4hp compressor and power supply

[SteveDon'tKnow]

Looking at buying this:

http://www.costco.co.uk/view/product/uk_catalog/cos_8,cos_8.1,cos_8.1.2/153416?referer=carousel

• Power: 230v (18amp)

• Motor: 3kw (4.0hp)

What would be the ideal power source for it, a 20amp or 35amp connection?

I'm trying to plan electrics going into my garage, just moved into the place and am utterly clueless.

 

[SteveDon'tKnow]

Hmm. Reading up on it, sellers are saying it's an 18amp motor that will run off a 16amp supply.

I'm confused.

 

[Petefeet]

hmmm - i think we can assume its a 3kw motor which is 3000 watts. I recall that power = volts times amps, changing the formula around we know the watts (ie the power) and we know the volts (240v) so the current draw should be 12.5amps - with a spike on startup i would have thought. I had the same problem in my garage running two compressors (one for spraying and one for air ) and the circuit would trip on the 16amp MCB. Changing it to a 32amp MCB (its via decent 2.5m HITUF cable) not had a problem and nothing has burst into flames either!

 

[Sidewinder]

OP,
To be honest, I would expect it to run OK off a C16 breaker, though it may need a D and even up to a D20, I would change the curve before the rating as long as your earth fault loop impedance is OK to ensure that you get automatic disconnection of supply in the event of a fault.

Petefeet, the nominal voltage in the UK is no longer 240 Volts, no matter what is measured!
All design calculations for electrical installations to BS7671 & machinery, must now be undertaken using 230V.
Another feat of EU harmonisation that has resulted in...
I won't go there.

By the way, just for you to know 2.5mm sq hituf is not rated at 32A, only 25A, it will run fine, until it goes wrong!
The reason that we have cable ratings and protective devices in the fixed installation is to protect the cable in the event of a fault/overload.
A 32A breaker will not protect the 2.5mm sq Hi-Tuff you have connected to it, so in the event of a fault, then it could catch fire.
Under normal load conditions then it is doubtful that the cable sees its full current carrying capacity due to diversity.
The only thing you need to be careful of in your own home, is that IF something goes wrong, then your house insurer will do anything they can to wriggle out of paying these days!
They are a pain.
Just be careful, please.
If it is in a work scenario then a circuit like this would be in contravention of EAWR.

Oh, and Ohms law does not quite work with a motor as it is not a resistive load, it has "efficiency" and "power factor" so it will always draw more current than a simple Ohms law calculation will identify, when delivering full power to the load.
It is rare however for motors to run at the full rated load for long, especially on a simple air compressor such as the one in the OP, they need to be oversized for the running load to cope with the required starting torque.

 

[SteveDon'tKnow]

Here it is running, assembled in Italy but Chinese motor. Bit noisy?

 

[Petefeet]

i stand corrected mr sidewinder! my schoolboy recollections are not really a substitute for being a pro i know, but i'm thinking that each compressor is starting independently so hopefully the starting surge rarely coincides, and each is plugged in via a 13amp socket and fused to 13amp which should protect the cable. And the cable is underground thereby offering cooling.....

what with the trip hazards, the 2k paint, the powertools and now the firerisk, its a wonder i am still here!

seriously - thanks for the advice; we are all here to learn and i know i need to watch it!
ta
pete

 

[Sidewinder]

Pete,
Actually...
Buried cable sometimes must be de-rated!

Only one comment, when I left school I worked in a motor factors before I started my apprenticeship, one of the customers swapped over to 2k paint, his sprayer was dead in a week!

Can't remember why, however, the isocyanate in 2k paint, unless it is isocyanate free, is a known carcinogen, the risk is known and present, it is just the dose and the exposure to worry about, as I said, please, just be careful and look after yourself.

As far as the trip hazards go, look where you are bloody going!