I am in danger of becoming the latest victim of "a little knowledge is
a dangerous thing".
I want to operate a 3-phase lathe and lighting in a new outbuilding
and am trying to sort out what I need to do in the face of apparently
conflicting advice. I suspect I do not have enough electrical theory
to understand what I am being told.
Basically, the outbuilding is 25m from the house mains supply point. I
think I need to run 3 core, 4sqmm armoured cable in a 450mm deep
trench with a warning strip buried at 150mm. So far so good.(I intend
to use 4mm as the extra expense over 2.5mm in negligible).
The 3hp lathe is to be driven by a 3kw inverter, the documentation for
which says it needs a 35A fuse.
I tried to work this through on (IMO excellent)K Zone at
http://www.kevinboone.com/domesticinstallations.html and a 35A fuse is
OK for the circuit.
The Regs, however require an RCD in the outbuilding. If I try the
calculation in K Zone again usine a 35A RCD, it tells me "NO", as the
RCD is above the "corrected nominal current". Same result with 16A RCD
So which RCD / fuse do I have to use?
Any advice respected. I intend to dig the trench myself but have a
contractor carry out the electrical work, I just want to understand.
On 20 Feb 2004 06:28:59 -0800, email@example.com (charles adams) wrote:
Remember that fuse/MCB ratings and RCD ratings are for different things - the
fuse/MCB rating is the
max continuous current before tripping. The RCD current rating (carrying capacity,
not trip current)
is the maximum it will break without damage, so on a circuit with a 35A fuse/MCB, you
to uprate the RCD to maybe 50 or 63A to ensure that a fault current that trips the
protection will not damage the RCD (e.g. weld the contacts together), compromising
In the case of a short-circuit (rather than overcurrent) fault, the
large current you mention will flow for only a very short time before
being cleared by any one of: the RCD itself (given L-to-E short), the
final circuit overcurrent device (MCB/fuse), or the service fuse. So the
total energy which flows through the RCD is limited (and it's not just
left to chance: this 'let-through energy' is calculated and taken into
account in the design of RCDs, consumer units, and supply fuses).
In the case of an *overload* - i.e. drawing more than the rated current,
but not massively more or in a way which causes an RCD to trip - the
extra load is sustained for quite a long time (given typical fusing
characteristics): for example, a 32A MCB will let a current of 40A flow
almost indefinitely (hours at least), and will take several minutes at
least to clear a 50A overload. So (at least when your loads are capable
of causing an overload, which motors sure as hell are!) you need to choose
the various elements in the circuit path appropriately. (Now, the people
who design RCDs for a living rate them in the knowledge of the characteristics
of protective devices they're likely to be used with, and a good-brand
'30A load' RCD is likely to be fine long-term with a load protected by a
32A MCB giving rise to occaisionaly overloads; but it's better practice,
especially as there's little difference in the price, to use an RCD with
a higher sustained-load rating than to push the margins).
Just because it's often the same device (fuse/MCB) which provides both
short-circuit and overload protection, doesn't mean they're not conceptually
separate, or that it'll always be the one device which does both...
Can I assume the inverter is in the outbuilding and you have a single phase
supply to it?
35A is an unusual rating. I suspect a C32A MCB would provide suitable
protection, although check with the manufacturer. As it will be pretty
efficient, its normal current draw (not peak) will be less than 3kW, so
there's plenty of diversity to get lights and sockets running too. However,
you could bump up the connection to 40A if you're worried.
For a single phase 32A connection over 25m, I make it 4mm minimum. For 40A,
6mm minimum. At the outbuilding, you'll want a consumer unit with RCD, and
I'd go for B/C32A for the inverter, B6A for lighting and B20A for some
sockets. The inverter will only pull more than a few amps or so when the
lathe is spinning up, so there is more headroom than you'd think at first.
Also, you need to check what the inverter really wants in the way of
protection. If they really do insist on fuses, you may need to choose a
consumer unit that will accept HRC fuse carriers and do it the way they say.
Wylex (and probably others) do DIN rail 35A HRC fuse carriers.
As this is intended for reasonably heavy use, you should use a split load
consumer unit. You replace the switch incomer with a time delay 100mA RCD
(63A/80A). The lighting goes off the "unprotected" side. Then you have a
fast 30mA RCD (63A/80A) for the socket circuit and the inverter.
The consumer unit side should all be single phase. Only the connection
between the inverter and the lathe should be 3 phase. However, the lathe
might require additional protection, in the form of a triple pole MCB (i.e.
6A) mounted in a separate enclosure. Read the lathe instructions and
inverter instructions to determine if additional protection is required.
You are obviously intending to install TT earthing. Make sure you measure
the resistance using a proper tester, not a multimeter! It must be below 200
Inverter manufacturers often are very specific about the required
protection (speed of intervention may well be an issue). Siemens may
well recommend the exact protection required, including a part number
for the Siemens component (I assume you mean 'Sinamics' G110?) - the
drives we use certainly do.
One thing to double check is that the inverter is happy for you to
switch the load side, either in fault conditions or just when stopping /
starting the lathe - certainly with older drives I have dealt with this
is a definite no-no, though the G110 may well be different. Comments
like 'automatic restart facility following power failure or fault' from
the Siemens website mean that you need to make sure the inverter is
setup right (yep, there are pages of parameters, most of which you can
leave at default).
Typically there will be restrictions on the maximum number of stops
/starts per hour as well.
A few more things that have occurred to me (you may already know them,
but just in case):
The Quick Start Guide for the Simanics G110 3KW drive specifies a 50A
fuse (part number 3NA3820). It also quotes an input current of 35A. The
2.2KW (3 HP) variant specifies an input current of 27.2A
A lot of inverters run with high earth leakage currents on startup
(particularly those with EMC filters built in). You may have problems
with nuisance tripping on the RCD.
I assume the motor on the lathe is dual voltage (ie 230VAC/400VAC, or
240VAC/415VAC as was)? It will need to be wired for 230V / 240V
operation, as most (all?) inverters will only change 1 phase to 3 phase,
they won't lift the incoming voltage from 230V to 400V.
I am sure it is obvious but just to clarify things like automatic
restart after fault must be disabled.
The 10A will be the continuous rating. When the motor is starting, it will
draw massively more current than this.
Best to use 6mm SWA. It may be hard to work with inside a building, but it
is easy enough to lay in a trench. Drop to 6mm T&E as soon as you enter the
buildings at either end.
6mm will give better voltage drop and more headroom for expansion.
It's an underestimate. 3 HP is about 2.2 kW. When you factor in the motor
efficiency (say 85%) and power factor (say 0.8), the circuit loading becomes
about 3.3 kVA. 15 amps is nearer the mark.
The inverter may well have a soft-start mode which will reduce the starting
current a fair bit.
I'd suggest a minimum of 10 mm^2 for this. It's a fairly long run, 25+
metres from the house, so 30 m of cable, perhaps. The lathe with a 3-horse
motor suggests that this is a fairly serious workshop installation. There's
probably going to a be at least a bench drill and a grinder, and maybe some
electric heating to consider too. A 45 A supply from the house (from a BS
1361 fuse) seems appropriate. The cable size is determined be the need to
keep the voltage drop low - you need a good stiff supply from the submain in
cases like this, leaving most of the voltage drop allowance avaialble to be
used in workshop wiring.
Firstly, thanks to those who have gone to a lot ot time and trouble to
The lathe in question has mechanically variable speed and is further
equipped with a clutch all of which mean that the motor will start in
isolation from mechanical loads. How much (if any) would this
alleviate the cable rating?
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