RzB wrote:

Design current first.
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This is design current of the workshop. I'm not
using diversity because I am very clear about
what the max current will be and that it will occur.
What I don't know, is what to do about motor
startup currents. Do I have to take them into
account for Ib? Some of the machines
(Eg.. 3Kw Hammer Sawtable) have soft start
so I imagine the startup currents are handled by
the electronics in the machine. Others, like the
2.2Kw Axminster ADE4000 Dust Extractor
don't have soft start. Axminster have told me
that I should double the running current and add
another 15% in cold weather.

Individual motor circuits for all but tiny motors I presume? Ib can be
taken to the motor's full-load running current (FLC) as given on the
rating plate. That's certainly OK for cable sizing. It's only when you
get to really big motors (10s of kW) that you need to worry about the
effect of starting current on cables. That said, don't skimp on cable
size, it's not a major cost and keeping the impedance down helps with
starting.

MCBs
-------
My thinking is that I may need Type C MCBs in
the workshop CU. I am really gozzy on the
differences/benefits/requirements re Type B and
Type C. My only understanding (apart from the
various graphs/charts in the Regs) is that the Type C
is less sensitive to startup currents.

Type B will trip "instantaneously" at between 3 and 5 times In (the
nominal current). For type C it's 5 to 10 times and for type D 10 to 20.

How do I determine if Type C MCBs are needed?

As a starting points use type C for all motor circuits and type B for
your lighting, 13 A sockets circuit and for any electric heating. If
you experience motor-start tripping you can either increase the MCB size
by one step, or go to type D, after checking that the cable is s/c fault
protected (adiabatic equation). This assumes overload protection is
provided by an electronic or thermal overload device in the motor
starter, which should be the case.

Also, if I have Type Cs in the workshop, does that mean that I must
have a Type C for this circuit in the house CU.

The best option at the house end, discrimination wise) is an HRC fuse
(cartridge fuse) in a separate switch-fuse unit. (Tee into the meter
tails using a service connector block.)

If so I think I might have to run a separate (larger) CPC to satisfy
the Shock Protection requirements.

Total design current? Cable type, size and length? Is house earthing
PME or TN-S?

Thoughts?

Wire your workshop in conduit and trunking so that it's easy to make
mods. Changes will be inevitable over time. A good strategy is a ring
of trunking (say 50 x 50 mm) round the walls at high level, with conduit
drops to sockets, switches and motor starters, and upward runs to lighting.

13 A sockets should be on an RCD protected circuit(s).

--
Andy