotex, Rockwool...

The stuff between joists in floors/ceilings, and inside partition walls.

http://www.tlc-direct.co.uk/Book/4.3.6.htm

 

[LeeBuckman]

Ok thanks.

I understand what you mean by these things but don't know enough to be able to calculate what is right or wrong. I guess it's a case of getting a professional out to advise unfortunately for me.

I was hoping most people would say a 20A would be fine. I was also adised myabe to change to a dfifferent rated MCD ie. to a D but again don't know what the difference is between them.

Cheers

Lee

 

[ban-all-sheds]

I was hoping most people would say a 20A would be fine.

Without knowing the size of the cable, the length, how it's installed etc, it's not possible to be certain.

I was also adised myabe to change to a dfifferent rated MCD ie. to a D

No chance that a Type D would be OK for a socket circuit.

but again don't know what the difference is between them.

• //www.diynot.com/wiki/electrics:mcb2

 

[JohnW2]

I was also adised myabe to change to a dfifferent rated MCD ie. to a D

No chance that a Type D would be OK for a socket circuit

As you know, I was airing the possibility of a Type C ('or possibly even type D'), albeit without looking up the figures. Having now 'looked up', a C16 could well be feasible, with a max Zs of 1.44Ω for 0.4 secs - but, as you say, a D16 would be a no-goer, with a max of 0.72Ω for 0.4 secs.

Kind Regards, John.