Dimming Lights & Voltage Drop when using 12kw shower.

[puranah69]

Hi guys,

Recently installed a 12kW electric shower for a customer.

Customer has called me back because the lights dim quite noticeably every time he uses it.

Took some readings at the CU Main switch. Shower off = 248V, shower on = 236V, a voltage drop of around 12V (although still in tolerances 230V +6/-10%). This drop seems quite high, as I've installed the same shower in other properties and not seen any noticeable voltage drop or lights dimming (although I know 10m is quite a long run).

Ze at the service cut-out fuse is 0.15ohms, TN-S.
Incoming is 100A BS88. Meter tails are 25mm.
Cable size is 10mm, length of run is 10m, no grouping, clipped direct, no insulation.
Circuit is on 80A RCD / 50A MCB with no other siginificant circuits. No other significant power users whilst testing - just shower & lights. Lights are on the other RCD.
R1+R2 between CU & shower is 0.11ohms.
There is a 50A isolation switch about 2m from the shower. Current reading when shower is in use is about 55Amps (which seems about right). According to my calcs, 10mm @ 10m should result in voltage drop of no more than about 2.5V.

(ps, I know that 50A MCB + isolation switch is less than 55A looad, but using the 1.45x / hour rule and the NICEIC diversity/heating circular regarding average length of shower v heating effects - anyway, MK don't make a 63A MCB or a 63A isolation switch as far as I'm aware - just before everyone jumps on their hobby-horse about this)...

Anyone got any ideas why there should be such a large voltage drop (customer is worried about his dimming lights)?

Some threads seem to think it may be a DNO transformer on the way out, others that there might be a high resistance problem with the isolation switch (which I'm going to swap out anyway, just in case).

Thanks

Sparkie Stu
NICEIC Registered Domestic Installer

 

[telectrix]

let's look at the situation logically::::

problem: lights dim when shower is in operation.

solution: have a shower in the daytime.

Q.E.D.

 

[mark80]

Could it not be a loose connection in the cut out or meter?? Seems a bit extreme to suggest the supply transformer could be on its way out

 

[GLENNSPARK]

I thought maybe loose tails mark....

 

[mark80]

Ah right sorry i missed that!

 

[Knobhead]

At least we’ve got away from up rating the shower cable. It’s a supply problem, pure and simple. A bit of judicious jiggling with your test leads will get you on to the meter output terminals. Check the voltage there. If it’s within tolerance I’m afraid your pi$$ing in the wind.

 

[zaax]

and who in their right mind would buy a 12.5kW shower. it don't get hotter. it don't get wetter, just uses more juice. i blame the manufacturers marketing cons. proven by the fact that john smiths extra smooth chemical muck is still being sold.

Surely more hot water though the head during the winter as the mains water is colder than the summer