# single phase compressor

#### Bish2

The power supply to my Garage is via 2x100ft lengths of 2.5mm Armoured cable Terminating at a second consumer unit . One line supplies power for six lights , the 2nd supplies power for 4 socket outlets for light power tools.
I would like to connect up my new 3hp single phase compressor which is fused at 30amps.
My question is;- is the existing wire 2.5mm suitable with a 32amp mcb if not, could i utilise both power cables running them from a single 30amp fuse at the household consumer unit to a single 30amp mcb in the garage effectively two cables becoming one and would it effectively reduce the loading on each cable
I would really appreciate some advice as to the correct route to take.

Bish2

as long as the cables are the same length and are run together following the same route, then they meet the requirements for parrallel conductors, and you can combine the current rateings of them, obviously after that you have to correct for cable grouping, without doing the calculations I can't see any problem with using the cables like this on a 32A breaker, and you might be able to use a 40A, but things are maginal around there, actually no, you'll have a hard enough time keeping the volt drop in spec with a 32A breaker at that length, remember to check your Z's as well, and a 32A breaker won't discriminate with another of the same, oh and I'd use C types for both the submain and the motor circuit

as adam say you can double them up and use a 32A MCB. but be aware that whilst the compressor is runnin, you will have extremely limited power.(i.e 1 or 2 lights, no sockets). voltage drop should be around 10V so you should be OK for that. you could run it from 1 of the armoureds but youd be OTT on voltage drop

Why is the compressor fused at 30A when it is a mere 3hp which is 2230W,(1 HP = 743.6w) which corresponds to a current of 9.7A?

The protection for this machine should be by a 10A Type C MCB, NOT a 32A MCB.

andy said:
as adam say you can double them up and use a 32A MCB. but be aware that whilst the compressor is runnin, you will have extremely limited power.(i.e 1 or 2 lights, no sockets). voltage drop should be around 10V so you should be OK for that. you could run it from 1 of the armoureds but youd be OTT on voltage drop

Incorrect. The current used by the Compressor is 9.6A, so say 10A, the Volt drop per ampere per meter for 2.5mm SWA in single phase in 18mV

So. 18*10*35(length of run)/ 1000 = 6.3V. The permissable maximum is 4% = 9.2V

With both cables in parallel and assuming 30A loading you get

(18*30*35/1000)/2 = 9.45V, so this would be outside the 4% limit in the Regs for compliance of volt drop.

However the compressor, as the first calculation shows, will clearly run from a single 2.5mm SWA feed.

for some reason i thought it was 30A...

The compressor is a clarke se16c 150litre light industrial. Machine Mart Tech dept informed me it was fused at 30amps.The Clarke instruction booklet quotes 30amp.
Phoned Clarke International,told them what you guys had said, their reply was and i quote:

" Perhaps 30amps is a little over enthusiastic , try a 10amp D MCB"

Incredible !

Big Spark, Andy, Adam i am indebted to you

Many thanks

Bish2

Bish2 said:
The compressor is a clarke se16c 150litre light industrial. Machine Mart Tech dept informed me it was fused at 30amps.The Clarke instruction booklet quotes 30amp.
Phoned Clarke International,told them what you guys had said, their reply was and i quote:

" Perhaps 30amps is a little over enthusiastic , try a 10amp D MCB"

Incredible !

Big Spark, Andy, Adam i am indebted to you

Many thanks

Bish2

you should go down the route described as big spark, altho a D seems a little OTT. i would go with C10 (can take the initial surge)

Bish2 said:
:

" Perhaps 30amps is a little over enthusiastic , try a 10amp D MCB"

Not sure D type breakers are readily available for consumer units, and even if they are, you are going to have to be very careful with your earth fault loop inpedance, i'd suggest that a C10 might be more appropiate (still have to watch your ELFI, but it doesn't be so hard as with a D type)

*kicks himself for not twigging the hp thing like big spark did*

*kicks himself for not twigging the hp thing like big spark did*

me2.

C10 ELI: 1.92
D10 ELI: 0.96

due to length, and supply Ze, you might be close with the D10. but still, C10 will be best

D Breakers are not rated for use in domestic boards, they are purely for industrial applications that require momentary heavy loads at start-up and are usually used to replace BS88 and BS1361 type HRC or even industrial size BS3036 rewirable fuses when controlling motors that startup under load conditions, such as lifts, fire pumps, hydraulic systems etc.

Whilst the compressor may be starting under a load, it is minimal, and there is no need to use a type D MCB for this. The Type C will be sufficient.

Just connected and run the compressor without any problem.
Big Spark,Andy, Adam thanks again for all the advice.

Bish2

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