|
|
|
3 phase electrics |
|---|
| message from jacob on 17 May 2004 |
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?
thanks
Jacob
|
| Lurch replied to jacob on 17 May 2004 |
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.
|
| BigWallop replied to jacob on 17 May 2004 |
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.
|
| jacob replied to BigWallop on 18 May 2004 |
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?
thanks for info
Jacob
|
| BigWallop replied to jacob on 18 May 2004 |
<<<snipped>>>
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.
|
| John Rumm replied to BigWallop on 17 May 2004 |
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?
|
| BigWallop replied to John Rumm on 17 May 2004 |
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 replied to John Rumm on 17 May 2004 |
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.
|
| Lurch replied to Lurch on 17 May 2004 |
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!
|
| nightjar replied to BigWallop on 18 May 2004 |
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.
Colin Bignell
|
|
