What size flex 3 metres long for a 2.2kw 3 phase combi woodwork machine? What size cable to workshop (about 6 metres) to supply say max at one time 6kw 3 phase machines? Would this need to be armoured or just conduited or what? Separate single phase elsewhere, but could I run a spur from a 3phase wall socket to a single phase socket (same phase as other single phase?) or is this against the rules? I've got some sheathed and armoured 3 core cable. Does this need a separate earth for 3 phase use, or do you earth the sheathing, or both, or neither?
On 17 May 2004 06:49:23 -0700, in uk.d-i-y snipped-for-privacy@jpbutler.demon.co.uk (jacob) strung together this:
1.5mm2.
6 or 10mm2, 32A supply as a minimum.
Yep, any of those.
Nope, you need to run the three phase submain to a 3 phase distribution board and run you 3 phase and single phase circuits from there.
Both, but if you want to run single phase through it as well you'll need a 4th core for the neutral. Some three phase machines will also need a neutral.
Is this 3-phase installation at home ? Do you already have a 3-phase supply to the property ? If you don't already have 3-phase, are you thinking of using a converter from the single phase domestic supply ?
If you already have the right supply at the property, then it is best to make the outbuildings a totally separate sub-mains supply from an isolator switch at the head end. You will need to have the supply split to take this but it is will be worth it for your own safety and convenience.
If you think you're only going to run up to 6 kW running ampage, then you'll need to treble that amount to take the start up ampage of the motors on the machines all running together. This will also allow for any strain on the working motors under their normal use.
Taking all that into account, you're talking about a supply of 16 mm on each conductor for the lengths you're talking about, and you may also have to double the neutral to take the start up load until the motors are all running to speed.
With that supply it will allow you to run lighting and a few sockets from it as well.
Not any expert on 3 ph setups, but I was under the impression that you designed cable sizes for the working load and not the startup current since it is only a short duration transient?
On Mon, 17 May 2004 21:30:39 +0100, in uk.d-i-y John Rumm strung together this:
Quite right, any motor and most inductive loads will take a large surge at startup. The continuous circuit current is the one that determines the cable size. You would also provide motor rated MCB's or fuses to cope with the startup current.
Generally you would only allow for the motor start up surge but these motors will be under strained load conditions while they work and not just start up and run at a constant rate. Three phase gives you a motor that can keep torque even though it is slowing under tension loading. As an example, a lift motor can reach the same load rating while lifting a heavy bin as it does when it first starts dry. So you don't just rate loading on three phase commercial tooling machinery as you would with a static motor on say a ventilation system or the like.
"Lurch" wrote | John Rumm strung together this: | >Not any expert on 3 ph setups, but I was under the impression | >that you designed cable sizes for the working load and not | >the startup current since it is only a short duration transient? | Quite right, any motor and most inductive loads will take a large | surge at startup. The continuous circuit current is the one that | determines the cable size. You would also provide motor rated MCB's or | fuses to cope with the startup current.
Wouldn't you need to check voltage drop at the start current though, to ensure there's enough voltage to get the thing started (or does a starter controller take care of that)?
On Mon, 17 May 2004 23:07:33 +0100, in uk.d-i-y "Owain" strung together this:
Bit of both, a decent starter will drop out if the voltage is too low but if you've sized up the cable properly with all the correct volt drop calcs and the like then it shouldn't drop!
Personally, I would run it in 25mm, on the principle that it is easier to build in spare capacity at the beginning than it is to add it later. One of Murphy's Laws must be that you always want more power in a workshop than you first designed for.
Is chapel conversion with existing 3 phase supply. Will be single phase throughout except for connection to basement workshop. This is how it was before - the 3 phase only powering the organ blower.
6kw only occasionally likely, and simultaneous start-up of all machines very unlikely. Separate single phase circuit for lights and sockets. So 16mm sounds excessive to me. 10mm perhaps?
If you can be sure that the machines are going to be running individually from each other, then you might be able to run the supply in 10 mm csa' but if you are going to have one machine running on its own, then start another machine, then turn a heater on. Your talking about un-plugging one machine before you plug another one in to be sure that you're not going to overload the supply. Three phase has an awful reputation of kicking you in the ass if you don't get it right first time, remember.
Any imbalance on the loadings of the three phase conductors causes heat, this heat creates further imbalance, this further imbalance creates more heat, and so the cycle continues. The start-up ampage of some three phase quick start motors can jump to nearly five or six time the running ampage, so if one machine is under strain loading taking a slightly higher ampage to do its work, then you start another machine with a quick start motor, the ampage across the supply is huge.
Even if the load is for a few seconds at a time, it causes improperly sized conductors to degrade very quickly, so it's best to err on the safe side of caution when you're dealing with phased supplies. Believe me on this, I've been through it many times with industrial installations.
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