I have an existing 8.5KW shower in the bathroom and am considering installing a 9.5KW (or possibly 10.5KW) in the new ensuite. My fairly new CU (< 2 years old) has a 100A breaker.
If both showers are on at the same time will this cause an overload?
Thanks
Fergal
No. Provided you aren't drawing excessive loads elsewhere, such as using multiple fan heaters and immersions.
An 8.5kW and 9.5kW @ 240V actual voltage will draw 75A, leaving you a good
25A for other stuff, which is plenty. Even with an 8.5kW and 10.5kW @ 240V, you will draw less than 80A. Even with the oven, washing machine and tumble dryer on as well, you'd be unlucky to trip it, due to the action of thermostats. I wouldn't use an electric hob at the same time, though.Christian.
Wouldn't one shower take water pressure away from the other?
There are some specific rules about diversity which you or an installer should check and follow. Installation notes typically ignore this.
If you are careful that your total current drawn will not exceed the company fuse rating of 100A or 80A then you are fine. Initially fuses blow at 150% rating but aging can reduce this.
Does your consumer unit really have a 100A breaker or is that the switch rating of the RCD or main switch?
Each draws so little water, that your mains supply would have to be absolutely pathetic for this to occur. Each shower will give around 4 litres per minute. A good mains supply should manage above 40 litres per minute.
Christian.
Anything less than 60 A is fairly unusual, IME, other than in installations that haven't been rewired since the 1950's.
Since showers are fairly short-term loads (and an overlap period between two of 'em probably even shorter) you would fairly certainly "get away with it." NB I am NOT recommending doing this! The 60 A fuse would pass 80 A more-or-less indefinitely, but things might get quite warm, leading to the characteristic fishy smell of gently cooking bakelite (and hot PVC if, as somone else mentioned, the tails are only 16 mm^2, or old 7/0.064). Also the voltage drop on the supply will be considerable - 0.2 ohm supply impedance isn't uncommon and that's 15 volts drop with both showers on, leading to dim lights and the showers running at 10 - 15% less than full power.
Definitely.
I've noticed that too. Unlikely to be just the busbar, in view of the total
100 A rating. Might be something to do with total heat dissipation in the box, or local heat from the MCBs of two 40 A ccts affecting the performace of adjacent MCBs?ITYM the rated capacity. The breaking capacity will be in kA! But, to repeat the mantra, there's not usually any need for an electric shower to be RCD protected - especially after upgrading to a shiny new 100 A PME service. The primary measure that ensures electrical safety in the bathroom is the local supplementary equipotential bonding.
There seem to be a lot of 60A fuses / 40A meters around here :-)
I think that *planning* an installation such that it is likely that 80A will be drawn for any length of time through a 60A fuse is dodgy at least. Yes, while the thing is new it should work, but over time the heat stress may well bring that fuse closer to breaking at 60A... and the hassle of getting such a fuse replaced can only be imagined. As for the tails, Western Power moved my meter a couple of months ago and stuck in a 100A fuse. Presumably their cable (co-ax from a pole) can cope, but the tails from fuse to meter and meter to isolator are *definitely* smaller than the 25mm2 tails I've just installed from the isolator to the CU. But then they're only about a foot long...
The way I read Fergal's OP was that the possibility of both showers being on at the same time was reasonably high. Without knowing his details, one must wonder *why* he is installing a second shower. Chances are that the existing one is in high demand and he wants to be able to get ready for work even when one of the teenagers is taking a bit longer than necessary in the bathroom :-)
Yes, hadn't thought of that. Makes you think twice about the +/- 10% thing at the end of a long circuit too...
The instructions I recently saw for a GE CU implied this - in the tightly-packed confines of a CU I can see how this might be a problem if two or three high-current circuits used adjacent MCBs.
Ummm... yes, whatever :-)
Which is why I didn't say "since you will be running...". Having said that, there are plenty about who prefer the "belt and braces" approach. I've done it both ways myself.
Hwyl!
M.
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