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 1 reply
  • Subscribers 31 subscribers
  • Views 501 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

Submain Design Current

fiftyhertz
Hi all,


In terms of calculating the overall maximum demand on the whole installation i use w/m2 figures, however i do not feel comfortable doing this for individual submains. Currently my methodology usually follows looking at the connected load and applied a diversity of say 0.7 to get a circuit breaker size and thereafter put the design current to 90% of the breaker. Ie 80A breaker, design current of 72 Amps, i have done this to allow spare volt drop for any future loads on the installation, however i am not sure this is the best way to tackle building the Amtech model as it seems very wasteful.


Ive seen various posts about using w/m2 for submains, but is this figure only for use throughout a full building. Ie looking at BSRIA rules of thumb, gives 160 w/m2 for a shop. If i applied this figure for a 50m2 commercial unit within a building, this would only require a submain design current of 11.5A TP&N, which seems very small to me.


How are other designers tackling this?


Thanks
  • 0
    In my view, estimates of load based on floor area are fine as an average over a large area but are not directly applicable to small areas.

    I have found that it can be helpful to take an accepted load per unit area, but to add a fixed allowance of say 3Kva no matter how small may be the area.

    If the calculated capacity of the three phase sub-main is less than about 30 or 40 amps, then in my view single phase should be considered.


    A three phase supply of 11.5 amps is not of much use if someone plugs in a 3Kw kettle. 


    A single phase supply of 34.5 amps avoids trying to balance single phase loads on a small three phase supply. 

    Adding my suggested allowance of say 3Kva/13 amps would give a single phase supply of 47.5 amps, or say 50 amps if rounded up to the nearest standard size fuse.


    I would expect a single phase 50 amp supply to suffice for a unit as described, but three phase 11.5 amps, no way !

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