Shower Circuit

[howto1]

Hi All,

I've got a problem with my shower circuit. Inside my CU there's a 30A terminal block which extends the Neutral connection from the flat T&E coming from pull cord/shower to the neutral bar inside the CU. This terminal block has burnt up and in the late stages of melting..... Before just simply changing it for a new one i've took the cover off the pull cord switch, all connections in there are tight and show no signs of overheating. I've took the front of the shower off and done the same to the connector block connections in there, all sound. Although I did notice the Earth from the T&E isn't sleeved.

Some details:

Shower rating: 8.7-9.5kW, 230-240V

I've done some ohms law (using 230V) based on the showers wattage rating and calculated the current to be between 38-41A(Ib).

Unless my maths is wrong, the terminal block is under-rated for the application. The circuit is protected by a 32A MCB, which is also under-rated, but never tripped? Now looking at the on-site guide (In should be greater or equal to Ib), which makes sense.

The supply cable into the shower looks the same as the (L)internal wiring. I suspect it to be 6mm. Now as the circuit is protected with a 32A circuit breaker, i guess this "sort of" supports my theory of it being 6mm cable (please see pic)?



My other worry (without doing external/de-rating factor calculations) first the supply cable is under-rated too.

Any thoughts?

Many Thanks

 

[Scoby_Beasley]

Inside my CU there's a 30A terminal block which extends the Neutral connection from the flat T&E coming from pull cord/shower to the neutral bar inside the CU. This terminal block has burnt up and in the late stages of melting.....

Are you talking "choc block" within your CU? 30A also (for the choc block) is low. Plus thermal variations will make the connection loose and speed up the burning out of the under rated connection.
Sorry. Read post to end. I'd say 10mm for that size cable and 40A + breaker to take the current.

But I've been to me bro's birthday bash and a few SF's are under the belt!!

 

[EFLImpudence]

8700 / 230 = 37.83A are the numbers to use. Not that it helps.

You will have to replace the connector with something correctly rated.

The mcb is also underated.

6mm² is sufficient depending on installation method.
I.e. if not running through any thermal insulation.

Sleeve the cpc.


Or, of course replace the shower. Max. 7.5kW @ 240V

 

[howto1]

Hi Both,

Thanks for the replies.

Are you talking "choc block" within your CU? 30A also (for the choc block) is low. Plus thermal variations will make the connection loose and speed up the burning out of the under rated connection.
Sorry. Read post to end. I'd say 10mm for that size cable and 40A + breaker to take the current.

But I've been to me bro's birthday bash and a few SF's are under the belt!!

Read more: //www.diynot.com/forums/electrics/shower-circuit.344990/#ixzz2Cgq9rPqV[/QUOTE]

Yes I was thinking more along the lines of 10mm and a 40A~ breaker.

8700 / 230 = 37.83A are the numbers to use. Not that it helps.

You will have to replace the connector with something correctly rated.

The mcb is also underated.

6mm² is sufficient depending on installation method.
I.e. if not running through any thermal insulation.

Sleeve the cpc.


Or, of course replace the shower. Max. 7.5kW @ 240V

Yep that's the equation I used and just rounded rounded up, then did the same for the 9.5. I guess it's a range (8.7kW-9.5kW) depending on the setting you have it on. Think it's best to stick with 41A (9.5kw/230) as my design/ F.L.C? If so i'll have to choose a connector block with a rating of 41A+.

I wonder why the MCB has never tripped, the shower is usually used at 1 setting below max, so I would say it's easily pulling more than 32A. I know the MCB won't trip at 32.01A, but think what the shower is pulling according to the maths should easily trip it?

Yeah, I guess i'll do the maths for Ca,Cg,Ci,Cf (Cf i'll use as 1).

I'll take into account voltage drop too.

Will get some earth sleeving on that cpc.

Well replacing the shower might be more cost effective and less effort.

Many Thanks

 

[EFLImpudence]

8700 / 230 = 37.83A is the only one that matters.

The figures mean 8.7 @ 230 and 9.5 @ 240

The current varies with the voltage but 230V is used for calculations.

 

[Scoby_Beasley]

The current varies with the voltage but 230V is used for calculations.

I'll not disagree with EFL with regards to our "new" 230 voltage. But calculations should always be done on the actual voltage.
Without grabbing the BGB, I think 240v was +-5V and 230 is +10v -4v.

Please, anyone remember the page, correct this.

 

[howto1]

Hello,

Once again thanks for the replies. Yup I was having a blonde moment there! Just looking in the book it shows:

Pre 1995 (240V) +6%/-6%
1/1/95 onwards (230V) +10%/-6%

Just going to have another look over these replies and decide a POA.

Thanks

 

[EFLImpudence]

The current varies with the voltage but 230V is used for calculations.

I'll not disagree with EFL with regards to our "new" 230 voltage. But calculations should always be done on the actual voltage.

I will disagree. That's the whole point of 230 as the nominal voltage.
What the actual voltage is at the time is irrelevant.

In this case all calculations should be made with the current of 37.83A.

 

[Scoby_Beasley]

I will disagree. That's the whole point of 230 as the nominal voltage.

Lowest voltage I've ever seen is 232v at the end of external lighting. At source (a pub on an evening, in the accommodation area) 245v.

Mind I would say to work on the minimum of 230v tolerance adds a safeguard.

 

[bernardgreen]

Calculating the current for a given load using 230 volts will give a design value for cable sizing and protective devices. Do the design guidelines for selection of cable and proctective devices allow for the current that the same load will take when the voltage is 253 volts. ( 230 + 10% ).

 

[Scoby_Beasley]

Hello,

Pre 1995 (240V) +6%/-6%
1/1/95 onwards (230V) +10%/-6%

Cheers Howto1. Lost the page but wasn't too far out!!

 

[EFLImpudence]

Calculating the current for a given load using 230 volts will give a design value for cable sizing and protective devices. Do the design guidelines for selection of cable and proctective devices allow for the current that the same load will take when the voltage is 253 volts. ( 230 + 10% ).

Yes.

 

[howto1]

8700 / 230 = 37.83A is the only one that matters.

The figures mean 8.7 @ 230 and 9.5 @ 240

The current varies with the voltage but 230V is used for calculations.

Hi,

Just taking this figure and linking back to my question, wouldn't you expect the circuit breaker (32A) at present to trip? I know were using 230 for our calculations, so the actual voltage may be higher, causing the current to be lower, but still the circuit I'd have thought was overloaded and should be tripping?

Thanks

 

[EFLImpudence]

Circuit breakers will never trip at 1.13 x their rating and
not until one hour at 1.45 x their rating.

Running with 46A (32 x 1.45) your 32A mcb would not trip for an hour.

A shower at 39A will take proportionately longer to trip it.

So 10/15 minute shower will never trip it but it will not do it any good.


And - no, the current increases with voltage as it is the resistance of the element which is the only constant.

9500W @ 240V means the resistance is 6.06Ω
6.06Ω @ 230V = 38A
6.06Ω @ 240V = 39.6A
6.06Ω @ 250V = 41.3A

http://www.tlc-direct.co.uk/Technical/Charts/OhmsLawCalculator.html

 

[howto1]

Circuit breakers will never trip at 1.13 x their rating and
not until one hour at 1.45 x their rating.

Running with 46A (32 x 1.45) your 32A mcb would not trip for an hour.

A shower at 39A will take proportionately longer to trip it.

So 10/15 minute shower will never trip it but it will not do it any good.


And - no, the current increases with voltage as it is the resistance of the element which is the only constant.

9500W @ 240V means the resistance is 6.06Ω
6.06Ω @ 230V = 38A
6.06Ω @ 240V = 39.6A
6.06Ω @ 250V = 41.3A

http://www.tlc-direct.co.uk/Technical/Charts/OhmsLawCalculator.html[/QUOTE]

Ah right, thanks for taking the time to explain, that makes sense now.Would you agree looking at the pic the feed cable is nearer 6mm than 10?

I think the best bet is to fit an MCB of the correct rating and replace the damaged neutral connector block, with one of the correct rating again. Seeing what damage has been caused to the connector block, due to a loose connection, overload or the combination of both, my worry is if the contacts on the MCB are in a similar condition, which may affect it's operation in the event of a fault or its integrity when used as a means of isolation.

Thanking you