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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
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    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 !
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  • 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 !
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