Fixed 110v installation advice - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

Fixed 110v installation advice

Hi all,

Just in the process of planning a 110v install in a small workshop.

Using a 2kva (centre tapped 55-0-55) wall mount transformer, first question is regarding the primary protection for the transformer, the primary current is around 9a but seem to be getting conflicting views regarding the appropriate MCB to feed the primary. Seem to see figures between 125% and 250% to allow for inrush - I was thinking a 20a D rated MCB should do the job but would appreciated thoughts on this!

The transformer will feed 8 (not all will be used at once - only a one person workshop) 16a commando sockets (radial). The transformer has no built in overcurrent protection on the secondary so was thinking of a small db with a double pole mcb. Even though the commando sockets are rated 16a is it suitable to use a 20a C rated mcb as it is a radial (and transformer output is around 18a)?

Thanks in advance for your input - not used to this 110v!

  • Problem with your suggestion is that Zs secondary will be restrictive in terms of cable length. Using C20 would result in a max secondary Zs of 0.227*1.09 which, if I am correct, would restrict a 2.5/2.5 circuit to around 16m. Anyway, quite what the short circuit characteristics of a 2KVA tx are, I could only guess but I can’t see 200A materialising. 

  • Indeed, I think,  but in a system with a maximum exposed fault voltage of 55RMS to ground you do not need 0,4 second shock safety disconnection, arguably you need none at all, as the voltage will probably drop below 50 during faults, and the thermal slow part of the breaker is enough for protection of cables. A 2KVA tx will probably droop 10% or less on full load, so 20A full load secondary and a 200A- 300A  PSSC is about right.

    If for some other reason you need prompt secondary side ADS, perhaps a salt water risk, then an RCD  is in order.


  • I had intended to use 4mm to connect up the sockets, longest run is about 12m (need to measure though to confirm!), however not sure if the commando sockets will allow for 4mm in and out in the terminals..

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