According to all the information and things need to be taken into the consideration in terms of this 110V ring circuit I am not sure what to do now.

The customer in now willing to share any more info to me apart from the fact that the transformer is 5kVA and protection device they use for this circuit is 32A MCB type C

I am not going to argue with them as end of the day they are going to do the final connection to the LVAC unit so their expectation from my side is to fit 8 of 16A sockets into each room. I am going to use 6mm2 cable instead of 4mm2 to decrease VD on this distance. Do you also thing it would be a good idea to use 16A or 20A RCD sockets to protect every individual socket and have some discrimination as well?

Do you also thing it would be a good idea to use 16A or 20A RCD sockets to protect every individual socket and have some discrimination as well?

In general, if the sockets are fed from a 32A circuit, I'd certainly have 16A overcurrent protection at each outlet, so you're probably looking at sockets with a modular DIN rail enclosure built-in - in which case adding an RCD in there too (either as a separate device or an RCBO) might well make sense. That said, the shock risks from a 55-0-55V system are much reduced anyway, so there's much less of a need for 30mA RCDs than for 230V sockets.

But you probably first have to decide how the main circuit provides ADS (if you're going this to BS 7671 so need at maximum 5s disconnection time). Presuming it's a typical 110V CTE RLV system - i.e. 55V to Earth, 32A C type has to have Zs below 0.16Ω (BS 7671 table 41.6) - if you have a 6mm² copper c.p.c. as well as line conductor you're talking about a resistance of around 7.3mΩ per metre - so 100m worth divided by 4 gives 182.5mΩ or just over 0.18Ω for R1+R2 if you like - so you're over the limit even before you start thinking about adding on the transformer impedance and the L-N impedance of the 230V supply circuit (the equivalent of Ze or Zdb). If you used T&E with a reduced c.p.c. it would be even worse of course. So you might have to think about an RCD at the transformer end ... which then begs the question of do you need some at the sockets as well? Of course the supply end one needn't be 30mA instant type - it could be say a 300mA time delayed (S-type) so that 30mA units at the sockets could discriminate with it - but then your costs are increasing and there's a trade-off between capital costs service convenience to be considered. Or a radial per socket with 16A overcurrent protection (and perhaps an RCD) at the supply end might start looking attractive.

If you're going to do the work and they aren't supplying a detailed electrical design, then I'm afraid you're the designer and you'll have to do the maths. The OSG and many of the usual rules of thumb aren't much use for 110V circuits as they're all based on 230V and often the results are very very different.

   - Andy.

well then they need to spec the cable as well, if they are holding the design data.

by the way a "5KVA transformer" may mean one where the output has dropped by a few % at 5KVA not one rated for 24hour load at that level.
This example is 25% duty cycle, i.e. 5 minutes on / 15 minutes off.. and is more than 10% voltage drop at that load point.

The way transformers are specified application note

Mike.

the transformer is 5kVA and protection device they use for this circuit is 32A MCB type C

Is that MCB on the primary or secondary? (i.e, 230V side or 110V side?) - it seems a bit small for 5kVA at 110V and if on the primary might suggest they're expecting >>32A to be available to the 110V sockets in total (so maybe not a 32A ring then...)

   - Andy.

if its on the primary side, and a real (24 hour rated) 5000 watt unit, the first thing it will probably do if the transformer is half decent is to trip on the inrush when the mains is switched on as the inrush will probably be of order ~ 300 amps

ref.

If so will either need surgegard type current limiting arresters in series or maybe a D type breaker.

Mike.

if its on the primary side, and a real (24 hour rated) 5000 watt unit, the first thing it will probably do if the transformer is half decent is to trip on the inrush when the mains is switched on as the inrush will probably be of order ~ 300 amps

ref.

If so will either need surgegard type current limiting arresters in series or maybe a D type breaker.

Mike.