What size MCB for my 12.8kW instantaneous water heater

[puranah69]

Guys,

Further to my last post, I'm trying to work out what MCB I should be buying for my instantaneous water heater.

I have a Hyco which is rated at 12.8kW. It has 3 metres of 10mm cable running to it from the CU - 1.5m is clipped to the wall of the airing cupboard, the other 1.5m runs in 3" trunking which it shares with the central heating boiler (9kW), but they won't be on at the same time.

I did the voltage drop calcs, using ambient temperature of 25degrees, voltage at 240v (measured) and the numbers came out at

Cable Size 10mm
Voltage Drop 0.704v
Percentage Drop 0.29%
Current Load 53.3amps
Max Cable Load 65.9amps

Whilst I understand the MCB is there to protect the cable from overloading and overheating, I also understood that the MCB had to be rated in excess of the anticipated demand. My problem is, I can't find a 55amp or 60amp MCB for my MK Sentry.

The MCB currently protecting the circuit is only 45amps. I would have thought it would have tripped as soon as the heater came on, but to date it hasn't at all. Is it most likely that although the heater is rated at 12.8kW, it's using less or might there be some other reason (I'd really like to put a clamp meter on and see what it's actually drawing - I guess I'll just have to invest the £50 or so and get one)!

Thanks guys

stu

 

[puranah69]

Oh, and should I replace the 45amp DP isolation switch for a 50amp one (I think MK do a 50amp one now)...
thx

 

[malcolmsanford]

It would seem to me whoever wired this flat didn't design it very well. a 12.8KW load in my opinion should have been wired with a 60amp MCB protection device, for instance Hager do them.

By fitting that 45amp breaker the sparks did not adhere to reg 433.1.1 (i) which states basically that the protection device should not be less than the design current. Now it may not have been his fault as the design may have called for a 45amp supply and he did that with a 10mm^ cable., which is correct as he adhered to 433.1.1 (ii)

So was it the fault of who installed the boiler as he should have noted that the 45amp protection device would contriviene the regs, with his 53 amp boiler.

Who ever or what ever it's wrong and needs to be sorted out. The reason your boiler is still operation is that most MCB's are designed to run at 1.45 it's capacity for an hour, so your 45amp breaker will carry a 65 amp load for an hour, and again that 10mm^ will just about comply to the 433.1.1 (iii)

Yes you also need to replace that 45aqmp switch, and in all honesty the 50amp is not suitable really. IMO though it would be safer than the 45amp but really you will need to get something that will be man enough to do the job. I would fit a small enclosure with a 100amp double pole isolator, the same type you have in a CU and these are readily available and less than 10 pounds at most wholesalers.

 

[puranah69]

Thanks for your reply.

I've ordered an MK 2 way enclosure and a 100A DP switch for the water heater to replace the switch in the airing cupboard. As the water heater is already on an 80a RCD in the main consumer unit, I'll just replace the MCB in there with a Hager 63amp MCB.

It's a great idea to use a standard DIN enclosure, as I'm also going to use one of the spare slots for the boiler control relay - RSComponents do a Finder 20A SPST-NC relay that's DIN mounted, so this should be perfect.

Thanks so much for all your assistance.

 

[nickblake]

What cable installation method are you using , i would never use the max current rating always look at how its installed wether its covered in insulation etc you could well be looking at a 16 mm cable requirement