IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Is this cable too small? ....allowing diversity
Topic Summary:
Created On: 18 November 2013 07:27 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 18 November 2013 07:27 PM
User is offline View Users Profile Print this message



misterben

Posts: 415
Joined: 11 June 2007

Hi all,

I have been asked to take over a job second fixing some flats they have a 16mm T+E submains run to each one. No services require bonding as water pipes are all pvc and definitely not extraneous.
there is no gas supply.
The owner wants the following:
1.9.5kw shower
2.9.5kw water heater
3.Cooker - unsure of size yet but assume worst case
4.Ring Final - approximately 8 sockets on to incorporate a kitchen which will only have a fridge and no washing machine or dishwasher.
This is also feeding a convector supply for a storage heater.
5.lighting circuit - standard pendants so am assuming 100w minimum load x 8 points.

I am concerned that it will not be substantial ,I have worked out the demand to be approximately 122Amps is this correct?

any help appreciated

Misterben
 18 November 2013 07:50 PM
User is offline View Users Profile Print this message



perspicacious

Posts: 7235
Joined: 18 April 2006

"I am concerned that it will not be substantial ,I have worked out the demand to be approximately 122Amps is this correct?"

Money shouldn't be a problem if the client can afford to spend £4 every hour on electricity usage

Regards

BOD
 18 November 2013 08:12 PM
User is offline View Users Profile Print this message



peteTLM

Posts: 3152
Joined: 31 March 2005

What containment is the sub main run in?

It won't work out anywhere near as high as you suggest, personally I hate t&e sub mains to flats. There is always a nail/ rodent/ cheapskate builder to ruin it.

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 18 November 2013 08:20 PM
User is offline View Users Profile Print this message



misterben

Posts: 415
Joined: 11 June 2007

Hi,

The submains is clipped direct and not in any containment.

regards
Misterben
 18 November 2013 08:26 PM
User is online View Users Profile Print this message



Zs

Posts: 2909
Joined: 20 July 2006

Some questions for you.

That water heater; is it instantaneous?

Would it be in use while the shower is on?

Would the cooker be heating up with all of the electrical components heating at the same time? 4 rings, 2 ovens etc.

What Amps have you allowed for the sockets? I'd say about 5.

Would you really have 800W on the lights?

Do you know what the real voltage is? I expect it is higher than 230.

And so on.

I think that this is well within limits. But this is a designer decision based on your opinion of how the circuits are going to be used. It has to be your call and you are the one who will be expected to explain and back it up. If it helps, based on what you have given us and what I consider to be sensible diversity for a small home, I'd be happy to explain a decision that a 60A main fuse on 16mm cables is suitable, to any judge.

Assuming of course that this isn't a health club with glitter-balls, revolving floors and steaming shower as well as a restaurant serving steak and freshly baked potato 24/7.

Zs
 18 November 2013 08:36 PM
User is offline View Users Profile Print this message



misterben

Posts: 415
Joined: 11 June 2007

Hi ,

Zs, the water heater is instantaneous, and is quite possible that it could be used at the same time.
The cooker like most probably rarely runs at its full capacity, the lights will be pendants but I went with the OSG at 100w per point minimum? should this be 66% of total current?
Socket circuit again went with OSG which I have read wrong! It seems its 40% of current demand. Is this based on what you think will be used or the rating of the MCB?

regards
Misterben
 18 November 2013 09:09 PM
User is online View Users Profile Print this message



Zs

Posts: 2909
Joined: 20 July 2006

Misterben, your are posting some good stuff today... I don't have it in front of me but right at the beginning of the OSG diversity section, does it not have a paragraph that says it is not to be considered as cast in stone and that the designer must decide? Only in posh IET style words of course.

So, my rationale on the water heating is as follows...

They are both instantaneous water heaters even though one is called a shower and one is called a water heater.

They are not going to be running at full tilt together for more than a couple of minutes.

On the lighting....you cannot buy 100A lamps for pendants easily any more and nearly all lampshades carry a maximum of 60W, so I would be reducing that massively. I would also be installing low energy as required by the building regs and calculating in accordance with that, which will drop most of the lighting load for you.

The convector heater will be on a thermostat and not likely to be used much because it costs so much.

If the cooker does not exist yet, you are in a position to dictate how much welly it has.

Etc.

Massive room for massive diversity. IMHO.

Zs

Edited: 18 November 2013 at 09:17 PM by Zs
 18 November 2013 09:21 PM
User is offline View Users Profile Print this message



misterben

Posts: 415
Joined: 11 June 2007

I have been very busy recently and not had time to catch up on all the things I wanted to query. I should make more time to come on here, as it is a useful resource!

I suppose diversity and max demand is common sense really, as long as the cables aren't overloaded and fuses/MCBs aren't tripping all the time then I suppose it can be over thought...

regards
Misterben
 19 November 2013 08:48 AM
User is offline View Users Profile Print this message



broadgage

Posts: 1294
Joined: 07 August 2007

IMHO, the main problem is the 2 water heaters. I consider it entirely possible that one person would shower whilst someone else washes up.

Also how is the flat to be heated ? In the absence of a gas supply, I would expect a relatively high loading on the ring final due to the probable use of electric heaters.

And what about a washing machine or tumble dryer ? although it is stated that no such are to be installed, it seems probable the occupier will add at least a washing machine.

IMHO, nothing less than a 100 amp supply and sub main to suit will suffice for an all electric flat with two large intantaneous water heaters.

The two waters alone are about 75 amps, leaving only 25 amps for the cooker and for electric space heating.
I would not go so far to plan for 125 amps, but would regard 100 amps as prudent.
It is true that the 100 amp cable and fuse can take more than 100 amps short term, but this might well be needed in any case.

Both water heaters=75 amps
two medium size electric heaters =20 amps in total
Estimate lighting and small appliances at 5 amps
That fully utilises a 100 amp supply.

Any use of kettle, cooker, toaster, hairdryer, microwave, or another heater would push the total to significantly more than 100 amps. Probably OK in the short term, but shows IMHO the unwisdom of less than 100 amps.

Edited: 19 November 2013 at 12:58 PM by broadgage
 19 November 2013 10:31 AM
User is offline View Users Profile Print this message


Avatar for Parsley.
Parsley

Posts: 1007
Joined: 04 November 2004

I think a careful look at 433.1 is required.

I wonder what size supply the DNO will install for the block of flats?

I would have thought these flats are small and the occupation based on 1 to 2 people not families. In reality is the proposed 16mm2 submain ever going to reach max operating temperature?

Regards
 19 November 2013 10:51 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11467
Joined: 13 August 2003

To my mind there are two parts to diversity - the first (and probably the most obvious) is that not everything will be switched on at the same moment in time. The second (often overlooked) is that cables (and fuses & MCBs) are rated for continuous operation - and can carry significantly more than their nominal ratings entirely safely for a certain period of time. It's what the GNs call thermal equivalent current. If you had a cold cable and shoved 100A through it for say 5 minutes, it would warm up to a certain temperature. If instead you loaded it with 80A it would take longer to reach that same temperature, perhaps 10 mins. 60A would take even longer. At some point, perhaps 50A, you could run the cable continuously.

MCBs and fuses are essentially thermal devices - i.e. their internal temperature mimics the temperature of the cable itself - that way they can provide overload protection that's matched to the heating of the cable, regardless of the instantaneous current (provided it remains below fault levels of course).

I'm with Zs - a 16mm2 cable will be able to carry significantly more than Iz for probably several tens of minutes, without problem.

- Andy.
 19 November 2013 01:04 PM
User is offline View Users Profile Print this message



broadgage

Posts: 1294
Joined: 07 August 2007

Originally posted by: Parsley

I think a careful look at 433.1 is required.
I wonder what size supply the DNO will install for the block of flats?
I would have thought these flats are small and the occupation based on 1 to 2 people not families. In reality is the proposed 16mm2 submain ever going to reach max operating temperature?
Regards


The DNO would certainly not allow 100 amps for each flat, but I suspect they would probably fit a 100 amp cut out fuse for a flat with 2 large water heaters.

Diversity would greatly reduce the average demand per flat, in say 12 flats it is is most improbable that all 24 water heaters would be used at the same time.
In one single flat though IMHO one should allow for the two largest loads being used at the time.
 19 November 2013 01:55 PM
User is offline View Users Profile Print this message


Avatar for Parsley.
Parsley

Posts: 1007
Joined: 04 November 2004

I think the 16mm2 conductor protected by a 63A BS88 fuse will be fine. I personally wouldn't use T&E though.

The two instantaneous water heaters aren't going to be on simultaneously for any real length of time.
BS7671 allows overloads of short duration (less than 1 hour) I2<1.45 Iz see 433.1.1 and 433.1.100.

The 16mm2 clipped direct T&E has an It (Iz) of 85 Amps assuming there are no correction factors, I think it will be OK.

Regards
 19 November 2013 03:30 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19673
Joined: 23 March 2004

A 16mm2 will cary 80+ Amps forever, and it'll take about an hour to reach 70C

Look at it from the other end - it's a flat - say 50m2 - and you need to stop thinking in terms of peak loadings

It'll have an all up electrical demand averaged over 1 hour of circa 30W/m2 at most.

It'll have an electrical demand for heating of about 35W/m2 and probably much less.

That gives you around 65 - 70W/m2 = 50m2 x 70 = 3.5kW.

Even if you go mental and double that to 7kW equivalent demand, that's only 28A

If you want to go for peak loading, take your inital estimate of 120A and then apply a diversity factor of about 0.4 - so about 48A

A 16mm2 T&E and a 63A fuse in the KMF will absolutely eat it whichever way you look at it.

Regards

OMS

-------------------------
Failure is always an option
 19 November 2013 05:49 PM
User is offline View Users Profile Print this message


Avatar for JZN.
JZN

Posts: 559
Joined: 16 November 2006

Someone is in the shower using 9.5Kw. Someone else in the kitchen turns hot water tap on using another 9.5kW. Both are drawing water directly form the same water supply. Pressure in shower drops substantially. Shower user screams "turn the freakin' tap of" or words to that effect.

Suddenly demand drops by 9.5kW. Problem solved! In other words I doubt they'd be on simultaneously for more than a few seconds and as suggested above, even if the water pressure was not an issue then both are unlikely to be run for together for more than a few mins.

John
 19 November 2013 08:11 PM
User is offline View Users Profile Print this message



ady1

Posts: 766
Joined: 19 April 2005

A new installation in France....
7 x 3kw heaters, 3kw immersion, 7kw cooker, approx 12kw of appliances, lighting etc etc and the supply company were amazed we asked for more than a 20A supply. Ended up with 32A......never tripped once !!!!

Maybe were getting a little over excited over here.

Regards
Ady

-------------------------
Resistance is futile.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.