Doubt if that cable size would be any problem. But a suggestion; if it is a wood stud wall; the studs are spaced at say 16 inches (or closer?)? Is there any way you can avoid putting foam into the one particular stud space that has the cable. This house 40 year old house (eastern Canada) has 'two by four' wall studs (today that would be two by six) basically on 16 inch centres! With f.glass batt insulation between and fairly good vapour barriers on the inside (warm) side. Idea anyway? BTW: 230v plus 10% brings us up to around 250+ volts. But I do remember, in the UK, some transformer type valve/tube radios that had IIRC 250 volt taps on the input windings of their power transformers! Cheers.
The point of the question was really to try and get some justification as to why a 400VA transformer was suggested for this job rather than the
700VA which, like a 16mm2 cable, would be required for continuous operation in this application.
The answer given was that there's plenty of thermal mass in the smaller transformer to cope with intermittent use in feeding a shower but it seemed to be given in somewhat hand waving, "oh, that will be all right for 25mins" type terms. This didn't inspire confidence so I thought I would ask why a 400VA transformer, why not 200VA, why not a bobbin wound on a nail (bent into a toroid ;-)? All to help the unqualified work out why that specific rating would be all right for 25mins (more, or less?).
The initial suggestion of using a 35A choke as a series dropper caused some concern as well. How big?
Its all a bit academic now anyway. I asked them if the would need to drill from inside my (attached) garage and the said they would. I said no since I have three lathes and a milling machine in there and did not want a layer of brick dust over them. They then suggested top filling with polystyrene beads but that I understand has a detrimental effect on the cable (now 10mm) insulation so I said no.
BTW
WTF does DOGs mean. Goggle just turns up hit on our canine friends.
I was only trying to correct the misunderstandings about transformer operation. The notions that loading a transformer increases the magnetic flux and that overloading might lead to saturation seem to be quite common, but are completely wrong.
Anyway the idea of trying to rescue the shower cable rating by reducing the voltage is utterly daft. Surrounding the cable in thermal insulation will halve its current rating and would require the shower to be working at a fraction of its rated power. Its controls probably wouldn't work properly at reduced voltage in any case.
The only option here is to get the cable out of the cavity, where it should never have been in the first place.
Interestingly, (or not) if you do a heat rise calc on the mass of copper in a 10mm2 the cable and assume that it was perfectly insulated then it takes about 25mins for the cable to rise from say 30deg to the max operating temp of 70deg.
Anyway, perhaps time for this thread to draw to a close . . . .
Well it's how things vary here in a remote and rural place with significant heavy industy, just houses and farms. We are on the middle of a section of 11kV line about 3 miles from the substation that is fed by about 25 miles of 33kV line. There may be no more than a couple of dozen houses on the whole 5 miles of line, the end of which is a normally open air switch.
There must be others out there who could log the data from a UPS and plot it out. It would be interesting to see what it's like in urban areas with/without industry.
Interesting - what current was that for, BTW? - but of course if you continue to assume adiabatic conditions the cable won't cool down after switching off and the next shower will start with the conductors at 70 deg. For a more realistic model you'll need to include the thermal conductivity of the insulation and calculate the cooling time constant.
The OP said it was a cavity wall, not a stud wall. The cavity will be between an outer skin of brick and an inner skin of either brick or block depending on age. There are no studs and no individual spaces between them.
Sorry, 2 screw ups in my calcs, time is 8mins for 40deg rise at the o/ps original load of 40A in perfect insulation. Just from power dis in cable per m and a specific heat capacity calc.
Wouldn't have mentioned it if it was that short.
Agreed. I thought a favourably long time for a worst case temp rise might give room for some guesstimation of an acceptable duty of operation but I agree that a full thermal calc would be necessary to show what was going on.
10mm2 reports 32A max for an insulated stud wall (not touching) which was the closest I could see to the o/ps situation. Given that 40A represents a 25% overload on the continuous rating, I think that a full thermal vs time plot would show some interesting results.
Pallet shelves - easy to build, especially if the pallets are all the same size. all the junk can come off the floor and go up on the shelves, then you can get more junk / tools / toys...
I see you have had lots of replies regarding your current/heat concern.
Have considered the impact the insulation may have on the cable protection? I have read that the poly beads (if thats what you are intending to get) can cause problems with the cable insulation. I have no personal experience of this, its just something that I read.
If he keeps a cable in the cavity, it must be larger CSA in conduit.
- He can argue the cable is >50mm from the surface
- However anyone drilling (CWI) does not expect a cable
I have a suspicion you need 10mm 6242BH (90oC XLPE BS7211) or 16mm
6242H (70oC PVC BS6004). Obviously wiring accessories will not tolerate 90oC, but we are concerned with the length of cable in insulation. The same argument is used re 6242BH 90oC lighting cable run in domestic loft insulation.
Unfortunately Screwfix stop at 6mm in 6242BH, there isn't any 10mm or
16mm on Ebay either.
I know people do not like "conduit on outside walls". However in this instance if possible I would thro-drill to the outside and run 10mm 6491X & 4mm 6491X (or 10mm FTE) in 25mm black conduit from besa-box to besa-box w/gasket & brass screws to enter a thro- drilled hole in the bathroom. The larger 25mm size can actually work better than small sizes. If your downpipes are brown you are a bit stuffed of course, if they are white you may want to try white (BESA boxes & conduit cost nothing, hence the wasted cost is immaterial).
It allows CWI to go in without delay - you get the energy saving benefit this winter, which can offset the cost of a) doing it b) improving safety re cable in cavity and c) giving a good incentive to get the cable routed internally anyway.
I personally prefer polybead re waterproof-bonded over fibre.
When we last moved the removal men's estimator took our word for the garage being 'full'.
On removal day, when the garage door was opened, the removers immediately rang up for an extra van, and another pair of hands. Even with that, it was touch and go as to whether we managed to get everything away in time.
Our current garage is /nearly/ as full, hopefully it will reduce when youngest son moves out soon. I'm not holding my breath!
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