My current electric shower does say anywhere its KW rating. I intend to replace this next week and am wondering if I can determine the rating from either...
a) the rating of the circuit breaker
b) the size of the cable
c) some other means.
By looking at the cable I cant determine its size (mm2?) - is there a measurement that I can take of it to determine its maximum rating. Also a quick breakdown of cable size against shower rating would be most useful if anyone can help.
Thanks for the information. I tempted to phone Triton and ask them the KW rating of the current unit. No doubt there will be a range of ratings per model!
So if it's a 30A breaker, max. power would be 30 * 230 or 6.9KW.
But thats assuming 100% efficiency - you'll lose some to the cable at those amps which I think was mentioned earlier in the thread..
If it were me, I'd uprate the wiring and breaker to handle the biggest wattage shower I could get my hands on, having been fed-up in the past with dribbly little 8KW showers in the middle of winter )-:
On Tue, 7 Sep 2004 15:57:49 +0000 (UTC), "Charles Middleton" strung together this:
Yes, but just the cable. The MCB might not be the right size for the cable. The size of the cable determines the maximum rating of the shower and the size of the MCB.
Easy way is to turn off all major appliances (washing m/c, cooker etc but lights/video don't really matter) then measure the power usage on the electricity meter over a timed period. These things use so much juice that the rating should be pretty clear from say just 5/6mins of use. Meter will resolve 0.1kWhr at worst so 6mins on a 7kW will give 0.7kWhr, 8kW ->
Measure the temperature of the cold water from a tap on the same mains supply as the shower (kitchen will do) - let it run for a few mins to get a stable reading of the actual incoming water temp.
Now calibrate your bucket with a marker so you know where 5L is.
Turn on your shower at maximum temperature and let it run for a bit to get a stable temperature. Measure this temperature.
Now time how long it takes to fill your bucket to the 5L mark.
Now do some sums:
Find out how much energy the shower has added to the water. To do that multiply the temperature rise, by the mass of water (assume 1L = 1kg), and then by the specific heat capacity of water (4200).
Say the temperature difference between incoming and heated water was 40 degrees C you will get:-
40 * 5 * 4200 = 840,000 J
Now divide this by the time it took to fill in seconds. Say it took 80 seconds, you get:-
840000 / 80 = 10500 J/second (or Watts)
Thus you would have a 10.5kW shower.
(The above procedure will tell you the heat put into the water by the shower - it will not tell you the energy lost in the supply cable to it. So for example if you had a 10kW shower, you may only find 9.8 kW getting as far as the water).
Perhaps a measurement of the DC resistance across the electrical input terminal to the shower with a meter might solve the mystery. You would need to isolate the shower from the mains cable in order to do this. So the power is calculated by multiplying the 230v mains by itself and then dividing it by the resistance of the shower heating element.
Perhaps... I expect you will find a heating element is a bit like a light bulb in that respect, since its resistance when hot will be different from cold. Hence you may tend to "over read" the actual power based on the resistance looking lower than it really is.
If you are going to measure it, one way would be to pop the cover of the cu and locate the feed from the MCB to the shower. Stick a clamp meter round it and then go fire up the shower. The clamp meter will take a direct reading of the current without needing to make physical contact with the circuit.
There have been some very interesting and accurate posts on this thread, but I suspect that the "real world" answer to your problem is much simpler.
If the shower has been there some time, chances are that it will be wired in 6mm2 T&E (this stuff is about half an inch wide altogether) and fed from a 30A fuse or 32A MCB. If the shower has been rated at 240V then it *should* be at most 7.6kW (32A*240V) though it is common to see showers up to 8.5kW run in this way: a 32A MCB isn't going to trip for that amount of overload, and it is highly likely that the cable will handle it fine, depending on route and length. (Note that I would *not* recommend installing an 8.5kW shower on a 32A MCB, or worse still a 30A rewireable).
I am told that 7.5kW really doesn't do the job for much of the year.
When you buy your new shower you really need to get the most powerful one you can afford so you should also be factoring in the cost of running a new length of 10mm2 cable (it's about an inch wide: while you are in B&Q (or wherever) looking for your shower, check out the cable in the electrical aisle) and upgrading the MCB (and possibly your supply) to accommodate the new shower.
While you're at it, you need to think about the shower disconnection times and whether or not an RCD is required for it. This will largely depend on the type of earthing your system has and the length of cable from CU to shower. Don't forget to make sure your supplementary bonding is up to scratch too.
One thing I'm surprised people often don't think about though is the possibility of hanging a shower from the existing hot water system. It seems that if there's an electric shower already installed many people think that more of the same is their only option. Not so; most combi boilers (should you have one) will give showers at least as good, and in most cases better than most electric showers because a combi might be putting 24kW into the water whereas the biggest practical electric shower is only 10kW or so. Do invest in a thermostatic mixer though.
Even if you have a hot water cylinder system it is possible to run a very good shower from that. If it is one of the rare (at the moment) mains pressure systems just bung on a thermostatic mixer. If it is gravity fed then a shower pump will cost about the same as a new electric shower and potentially give a better result. The difficulty is re-arranging the plumbing.
For both the above though, the electrics are simplified. None at all are needed for a thermostatic on combi or mains, and for a pumped system, the pump takes a very small amount of power and can easily be run from an existing sockets circuit.
Erm... sorry. I wrote a lot more there than I was intending.
Your suggestion that 10 mm^2 cable is about twice the width of 6 mm^2 didn't quite ring true. According to the Pirelli data sheet 6 mm^2 T&E is approx. 13.1mm wide and 10 mm^2 is approx. 16.8 mm wide - so "just over half an inch" and "getting on for eleven-sixteenths of an inch" is nearer the mark.
And on the type of overcurrent protective device. For a 45A shower circuit the maximum voltage-drop limited circuit length is 49 m. With a PME supply there is no need for an RCD with any device type (other than Type C or old Type 3 MCBs on circuits longer than 23 m - but these device types aren't appropriate for a shower in any case).
For a TN-S earthed supply:
- with a BS 1361 cartridge fuse an RCD would be required for a circuit longer then 10 m;
- with a re-wireable fuse (or old Type 1 MCB) the full 49 m is OK without an RCD;
- for a Type B MCB the length limit without RCD is 35 m;
- other MCB types will require the RCD, regardless of length.
In all cases running a separate parallel 10mm^2 circuit protective conductor to the shower (taking the same route as the 10 mm^2 T&E) could also be considered.
Ok, you win. I have actually been using the Pirelli stuff this weekend, though I haven't gone so far as to measure it :-) Having said that, if it were one of those nothing-to-compare-against, instant-guesstimate things, I would probably look and say "half inch" or "inch". In the OP's case all he needs to do is to say "is it a half inch, or significantly bigger?"
Just thought... 4mm2 cable isn't *that* much smaller than 6, but then you wouldn't find a... erm... would you? ;-)
Yes, and we're probably getting way out of the OP's depth by now. I assumed he had MCBs when he was asking about sizing the shower by "the rating of the circuit breaker"... but I do tend to assume things which aren't necessarily the case.
49m is a heck of a long way (come to that, 35m is a fair old distance too), especially since 10mm2 cable gets extremely unweildy if you have to manhandle anything more than a 25m drum!
Not a particular enthuser over showers (difficult to take a book into the shower to read), but I am confused that the electric shower seems to be so common when there is often (granted not always) hot water freely available from other, cheaper, more plentiful sources.
The pump is a compromise solution for the bog standard gravity-fed HW system (yes, we have one). Most pumps (again, there are notable exceptions) are noisy and give little impression of solidity, but the cheapest pump on a gravity system can give even the most powerful electric shower a good run for its money, and at a fraction of the running cost - assuming the HW cylinder is in use and doesn't have to be heated solely for the shower.
If you want lengthy and wide-ranging discussion on this and similar topics, all you have to do is Google back into this group's past and look for (particularly) anything written by IMM and Andy Hall on the subject of hot water (try keywords such as "combi" or "two combis" or "heat bank" or "plate heat exchanger"). Just don't go looking unless you've a few hours to spare :-)
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