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Young & Keen · Nov 15, 2013

Hi All,

Any chance of some help? I am installing a mirror unit in an office which contains two sockets to be used for models to do make-up etc. Plugged in to these sockets will be hair dryers and the like. The plug is a 16A commando type and the nearest dist board is 55m away. I have done the calculations as follows:

4000/230=17.39A

In = 20A

Cg = .5 as there are a number of cables from the db running through a pearling.
Ci = .78 for running through 100mm of insulation.

20/.5 x .78 = 51.28

As per table 4D2A, it states that a 10mm cable should be used. Surely that can't be right or am I missing something???

Richard Burns · Nov 15, 2013

With a 17A design current (Ib) over a 55m distance you would need at least 6mm² cable to manage the volt drop.
2kW is a large load for a hairdryer, but if you know that they are going to be used you are stuck then.
If you were using a more likely 1.2kW then this would permit a drop to 4mm² for volt drop.
Cables passing through a ?Purlin? and then spreading out would be a limited reduction in CCC as they have the opportunity to cool after that and also, are they all fully loaded, as if less than 30% loaded then can be discounted. For the 100mm of insulation is this an insulated wall it is passing through or is it running under 100mm of insulation, if the former then place in conduit /trunking for the pass through, if the latter then get it out of the insulation.

Remember that the CCC is for a continuous load and hairdryers would be on for a short time, though curling tongs may be left on but I do not know the loading of those.

Young & Keen · Nov 15, 2013

Richard Burns said:
With a 17A design current (Ib) over a 55m distance you would need at least 6mm² cable to manage the volt drop.
2kW is a large load for a hairdryer, but if you know that they are going to be used you are stuck then.
If you were using a more likely 1.2kW then this would permit a drop to 4mm² for volt drop.
Cables passing through a ?Purlin? and then spreading out would be a limited reduction in CCC as they have the opportunity to cool after that and also, are they all fully loaded, as if less than 30% loaded then can be discounted. For the 100mm of insulation is this an insulated wall it is passing through or is it running under 100mm of insulation, if the former then place in conduit /trunking for the pass through, if the latter then get it out of the insulation.

Remember that the CCC is for a continuous load and hairdryers would be on for a short time, though curling tongs may be left on but I do not know the loading of those.

Thanks mate. The purlin is carrying the cables over a 15m length, not just passing through

Richard Burns · Nov 15, 2013

Strict application of the regulations does seem to go to 10mm cable, it is a long run and a reasonable load on the end of it.
Take account of all possible leeway, are both hairdryers on at the same time, are the cables spaced enough along the purlin, is the distance actually 55m, is there a local supply nearer that already has the CCC to also use another 17A.
If not then run 10mm to a joint box and then wire the room and sockets in 2.5/4mm, so long as the room is not huge.

Deleted member 72666 · Nov 15, 2013

Will the cable be grouped with others when in the insulation? If not then no need to de-rate cable as 0.5 factor will suffice.

Young & Keen · Nov 15, 2013

stantheman said:
Will the cable be grouped with others when in the insulation? If not then no need to de-rate cable as 0.5 factor will suffice.

No they'll be on their own. Thanks a lot!

Baker1988 · Nov 15, 2013

Couldnt u just cut the commando plug off and install a spare off existing ring and hard wire it into that

Rockingit · Nov 15, 2013

You won't fit a 10mm cable into a 16A ceeform!!

And in reality you'll also find the current draw will average naff all. Think about it - how long is a hair dryer actually on for? How long is the element in a pair of straighteners on for??? I've YEARS of experience dealing with this type of stuff, and I can 100% promise that you're over-thinking the problem.

Young & Keen · Nov 15, 2013

Rockingit said:
You won't fit a 10mm cable into a 16A ceeform!!

And in reality you'll also find the current draw will average naff all. Think about it - how long is a hair dryer actually on for? How long is the element in a pair of straighteners on for??? I've YEARS of experience dealing with this type of stuff, and I can 100% promise that you're over-thinking the problem.

Thanks mate, I knew that was the answer. The regs etc teach us to do things a certain and I knew they were being over-cautious but unfortunately they are all I have to go off. What would you do?

Rockingit · Nov 15, 2013

Young & Keen said:
Thanks mate, I knew that was the answer. The regs etc teach us to do things a certain and I knew they were being over-cautious but unfortunately they are all I have to go off. What would you do?

What's the building type? Is this a temp? Seems a bit of an odd set-up if it's a permanent install?? This has hallmarks of marquee parties????!!

Rockingit · Nov 15, 2013

Just putting the little apprentice to bed, back to help more later!

Young & Keen · Nov 15, 2013

Rockingit said:
What's the building type? Is this a temp? Seems a bit of an odd set-up if it's a permanent install?? This has hallmarks of marquee parties????!!

Commerical offices mate. It's a permanent install, at least until they change their minds again!! The whole of the first floor is made up of power circuits supplying computers, don't want to risk overloading or tripping the MCB's with hair dryers etc.

Rockingit · Nov 15, 2013

OK....well, for starters ditch the ceeform. It's too tempting to become unplugged if nothing else! Second, I'd just run it in a perfectly normal radial in 2.5 or maybe 4mm (by whatever cabling method is preferred, but even if you de-rate the whole lot by half you've still got 4 kW to play with) and then power each mirror via a 13A fused outlet, protect the whole lot on a B16 RCBO (assuming no other isolated RCD protection available). I really wouldn't worry about VD for the few lamps that they've got, it's just not mission critical enough. Oh, and a tip there: replace them for daylight colour LED - it's MUCH better for doing makeup!

Lastly, don't forget to rig up the covert CCTV cam back to the technical stores.

Young & Keen · Nov 15, 2013

Rockingit said:
OK....well, for starters ditch the ceeform. It's too tempting to become unplugged if nothing else! Second, I'd just run it in a perfectly normal radial in 2.5 or maybe 4mm (by whatever cabling method is preferred, but even if you de-rate the whole lot by half you've still got 4 kW to play with) and then power each mirror via a 13A fused outlet, protect the whole lot on a B16 RCBO (assuming no other isolated RCD protection available). I really wouldn't worry about VD for the few lamps that they've got, it's just not mission critical enough. Oh, and a tip there: replace them for daylight colour LED - it's MUCH better for doing makeup!

Lastly, don't forget to rig up the covert CCTV cam back to the technical stores.

The thought hadn't even crossed my mind!!

Thanks for your help, it all makes sense to me. I'll give it a whirl!