Shower cable in plastic trunking

I have a shower (7.8kW @ 230V, 34A) connected on to 15 metres of 6mm squared cable protected by a 45 amp MCB. The shower cable runs from the CU then up the wall on its own in a approx 2 metre length of white plastic trunking. The cable then goes under floorboards and then up the shower wall in a channel cut out of the plaster behind the tiles. Should I apply any derating because of the plastic trunking or other influences? The house is old and hence I only have insulation in the roof. Steve.

For 6mm squared cable clipped direct the current carrying capacity of the cable is 47A which is reduced to 41A when installation Method 3 conduit is used (Single core table 4D1A regs page 220), although the length is only two meters. The fuse rating is too high for the cable and I'd suggest you lower this to a 40A MCB

Sorry, I should have said that the cable is T+E, not single core. Steve.

Multicore page 222 4D2A then - clipped direct rating 46A method three reduces cable to 38A so suggest lowering Fuse to 35A MCB unless you can chase in the cable instead of using the conduit which will no doubt give you more work though will look prettier long term.

I guess I could remove the trunking and clip the cable direct to the wall and then I should be able to use a 40A MCB. Thoughts on that please.

You could make the case that its perfectly alright as it is (pending a few fag packet calculations for the adiabatic) as the design current (34A) is less than the current carrying capacity of the cable how it is installed (38A), and the shower is a fixed load and can't overload, that said I'd be much happier if the breaker was changed to a B40, incase a builder comes along and uprates the shower. Not sure I'd be ripping down the trunking... (although given half a chance, I'd install 10mm² to give the option of a bigger shower in future)

Adam_151 said:

You could make the case that its perfectly alright as it is (pending a few fag packet calculations for the adiabatic) as the design current (34A) is less than the current carrying capacity of the cable how it is installed (38A), and the shower is a fixed load and can't overload, that said I'd be much happier if the breaker was changed to a B40, incase a builder comes along and uprates the shower. Not sure I'd be ripping down the trunking... (although given half a chance, I'd install 10mm² to give the option of a bigger shower in future)

Click to expand...

Adam, so what is the requirement for the MCB selection? Can you use a 40A breaker to protect a cable rated at 38A ? and why is that possible? Thanks in advance. Steve.

PS planning on getting rid of the electric shower soon to fit a combi powered mixer. Just want to the electric one to be safe in the short term.

There are two types of overcurrent fault:

1) Overload, healthy circuit, but too much load placed on it, for example you plug in too many electric fan heaters

2) A large current flows due to with a short circuit or earth fault

Generally breakers and fuses protect against both, and when we size a breaker lower than the current carrying capacity of the cable to protect against the first one, and that breaker has sufficent breaking capacity for the fault level where it is installed, we can assume that it also protects against the second. Some loads are incapable of overloading, because there might be a fuse further down the line (consider the 60A switchfuse fed with 16mm² tails from a 100A company fuse), or because the load is not capable of overloading, like a shower would be, in this case the breaker only has to protect against the second type of fault, to check protection against this kind of fault we do a calculation called the adiabatic to compare the current that will flow, in conjuction with the time before the fuse or breaker cuts it off to the amount of energy that the cable will withstand before it becomes dangerously overheated.

Its breaking away from standard circuits, but there is nothing wrong with that as long as you follow the design rules in BS7671, it should get documented though in the design notes.

You really should be testing electrical work anyway... but skipping over that, I'm assuming you don't have instruments to measure ELFI and R1+R2 ? ...calaculating the adiabatic is not going to be possible without test results

Just keen to learn about the theory really. Don't think much thought went into putting the shower in (that was done by previous owners). So just wanted to make sure it would be safe. My job is electronics so I do understand a lot of what your saying about your adiabatic calculation. I guess you are integrating the power dissipated by the cable in fault mode (I squared x R) before the breaker trips and then working out how hot the cable will become due to that energy dissipation ....Or something along those lines.

Not much thought does go in, 95% of the time you keep things reasonably standard and that makes stuff so much easier, its just if you want to break away into slightly more unusual arrangements that are still permitted by BS7671 that you start having to do calculations, etc.

Basically the Adiabatic compares the let-through energy of the device (I²t) I = current, t= operating time, to the amount the cable can safely take (K²S²) k = special constant from tables taking into account resistivity, maximum temperature rise, specific heat capacity, etc, S= CSA of conductor. It appears arranged in serveral different ways, but basically its just about making sure the cable withstand is higher than the let-through as I said.

Yes, its firmly based on P=I²r, r is factored into the K factor though

You have to consider it for the worst case I²t, generally fuses will give the worst case I²t at the low fault level end, and breakers at the high fault level end, but to be safe, work it through for both extremes.

Most of the time we don't bother with any of this though, there are other ways you can design that don't require this working out, although thicker cables may be needed[/i]

Adam, thanks for the explanation. I just found a whole section in my library book that explains that (Electricians guide to BS7671 I think). I'll have a good read of that tonight. Steve.

Had a good read and then did some measurements to work out PSC

Used the shower as a 35A load and measured the supply voltage drop.

Unloaded voltage 235v - loaded voltage 231v (both measured at CU)

R=V/I R=4/35 R=114 mohms

PSC = 240/0.114 PSC = 2100 A (this ties in with the fact that the substation is about 80 metres away)

t = k^2 x S^2 / I^2 t = 115 x 115 x 6 x 6 / (2100 x 2100)

t=108 ms So cable should be OK as the tables show that a 40A breaker will trip in 100ms with a 200A load.

Also did calculation assuming the short was at the far end of the 15m shower cable. Got something like PSC 1500A and t 212ms.

Adam please give marks out of 10. Steve.