3ph run to garage.

1 Oct 2019
Reaction score
United Kingdom

After the great response to my 3ph supply question I'm looking to run swa armour to my garage.

Meter to garage 20m then another 20meters to compressor.

Load will be a 30amp Clarke compressor.

Electric supply company will connect to an isolator external to my external meter box.

Compressor requires 3x core and earth.

Online cable calculator sates 4mm 4x core SWA.

SWA will be externally clipped to house wall then internally clipped to garage wall.

32amp isolator.

1x way consumer unit in the garage, then another 32amp isolator by the compressor.

My electrician is having his first child so drooped off the face of the earth along with sleep deprivation .. :)

Just sold my single phase compressor and need to get air back up and running asap.

I want to do the first fix myself so he can hit the ground running.


Sponsored Links
This is the problem with female electricians, should use male ones, they don't have babies. But I see a number of problems, first is 4 mm² is not going to take a 100 amp supply, so some where some how at the house you will need an overload.

Next way is a poor description we normally refer to module, but I am reasonable sure you will need at least 4 module wide for 3 phases and neutral, and will not be a consumer unit, as they are single phase not three phase, you may be able to utilise the box, and use the DIN rail mount, but three phase normally looks like this
OK this one has an isolator in it not a MCB or RCBO but it is not called a consumer unit.

So step one is design, where is the over current device, house or garage, and what type, some times we use an overload like this
unlike a MCB it can be set to the motor size, you can see adjustment top left, but you looking at design, inrush, and it is not as simple as just plug it in, and to be frank I have had a lot of problems with compressors, they are not just a simple motor, and how it de-loads or starts will alter how it is supplied. You can use a soft start or inverter, but then the deloading has to be tied to it.

She will clearly not be returning to work for some time, so seems you need another electrician.
Hmm, some sexism going on here ? The OP did say sparky having his first child :rolleyes:

But I'll second what Ericmark says, compressor can be a right PITA to start. Many years ago (and long before I really knew what I was doing :whistle:) I helped a mate when he bought a new compressor - 2off 3kW 2pole motors driving separate compressors that started with a time lag. Oh what fun we had :mad:
Domestic supply to Wylex fuseboard in house, 30A wire fuse for garage supply, either 10 or 6mm T&E (I think it was 10) to garage (maybe 10m run), and another Wylex fuseboard. Ran 2x2.5mm T&Es for the compressor off a 30A fuse (yeah, some will be ahead of me here). If hot, and the receiver empty, it would fire up - one unit first, then a few seconds later (pneumatic delay on contactor in control box) the second would fire up. But from cold and with pressure in the receiver, it would more usually go : first unit fires up, second unit starts humming and creeps round a bit before the fuse blows :evil: The difference being that when the second unit fires up, the supply is already carrying the running current for the first unit. Of course, not just one fuse, but both 30A fuses in series - so a trip to the house (in the dark) to replace that one as well. The house end quickly got upgraded to "35A" :whistle: which fixed that problem.
Machine Mart sent out someone to look at it, he fitted soft start valves* and after that it was much better - although it could hiss a LOT until there was a bit of pressure in the receiver and the valves shut.
It also improved after being used for a while, once the compressors were "run in" a bit. It was for grit blasting, and we could keep it running continuously for as long as it took to empty the grit pot.

The fundamental problem is that right when you need full voltage to get the torque for the motor to start up while also spinning up the inertia connected to it, and against back pressure - the high current causes a dip in voltage, and that slows the acceleration which prolongs the high-current startup period and gives fuses more excuse to blow. In some cases (as above) it may make the difference between starting and not starting at all if the starting torque isn't high enough to overcome the mechanical load attached.

* Soft Start Valve - like a poppet valve fitted in the line between compressor and the non-return valve to receiver. When the compressor is starting, it lets air out of the line and reduces the back pressure for a few seconds before it pops out and closes.

OK, so back to the OPs cable question. Without doing any calcs, a combination of things stand out - 40m run, 30A motor (I assume that's full load running current) driving a difficult load for starting, 4mm² cable. That to me sounds like a recipe for excessive volt drop during starting and starting problems.
Ah, now I've read the other thread, it's not 30A, it's 7.5hp which by my fuzzy calculations is only 7.5A/ph FLC - rather a bit of difference there. But still starting DOL which will mean a startup current that could easily be between 50 and 100A - IIRC a quick rule of thumb says something like up to 10 running current during startup. If the compressor doesn't come with unloading during startup then you probably want to add it - it will help considerably with both reliable starting and minimising the time the neighbours' lights dim each time it starts :D
Do you have agreement from the DNO to start such a large load ? Probably not, and in extreme if neighbours complain about voltage dips you can be told the stop doing it - or face being disconnected.

And as to the supply questions. As mentioned, you'll need to split the supply (set of "Henly" blocks), fit an isolator and overload protection (i.e. MCB in a case, or maybe a switch-fuse which offers better discrimination), and only then does the cable connect to that. At the other end, you'd split out 1 phase for all the single phase stuff. You may want to reconsider 30A & 4mm² cable - you might find it limiting as to the single phase stuff you could run. You'll need your electrician to do the design, it's not just current carrying capacity of the cable (which depends on cable construction and installation method) - there's volt drop, prospective fault current and let through energy (c.f. adiabatic equation regarding heating of the cable before the protection trips), fault impedance (to ensure timely tripping), discrimination between protections (see above about my mates problems with two 30A fuses in series), earthing (you may need 5 core from house to workshop so as to have a neutral as the wire armour probably won't be adequate as the CPC), ...
There's an iterative element to this. Making educated estimates as to cable size, then doing the sums and see if it works - adjusting the cable size to suit and going back to the beginning till you have a solution that meets all the requirements.
Sponsored Links
  • 30cfm displacement - twin cylinder, 2-stage compressor pump
  • 270 litre horizontal air receiver (CE certified)
  • 7.5hp motor, 400V / 3ph continuously rated with overload protection
  • 10bar (150psi) maximum working pressure controlled by a fully automatic pressure switch.
  • Compressor pump has fitted inter & after coolers to provide cooler, drier air
  • Specification also includes safety valve, pressure gauge, air outlet valve & drain tap.
  • With a wired in starter (WIS)
  • Dimensions (LWH) 1600x520x1140mm - Weight: 140kg
Hi Guys, Gals, transgender, gay, straight, Bi, transsexual, or just plane confused like me.. :)

Female trade persons..... ! I'd rather see a girls builders bum than the hairy arsed heating engineers I work with..

Yes, my electrician is a bloke expecting his first child, above is the spec I have for my compressor.

I already have single phase running in my garage so no neutral required I believe, if 40m is going to be a problem I can site the compressor by the doors so more like 20m.

Consumer unit / distribution board / BIG turney off switch thing either end is what I attempted to say in my original post.. :)

In the spec it states (WIS) will this cover start up loads..?

The 30amp was taken from the manual supplied but I left this with my electrician.

Just want to run the swa, mount the isolators each end so my sparks can waltz in with as little to do as possible so he can get back to his new worry / panic / joy of his life asap.

Just to add to this, sold my single phase compressor today so now need to get some air online asap..

Do you have agreement from the DNO to start such a large load ? Probably not, and in extreme if neighbours complain about voltage dips you can be told the stop doing it - or face being disconnected.

Have I gone over the top with the size of the compressor..???

Thanks to one and all..

I already have single phase running in my garage so no neutral required I believe.
Your electrician can explain which bits of BS7671 that would be non-compliant with. Hint: You'll have two supplies with two different means of isolation - which isn't allowed in a domestic environment.
Have I gone over the top with the size of the compressor..???
You're the one with the requirement for compressed air - only you can answer that.
"You're the one with the requirement for compressed air - only you can answer that".

mmm.. not really from an electrical supply side, I require around 30cfm of air.

I found out I have 3ph coming into my cottage so started to see if I could utilise it to satisfy my air requirements.

I could have bought a 1ph compressor to satisfy my air requirements, but I believe 3ph would be the better option.

So I need to know now if it is possible to run the compressor I have bought without putting such a silly amount of draw on the supply I have that it could affect my neighbours.

BS 7671 kills it dead in the water any way no..? Or I scrap the existing single phase feed and connect the lights & sockets to the 3ph so I would now require the neutral ...?



Do you literally have a TP&N head and meter in your external meter cabinet and nothing else at all for the 3 phase installed yet?

If you could post a couple of photos it would be a good help so we can see what you’re up against.

I have spoken to Igloo who are happy to connect to an external isolation switch.

If the compressor is outside my garage in another purpose built shed would this satisfy BS7671..?


  • With a wired in starter (WIS)..... is this going to be a an asset..?
The WIS makes little difference - it just means you don't need to buy a starter (contactor plus overload in a box) to screw to the wall alongside the isolator at the load end of the cable.
IMO, the ideal approach would be to run the garage from the 3Ph supply and disconnect the existing 1Ph supply - that way all your garage stuff comes off the same supply. Once you have 3Ph in the garage, you'll suddenly find that there are more options for equipment - so if you did just run a supply for the compressor, you might find yourself "a bit annoyed" not to have made provision to use it in the garage.
What you could do is run a 5 core to the garage, then a 4 core to the compressor shed, and just connect them in a box in the garage. Size the 5 core according to what you might want in the garage, and the 4 core to suit the compressor, and fit protection at the meter end to suit the smaller cable. You've then got the option to upgrade your garage if/when you decide you want/need it without having to replace the cable.
Hi Simon,

yes I had thought about more equipment in the future. Boys and their toys.

this does bring me back to taking too much of the supply coming in at the meter though. There are three 60amp fuses at the moment Could these be upgraded to 100’s & we also have a 30amp AGA.

Obviously I don’t want to upset any neighbours too.

So running a 5 x core 10mm swa would satisfy the compressors needs with the option to add More machinery at a later date.

mill. Lathe. Spindle moulder. Tig. Just joking.

I’ll probably only use the compressor in truth.


Like most things, it's a balancing act between spending a little more now and getting more flexibility later. If you think wanting 3Ph in the garage is "likely" then the extra cost of a larger 5 core cable now will be less than the cost of replacing the smaller 4 core cable later. But if you never use that 3Ph in the garage, then that extra cost would be wasted.

As to using too much of your supply, well you only use what you are actually using. One thing you would do is make sure that the phase you tap off in the garage for your 1Ph loads is one that isn't used in the house. That way, your house loads 2 of the fuses, the garage loads the other one, while your 3Ph loads load all 3 fuses equally. So your 1Ph loads in the garage would not "compete" with the Aga for fuse capacity.

3Ph items take less current - because the load is split between the 3 phases. Your 7.5kW 3Ph motor will only take about of 10A on 3 phase, but if it were single phase (not that you'd ever get away with starting one of those :whistle:) would take about 30A.
If you're into welding, you may be interested to know that some larger welders can use 415V (across 2 phases of your supply) instead of 240V. Doing so would cut the current draw by a factor of 1.7. Some years ago a friend of mine rented a small industrial unit for his DIY doings instead of an open draughty barn at the farm - he commented that when he connected his welder (one of those big oil filled things) across 2 phases of a 3 phase supply, it ran very much better. I'd hazard a guess that it dimmed the lights less in the surrounding houses as well :D Probably applies to other stuff as well, but welders are the most common high current devices you might use.

The only thing I can add is that I am envious :mrgreen: Most of us don't have 3Ph available unless we want to stump up a lot of cash for the DNO to dig up the street and put us a new service cable in. So many things are just so much easier or just plain cheaper with 3Ph available.
Hi Simon,

Thank you for input, as the compressor runs total 30amp which is split into three I calculate the cable size on 10amp not the full 30amp..?

Doncaster Cable online calculator.

Multicore 90°C Armoured Thermosetting Insulated Cables
Tables Apply to: H6942XL, H6943XL, H6944XL, H6945XL, Tuff Sheath,

Voltage: 400V

Load: 5.54kW / 10A

Length: 40m

Method of Installation: Clipped direct

Cable Size: 1.5mm²

Doncaster Cable online calculator.

So if I run say 1 6mm 5x core happy days...??




DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links