Running 2 x 3HP compressors ?

Hi guys, got a poser for the electrical guru's on the group.
I've currently got a 3hp compressor that is run off the garage electrics. From what i can tell the garage is supplied by a radial circuit, via a 20amp MCB along fairly hefty armoured cable. I've wired in a 16amp socket in the garage and this is what the compressor runs off..
I'm now looking at getting another compressor as I want to do some sandblasting and don't think my single unit is going to have enough oomph.(it's rated at 10 cfm FAD)
I'm guessing that I won't be able to run both compressors from the same circuit as if they both kick in at the same time then the draw is going to be too much for the circuit and something is going to give.
So the ideas I came up with to run the both at once would be
1. Swap the 20A MCB for a 32A - guessing this isn't really acceptable as don't know what the wiring is rated at.
2. Run an extension lead from the house ring main to the garage. The new compressor runs off a standard 13 amp socket. Is this likely to cause any problems ?
3. Try and set the compressors to kick in at slightly different pressures. Presumably it's just at start up that causes problems once going they should both run together OK ?
Any views or suggestions most welcome
Thanks
Jim
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Jim presented the following explanation :

Maximum running load of each will be around 2.5Kw, or roughly 10amps - though the initial start up surge could be much higher than this. Some/many have a soft start, where the motor is allowed to get up to speed before the compressor comes on load.

Correct!
You need to determine which circuit the garage is actually fed from, then you could run the 2nd unit from a different circuit.

With no pressure, both would attempt to start at the same time. Up to pressure with air being consumed, it would be very difficult to synchronise both to start at exactly the same point - so basically start one, then the other after which there would probably not be a problem providing they are not both on the 16amp MCB.
Method 4 would be to add a larger air receiver, if you could work in shorter bursts of activity.
Are you sure 20cfm is adequate for sand blasting?
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You will also need a thick extension cord. For a 13A extension cord, 1.25mm² conductors OK up to 12m long, 1.5mm² conductors OK up to 15m 2.5mm² conductors OK up to 25m (but 2.5mm² won't fit in a 13A plug).
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Andrew Gabriel

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Probably not. Do you know what you do need?
It may be that use of a larger receiver as an add on would achieve what you need unless it is for very long bursts of operation.
I'm not sure that combining two compressors together is a very good idea. You would probably be better off selling the one you have - which is more than ample for a lot of applications like nail gun use etc. - and getting a single larger one with bigger receiver.
I had the same issue, although with spraying, and went for a larger single product.

You could find that out. Measure the diameter of the cable and look up the type - probably 4,6 or 10 sqmm.
You can then refer to the tables for the cable type; the two factors being rating and volt drop.
http://www.tlc-direct.co.uk/Book/1.1.htm
is quite helpful.
You may find that you can upgrade the circuit.

This suggests that it's a smaller one?

There is also a rise in current as the pressure approaches cut off because the motor is working harder.
Also, pressure switches on typical compressors are not adjustable.

I would look at the single larger compressor and extra receiver options first.
Have a look at this site
www.thorite.co.uk and ask them for a catalogue.
--

.andy


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Jim wrote:

Also AIUI 16A sockets have to be protected on a 20A circuit, so you would need to install a mini consumer unit with 2 x 20A MBCs one for each socket.
Can you use a generator for the second compressor, especially if it's only going to be for occasional use. (Usual warnings about mixing generator and mains electricity apply.)
Owain
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On Sat, 28 Jan 2006 10:08:58 +0000, Owain

The trouble is that
£ { compressor 2 + generator } > £ { new compressor - sale of existing on Ebay }
Most likely.
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.andy


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Andy Hall wrote:

Yes.
But £ { compressor 2 + occasional hire of big genny } maybe < £ {dig up garden and fit new supply to shed } FSVO { occasional }
Owain
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Jim wrote:

you need to find out. Another possible may be swapping the B type mcb for a C or D type. If you do run 20A of compressors on a 20A rated supply, it might be worth using a battery backed-up worklight.

probably not, but beware that long thin leads can cause v drop that can cause induction compressors problems. This can be got round with either heavy cable or a step up transformer. A transformer providing say 5v at 10A is not large.

If youre stuck with low supply rating, you might also be able to make some use of the fact that the compressor loads are intermittent.
NT
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First, what is the actual current draw? Is that 3 'real' hp, or a much smaller figure? As a first cut, I'd stick in a 5A fuse, and see if it blows.
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On Fri, 27 Jan 2006 23:15:32 GMT, Jim wrote:

<snip>
Hmm, how soon before, quite co-incidentally, we have someone who lives close by complaining that he can see this odd sort of flickering and dimming that happens at all sorts of odd times on his lights.....
:-)
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Ah the "Wanderer" having ceased to post in other UK groups, after making wrongful accusations of child abuse, has moved on to pastures new!........................notice he/she is just as unhelpful on here as other groups that have in the past been blighted by his/her prescence!
You would be able to run a small blast cabinet without any problems using a 10CFM (FAD) compressor, but the compressor would run continously, and you would have to give it time to recover, so would need to blast in small bursts.
If you need to run a larger cabinet, or need to work continously then the way to go would be to get hold of a twin 3hp motor 240v compressor, with a start box which phased motor starts so they didnt start same time. One of these would provide about double the CFM you have at the moment, and would run a small cabinet just fine.
If you have a larger size industrial cabinet, then you need to be looking at 3 phase compressors, or cheaper option if you have the space and noise is no problem, would be diesel towable compressor, of the type used for road breaker drills and the like.
k
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On 29 Jan 2006 00:31:31 -0800, Ken wrote:

Post the message ID Ken. But you can't, can you, because I have never made such a post. Once again you resort to out and out lies to try and prolong your totally inept trolling attempts.
My apologies to the regular readers of this NG now this creep has found his way here.
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the dot wanderer at tesco dot net

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"Wanderer" is one of several posting IDs used by this prat! His/her most recent posts have been threats against someone, who has had the audacity to point out the fact that "Wandrerer" appears to be a liar and a coward!
k
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Ken wrote:

take it somewhere else.
NT
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I have posted a helpful reply to the OPs orginal question, perhaps you and "Wanderer" should attempt to do the same?
k
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Thanks for all the input guys. After reading all the comments I think the sensible option is to see how I get on with just the one compressor.
Thanks again
Jim
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You should be OK with just one 3hp compresser. I use a 1.5 hp compresser with mine. Works fine with small parts. I tend to blast for 10sec wait ten seconds and so on. If you do decide to upgrade your compresser then the size of receiver do'st realy matter, it's the CFM that matters.
Mike Cole
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Jim wrote:

Its really down to the type of blasting you want to do. If you have a small blast cabinet, and dont want to use it continously then a 3hp compressor will work fine. On the other hand if you need to do anything much more than that, you really do need more air.
k
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Perhaps if you were to offer some context, we might have a clue what you were going on about?
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