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

 

[GLENNSPARK]

just done the calc mate and you have 50.84A=51A . Could be length of run mate as thats quite a bit of current to get along to point of use. By the way, why a 12 KW shower? I know you have to install what the client wants but it should never really be used to capacity. Have you checked all joints? Even at the dis board (including tails)? Try putting the shower on a lower setting and see if theres any appreciable difference........

 

[GLENNSPARK]

What do you think tel?

 

[puranah69]

He'd already bought the shower - Triton seem to be punting out these huge-load showers at the moment... biggest = best, I guess... Yes, I ran it at a lower setting, voltage drop was lower, but he thinks that's not the point.

I thought about upping the cable size to 16mm tw/e, but the point is, according to calcs, voltage drop should only be 2.5v and it's nearly 12v so something clearly not right. Possibly a fault in the shower itself? (btw, how do they get away with such thin poor cables inside the shower - is it because the lengths are so short?)

thanks
sut

 

[GLENNSPARK]

Short lengths mate. Dont forget that heaters are resistive loads which may account for the volt drop but check all terminations are good and do an R1 R2 and an R1 RN to be sure..........

 

[Monkey]

hi bud,

Dont do this at home

but I have installed electric boilers which take a huge demand but not seen volt drop or dimming but i never so no to any thing try this

run a temp 16 mm feed to it and run shower if prob goes you know y

if stays well its nothing yove done but speak to the supplier of the electricity

you ve then elimnated it the cable but start at one end and recheck all connections one by one

good luck

 

[puranah69]

Thanks. Will try that Monday and let you guys know

 

[telectrix]

you've also got the possibility of volt drop on the supply cable, esp. if it's in a rural area. some areas round here, it's a bonus to get over 215V at tea time.

 

[telectrix]

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.

 

[puranah69]

Yeah, I thought of that - although the place is at the end of the road, the substation is only about half a mile away. 0.15 on a TN-S is also a pretty low reading, so unless the transformer is under high load or undersized or breaking-down, not sure it's the rural factor. Would have thought that, if it was external factors, would have seen a low volts reading anyway, not such an obvious 12v voltage drop when the shower was switched on...

 

[puranah69]

Anyone think it's worth calling UK Power Networks and seeing if they can shed any light on it?

 

[GLENNSPARK]

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.

Mad innit? This "american" attitude that bigger is always better.....total b**locks

 

[puranah69]

On that subject, pity the poor ol' OAP's I went to a few months ago - they'd bought a 9kW shower, wanted me to install. They had an old GEC Ceramic 30A with a fused neutral as an incomer (and the meter had been replaced - disgraceful!). I called the DNO, but they had to replace the incoming main, and the old dears didn't want their petunia's dug up...

So I had to suggest they take it back to Screwfix and get their money back! lol

 

[GLENNSPARK]

Its not there fault. The avarege layman is largely ignorant to current demands, volt drops etc. I dont think manufacturers help when they advertise these things either, i mean how many times have i heard selv`s being refered to as "low voltage"?.......

 

[puranah69]

The fact that the DNO's call anything under 600v low voltage doesn't help!

 

[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.