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Topic Title: Shower
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Created On: 22 April 2014 09:19 PM
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 22 April 2014 09:19 PM
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goldenboy1818

Posts: 476
Joined: 22 February 2011

Hi random question here guys.

A 8.5kw shower....at what point does it actually pull 8.5kw ?? is it only when max pressure and maximum heat turned right up ??

Iv seen many 8.5kw showers on a 32amp fuse with 6mm....obviously at max a 8.5kw pulls more than 32amp

Thanks for any input
 22 April 2014 09:27 PM
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hifly

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turning up the heat only slows the water down, the current draw will be constant.

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 22 April 2014 09:30 PM
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alanblaby

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What is the voltage for the 8.5kW rating?
That is when it is drawing 8.5kW.

It's a fixed resistance element, so easy to work out what wattage it is using at a measured voltage.
And on the same subject, has any testing been done on shower elements to see if they increase their element resistance as they get hotter? Or, is the heating up that quick, that the resistance is assumed to stay roughly the same?
 22 April 2014 10:18 PM
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goldenboy1818

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Is it incorrect then for people to put a 8.5kw shower on a 32 amp fuse ? pushing the mcb to the limit. Also do people use 6mm in this case or 10mm ?. 6mm can not always go on a 40amp breaker depending on the reference method
 23 April 2014 07:08 AM
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alanblaby

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What is the maximum current draw for a 8.5kW shower?
What can 6mm T+E cable carry?
Do you think 433.3.1 (ii) may apply?

Have a read, and think about it.
 23 April 2014 08:49 AM
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broadgage

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The nominal 8.5KW loading is fixed regardless of the water flow, pressure or selected temperature.
A higher or lower water temperature is obtained by throttling the water flow not by altering the energy input to the element.

The actual current drawn will depend on whether the nominal 8.5 KW loading is at 230 volts or at 240 volts, and on what the actual supply voltage is.

If it uses a genuine 8.5 KW at 230 volts, then at 250 volts it will be significantly more.
If the rating is at 240 volts, then at an actual supply voltage of 210 volts the amps and the KW will be less.

Add to that the fact that actual showers are not precision resistances, but may vary by 5% from the nominal or calculated value, and it may be seen that the actual current is rather variable.

In practice the use of an apparently slightly under sized cable and OCPD is fine due to the short term nature of the load.
Not however strictly compliant with the regs in my view.
 23 April 2014 10:00 AM
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Thripster

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What about considering the shower as a fixed load not susceptible to overload? Therefore the MCB is providing short circuit protection only - does 433.1.1 (ii) apply then?

Regards
 23 April 2014 10:33 AM
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OMS

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Perhaps the other half of the equation you may want to think about is the time constant of the cable

If you take a 6.0mm2 cable at 30C and apply the 8.5kW load to it, how long will it take to get the insulation to 70C - will it be seconds, minutes or even hours.

Then consider the effect of enough current flow (ie not more than 1.15 x the CPD In) in terms of conductor (and hence insulation) temperature

Regards

OMS

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 23 April 2014 11:18 AM
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Thripster

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I should say that, when making my comment above, the MCB has assumed to change from 32A to 40A. Are we assuming anything other than Reference method C, at this stage? Sorry, do not mean to be interrupting - I'll butt out.

OMS - that is an interesting seed - off we go again!

Regards
 23 April 2014 11:53 AM
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OMS

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OK - The time constant of a cable is defined as the time taken for the cable to reach 63.2% of its final temperature after a change in the load.

The IET did some work with ERA looking at T&E cable surrounded by insulation and as a basic approximation, a 6.0mm2 T&E will take around 4 hours to reach 70C, it may reach 50C in an hour (ie around the 63% point

Not even teenager can spend that long in a shower.

Going back a while, I rigged up and proved by testing, the predicted time constants on a range of cable as part of a summer job with Delta Cables.

You can say with some certainty, that commercialy available cable will hit the 63% temperature rise point for full load current at the 1 hour point.

You could also say that a cable exposed to 150% of it's rated current will hit the limiting temperature point at 0.6 of the cable time constant.

Smaller cables will react faster and much larger cables will react slower - as you'd expect, but in general, it takes about an hour for any cable to rise from 30C to it's limiting temperature if carrying it's rated current for that installation method.

Small cables in air, tend to exhibit the shortest time constants but may well have the higher current rating under sustained loading

That should keep you out of mischief for a while

Regards

OMS

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 23 April 2014 12:13 PM
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Thripster

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'The IET did some work with ERA looking at T&E cable surrounded by insulation and as a basic approximation, a 6.0mm2 T&E will take around 4 hours to reach 70C, it may reach 50C in an hour (ie around the 63% point'

Was some assumption made for the current passed by the cable for those figures to be true?

Thanks
 23 April 2014 12:29 PM
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AJJewsbury

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Fig 2 here might be of interest: http://electrical.theiet.org/w...-ratings.cfm?type=pdf
- Andy.
 23 April 2014 12:50 PM
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OMS

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Cable FLC for the installation method is the assumed load

So a cold cable, unloaded at 30C start, then apply the cable rated current for the installation method and start the clock and measure the cable temp - theoretically stop the experiment at the 63% temperature point or leave it run until (or if) you reach the limiting operational temperature.

Regards

OMS

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 23 April 2014 12:51 PM
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OMS

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Originally posted by: AJJewsbury

Fig 2 here might be of interest: http://electrical.theiet.org/w...gs.cfm?type=pdf

- Andy.


Thats the document I was thinking of Andy - just couldn't find it

Regards

OMS

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Failure is always an option
 23 April 2014 03:28 PM
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Thripster

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Thanks for the link. Doesn't seem to be a statement of ambient temperature at start of test?

Interesting that for 100mm insulation and 2.5mm² cable, the current rating drops from bottom of joist to laid on ceiling , whereas for 6mm² the rating remains the same. So, you think to yourself, is that due to thermal inertia - then you look at the 16mm² figures and find that these seem to corroborate the pattern of the 2.5mm² tests - probably a reason for it somewhere........


Regards
 23 April 2014 03:58 PM
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AJJewsbury

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Doesn't seem to be a statement of ambient temperature at start of test?

Fig 2 suggests around 30 degrees at zero-time - which corresponds to BS 7671's normal assumption for ambient - so maybe 30 would be a reasonable assumption. (Perhaps it was a warm day in the Lab.)

whereas for 6mm² the rating remains the same

Humm, exactly the same. Very curious. If it was homework a teacher might suspect copying. Typesetting error perhaps? Oddly table 4D5 (which I think came about after this work) doesn't differentiate between those two positions...

On a slight tangent there's the suggestion that running suitable cables to 90 degrees within thermal insulation would be acceptable provided enough length is allowed for the temperature to reduce before accessories - with the suggestion of half a metre. How does that jive with Fig 3? As I read it, the conductor temperature starts reducing for the last half meter within the thermal insulation, (between 0 and 0.5m), it's barely over 70 degrees where the thermal insulation ends and half a metre away it's closer to 50 degrees (-0.5 mark). Taking Fig 3 directly would suggest that a terminal a few mm beyond the end of the thermal insulation wouldn't exceed the 70 degree limit. Presumably Fig 3 is based on having a 'long' length of the cable outside the thermal insulation to act as a heat-sink, which presumably wouldn't be the case if there was only a shorter length which terminated at an accessory (or re-entered the insulation). Are they presuming that with less cable to 'heatsink' the curve just moves left by half a metre? with no change of shape?
- Andy.
 23 April 2014 04:18 PM
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OMS

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Fig 2 shows the temp to be 30C

I wouldn't read too much into the absolute numbers in Table 1 - there will be some anomaly based on CSA to surface area ratio going on there possibly - basically the fact that the cable touches the joist on the major axis and the ceiling on the minor axis shows the potential for greatest heat transfer and thus highest constant current rating

Or the article simply copies the data for 6.0mm2 cable from the "bottom of joist" to the "laid on ceiling"

Regards

OMS

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Failure is always an option
 23 April 2014 07:56 PM
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goldenboy1818

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so would you guys fit or deem to be ok, a 8.5kw shower sat at 35amps on a 6mm and 32amp breaker ?
 23 April 2014 08:22 PM
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OMS

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Originally posted by: goldenboy1818

so would you guys fit or deem to be ok, a 8.5kw shower sat at 35amps on a 6mm and 32amp breaker ?


That's your call - but the dialogue above should suggest to you that 6.0mm2 cable will be OK - the 32A MCB might get a bit huffy once in a while and will potentially give up the ghost a nanosecond before it was designed to - maybe

Personally, without knowing what consecutive use the shower gets, but presuming normal domestic, then I wouldn't bat an eylid over it - particularly if it was 8.5kW at 240V rating - if the MCB was looking a bit faded, then a 40A or 45A device would be all I'd suggest

Regards

OMS

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Failure is always an option
 23 April 2014 08:38 PM
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goldenboy1818

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Its just depending on reference method as to whether 6mm cable can go on a 40 amp breaker
IET » Wiring and the regulations » Shower

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