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!

  • you are right to worry about inrush. A C or D breaker wil be needed, and even then, it may be worth adding a surge limiter in series - I have done this inside the box  on a bit of ceramic choc bloc in the past..

    This sort of thing, but selecting the right one needs a bit of thought. At room temp they add some ohms and over about half a second once power is  on they heat up to offer a far lower resistance. They do need to be kept away from plastics though.

    20A breaker feeding any no of 16A sockets is fine. The hard part is finding one that senses and breaks on both poles. If you want RCD cover on the 110V side, and you may well,  then be aware that models of RCD or RCBO with built in electronics sold for 230V may not work on 110, and the self test may not pass enough current to do the needful.


  • 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 suggest you buy and install a bigger transformer for handheld tools, my portable transformers are 3 KVA and I would go bigger than that for a fixed transformer in a workshop with a dedicated circuit.

    Based on experience a 3 KVA transformer should be okay on a B32, it’s okay in my garage.

  • Don’t forget these transformers have a duty cycle, typically 

    • Intermittent Rating: maximum rating for operating load at 25% duty cycle, i.e. 5 minutes on / 15 minutes off

    so you may only be able to use the transformer you are proposing to install for three 5 minute sessions in a hour, rendering it unfit for purpose.

  • The transformer is rated for 2kva continuous so should be fine for a one person workshop - just using grinders, drills etc, nothing big.

  • Thanks Mike, very interesting, I will check it out!

  • 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..

  • I have an older version of this Festool dust extractor which runs at around 1200 watts alongside the power tool being used, which I would have to take into account.

  • I second sparkingchip's advice

    we use 5kVa transformers to run 2.2kW grinders, anything less and they start to trigger the thermal overload, which takes ages to reset.

    If they use a shop vac on it, it will definitely trip out in fairly short order. 

    Screwfix seem to be the cheapest way to get Carrol & Meynell transformers

    the transformers have a continuous rating that is half of the sticker rating don't forget