The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 11 replies
  • Subscribers 31 subscribers
  • Views 2270 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

electric showers

Former Community Member
Former Community Member
Hello again, following on from the recent topic that discussed sizing electrical supplies to fixed equipment we have been asked to install 4 number 9KW electric showers into the same commercial premises as my earlier question on RCD protection to lighting.

Designer has asked for a 250A SPN sub board, (3 phase supply is available) and 4 x 60A SPN supplies to the showers with 60A DP isolators. When I checked the model MI they state a 40A supply is required. I queried this with the designer and his answer is that the showers draw 39A so wants the 60A supplies. Apart from the fact I can't find a decorative DP switch rated at 60A, am I missing something in that this feels unnecessary. No cable sizes have been discussed yet, 

Showers are on an office floor with 200 or so desks so assuming similar numbers of staff although not convinced that all staff will be lining up to use the showers on a daily basis. 

Again thanks in advance for any thoughts.
  • 0
    Presumably these are domestic electric showers that may be installed in a house or flat that has a 60-amp supply for everything, don't designers ever look around their own homes or do they live in a office?


    Andy B.
  • 0
    It does sound somewhat oversized for four 40A loads. 

    If the intention is that the RCBOs or MCBs + RCDs are all side by side, then it is not unusual to ue a 50A device for a 40A load, to reduce problems with one breaker heating its neighbours and skewing the thermal trip threshold.  The better way to do this is not to put fully loaded breakers side by side, but to leave a gap or to  alternate with light loads.


    Is the intention to add additional loads in the future ? If not then a 80A 3 phases would cover it better, and still have capacity for 6 showers.
  • 0
    No harm in over specifying the shower isolators, and arguably a good idea if remembering the failure rate of fully loaded shower switches. I would consider a rotary switch that can be padlocked off for safe maintenance.

    The OCPD should however be correctly rated according to the instructions with the shower, probably 40 or 45 amps.

    Electric showers can result in earth leakage currents, and it would be preferable in my view NOT to add showers to any existing RCD, for fear of total leakage currents tripping an RCD. Use a dedicated RCBO for each shower.


    Do double check that enough power is available to add a load of about 80 amps to one phase, and 40 amps to the other phases.

    Most office showers see relatively little use, but the installation should not rely on this. Heavy and repeated use is certainly possible. I recall an unfortunate episode during the great Dartford power failure some years ago.

    A multi tenanted office building had a 315 amp 3 phase service. The electric showers added by each tenant added up to about 500 amps per phase, not normally a problem with limited use of each shower.

    Due to the general power cut in the area, which did NOT affect this building, there was a queue for showers. The cut out fuses operated, repeatedly.
  • 0
    mapj1:

    It does sound somewhat oversized for four 40A loads. 

    If the intention is that the RCBOs or MCBs + RCDs are all side by side, then it is not unusual to ue a 50A device for a 40A load, to reduce problems with one breaker heating its neighbours and skewing the thermal trip threshold.  The better way to do this is not to put fully loaded breakers side by side, but to leave a gap or to  alternate with light loads.


    Is the intention to add additional loads in the future ? If not then a 80A 3 phases would cover it better, and still have capacity for 6 showers.


    Depends on how many sweaty cyclists there are in the mornings.


    Andy B.


  • Former Community Member
    0 Former Community Member
    BusterK:

    . Apart from the fact I can't find a decorative DP switch rated at 60A, am I missing something in that this feels unnecessary. No cable sizes have been discussed yet, 

     


    Hi buster k, why would you require a decorative 60A shower switch, is this listed in the specifications? 


  • Former Community Member
    0 Former Community Member
    All good points, this is an outline specification we have received and I know there is going to be an element of contractor design so was testing the water so to speak with the designer. 

    If we're going with the 60A supplies it may lead to the circuit cable being oversized for the shower's terminals if we have to use 16mm instead of 10mm. I can't see omitting overload protection due to shower not being able to draw more than 39A being accepted so would need to size the cable for the OCPD.  I have suggested using a 4 way 3 phase DB rather than single phase, that way we can connect the RCBOs 1L1, 2L2, 3L3 & then 4L1 giving plenty of room for heat dissipation if the showers are continually used. I don't know if the shower unit themselves would be capable of continuous use. 

    By decorative DP switch I meant a wall mounted flush unit rather like 1 used for a cooker (with no 13A socket) rather than having to fit a rotary isolator, I know MK used to do a 50A version so that on a 40A supply would seem to be more suitable to me, just was wondering if I had missed something on their response of current draw is 39A so install 60A supply.
  • 0
    Put them on 40 A final circuits. And do it with the three-phase board: better balance, less copper, room for two more, altogether neater, and maximum 80 A per phase. 


    The only reason I could see for wanting higher-rated circuits would be if an upgrade to e.g. 10.5 kW showers is considered, or the "9 kW" showers are really rated 9.6 kW and the designer insists on working with I = 9.6 kW / 230 V (in the days of 240 V it would have been just ok). 


    Perhaps (thinking of another thread recently, about ovens) the shower manufacturer would even require a lower protection rating than 60 A. 


    Try this example of an installation manual (manufacturer:pdf,  archived:pdf)  - see the top of page 7, where it sensibly declares 40 A as ok for the 9.5 kW rating. This implies that the power rating is at 240 V, and given that when fully on it's basically a resistor, a 230 V supply will take even less current.  It doesn't say "must be connected to a 40 A MCB, no more and no less": but I feel that connecting to 63 A would be harder to justify. 

  • 0
    60A sounds like a peculiar thing to specify anyway - aren't virtually all devices 63A these days?

       - Andy.
  • Former Community Member
    0 Former Community Member
    Yes Andy I would agree about the 60 A, we seem to get a lot of cut and paste specifications these days, you can usually tell when there's 200 pages of bumpf and about 4 lines of meaningful text about what is required. ?

    There's a few things missing, RCD protection to lights, no emergency lighting in the changing/shower rooms, only 1 lighting PIR in each changing room which meant that there's a lot of dead spots for detection. As it is only an outline spec for our pricing we have to be careful at this stage so we do not add in what we would think is required as this could make us uncompetitive so have just tried to clarify a few points but still received some strange answers.
  • 0
    BusterK:


    If we're going with the 60A supplies it may lead to the circuit cable being oversized for the shower's terminals if we have to use 16mm instead of 10mm. I can't see omitting overload protection due to shower not being able to draw more than 39A being accepted so would need to size the cable for the OCPD. 

     


    So as a future proofing exercise putting in

    60A capable/sized supplies, but Protected at 40A

    60A cables to a local Isolator  (40A min)

    40A cabling from the local (to the shower) isolator to the shower ... 


    would mean that if the showers were ever up-graded,

    minimal re-work would be required to bring the whole circuit to 60A without going back to the board.
    (would a competent installer re-use underated cables - esp when they inspect the local isolator)

    PLUS

    the specifier gets the comfort of knowing they are not running their major portion of the cables to the max 'today'


    Doesn't that end up being win-win ?

    the installation can always have notes on why it was done that way (future proofing), if needed.


    or does that transgress some rule that you can't over specify a design (or clearly visible portions of it)

     


Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines