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 2 replies
  • Subscribers 34 subscribers
  • Views 596 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Grouping of non-continuously loaded circuits

BrightSpark

Hi, I am new here :-)

I have a question about grouping factors…

The installation is a kitchen in a community hall with 2 x hobs (max 25A each if all four rings are on), 3 x ovens (12A each), a and dishwasher (8A). Each of these six appliances is having its own radial circuit, and there also will be a ring main for microwave/kettle/toaster etc. It's not really a commercial kitchen, in that it won't be used continuously, but it's certainly not domestic. 

Circuits will be wired in T&E plastered in the walls, going up to a (well ventilated) ceiling void, and then down in metal trunking to a DB on a wall at high level. The DB has an 80A supply and the client wants to be able to use all of that based on their worst-case maximum cooking situation. My main concern is the trunking section.

The grouping factors in BS7671 are specifically for equal circuits with prolonged 100% loading. If I use these, I will end up with cables that are one or two sizes bigger than normal, which will be expensive and probably impractical to install. But these circuits are not equal, and their loadings are not continuous - they will vary in a rather unpredictable manner due to thermostats in the appliances. Yes, if they switch a few things on together, several cables will be loaded at once, but that might last just 10 minutes or so before the thermostats kick in and the loading becomes intermittent. I suspect it will have a fairly low duty factor, but that's a bit of an unknown.

So what do I do? I cannot find anything published on how to calculate grouping factors when cables are not continuously loaded. Is there anything available on this subject?

I suspect many people would just use normal cable sizes (2.5mm and 6mm) and hope it will all be OK, but I don't want to take the chance. I am thinking of splitting the cables between two lots of containment (ie two trunking drops to the DB, which I can do without it looking ugly), as then, even with standard grouping factors, I don't think any over-sizing of cables will be needed.

I hope this makes sense, and I'd appreciate any comments..

Parents
  • 0

    Good question.

    I think my first stab would to be to apply diversity quite generously, and then see where that takes you.

    So think of an oven+hob as a “cooker” so 25+12A = 37A and apply diversity - 100% of the first 10A and then 30% of the remainder - so 18.1A in total - so similar in total to the hob drawing 12.3A and the oven 5.8A - so chuck those numbers into your grouping calculation and see if it's reassuring.

    If needs be apply diversity between circuits (40%?)

    then don't forget that the whole lot must be limited to 80A anyway - so if the sum total c.s.a. leaving the DB is significantly larger than the c.s.a. of the submain, then intuitively over-heating should be unlikely.

       - Andy.

Reply
  • 0

    Good question.

    I think my first stab would to be to apply diversity quite generously, and then see where that takes you.

    So think of an oven+hob as a “cooker” so 25+12A = 37A and apply diversity - 100% of the first 10A and then 30% of the remainder - so 18.1A in total - so similar in total to the hob drawing 12.3A and the oven 5.8A - so chuck those numbers into your grouping calculation and see if it's reassuring.

    If needs be apply diversity between circuits (40%?)

    then don't forget that the whole lot must be limited to 80A anyway - so if the sum total c.s.a. leaving the DB is significantly larger than the c.s.a. of the submain, then intuitively over-heating should be unlikely.

       - Andy.

Children
No Data

Community Rules & Guidelines