Cable Calculations - Diversity Applied

It is all guesswork. The hotel may let out a room all day and evening for business meetings etc. The lights may be on for hours. At busy times like Christmas the hotel may be full to capacity and everything used to excess. I suggest that you get your employer to book you in for a luxury weekend break at the employer's expense so that you can get a feel for the environment and take scientific readings.


Z.

I have a room where the lights are all on or all off through presence detection. I anticipate that within a typical day the lights will be on 6 hours. That is 25% of the day. Does this mean that I sould insert for this specific circuit an individual diversity factor of 0.25?

I've no idea of the Amtech side of things, but from a general principle point of view, I think there's not enough informtion to say. My thinking is that it's down to how the pattern of usage maps onto thermally equivalent current - i.e. how much the load warms the cables (and equally overload protection devices) - which depends not only on what proportion of the day it's on but how long each on period is. Conductors take time to warm up - at full load perhaps something in the region of ten minutes for small sizes (e.g. 1mm²) or maybe closer to an hour or two for larger sizes (say 16mm² or 25mm²) at their full load. So if your 25% of the day was say 6 hours on and 18 hours off then after an hour or two of on time all the conductors will likely have warmed up just as much as they would have with a continuous load - so there's not really any scope for applying diversity at all. On the other hand if it's a pattern on 1 min on and 3 mins off then the conductors will hardly get any warmer than if you had a 25% load running constantly - so a diversity of 25% would be fine. Naturally your situation will be somewhere between the two and probably with a fair amount of uncertainty, which doen't make it easy to give a simple answer.


    - Andy.

Well it depends if it is the same 6 hours on for all the lights. Generally I suspect not, but not fully staggered either -  I  suspect that nearly all the lights will be on at certain times of day say as folk are rushing around breakfast time... Certainly they will all be off at once - 3 AM probably?

Then we are into the dark arts of how long each day can you run things at 200% load for without actually having a problem - usually in practice, longer than you think, but it would be 'brave' to design to that.

Is this a real example ? Unless the area is massive,  one 16A 3 phase supply covers a lot of modern lights, and is normally set so you can isolate zones that are far from fully loaded, to ensure thta the effect of any one trip is not too serious. After all you do not want to shut down large chunks of the building after water gets into a single fitting from the room above. Having half a restaurant lit is far better than  no restaurant or kitchen... I assume you already use the true loading - so many hundred 25 watt LEDs or whatever, not the old 'assume 100W per lamp holder'.

You have already assessed that the lights may be on for 6 hours through the course of a 24 hour period, which isn't excessive, so I would follow table A2 which allots a 75% allowance for small hotels etc.