9.5kw shower

[meady]

Went on a callout yesterday, burning smell from shower switch. On inspection I discovered burnt neutral on load side. Checked the cable sizing which is 6mm and shower rating is 9.5kw. Am I right in saying there is no diversity on an electric shower? And if not why has the 32amp not been tripping since it was installed? (faulty breaker)? I told the customer that he needs a 10mm cable in or replace the shower to a lower wattage unit. I'm due back there to test the circuit later but I'm not sure if I'm missing anything.

 

[Apache]

How long is the cable and what de-rating factors?

Have you looked at the graphs in the BRB for the tripping curves of MCB's? They can hold more than they are rated to.

 

[meady]

How long is the cable and what de-rating factors?Have you looked at the graphs in the BRB for the tripping curves of MCB's? They can hold more than they are rated to.

It's a long run so there is the issue of volt drop plus buried cables. Not sure on exact install of cable and lengh so I'll pop back tonight to get a better idea and get the calcs done

I'll take a look at the graphs too when I get a spare miniute. Thanks for the advice.

 

[PC Electrics]

Assuming no derating required, then 6mm is okay as it has a ccc of 47 amps (subject to volt drop if it's a long run - you're good for 20m). I always clip 6mm cable for showers round the woodwork in a loft and where it passes vertically through the insulation layer I use a piece of 4 inch soil pipe to ensure an air gap around the cable.

Surprised they haven't had any MCB tripping issues - maybe they take short showers. You could change the MCB to 40A (9.5kW shower = 39.6A).

Burned neutral connection almost certainly down to loose connection. I've repaired many like this by crimping in a new piece of conductor (if there's no slack to pull through). I've also taken to putting cord end ferrules on the conductors to stop the wires spreading under the connection screws.

 

[meady]

Assuming no derating required, then 6mm is okay as it has a ccc of 47 amps (subject to volt drop if it's a long run - you're good for 20m). I always clip 6mm cable for showers round the woodwork in a loft and where it passes vertically through the insulation layer I use a piece of 4 inch soil pipe to ensure an air gap around the cable.Surprised they haven't had any MCB tripping issues - maybe they take short showers. You could change the MCB to 40A (9.5kW shower = 39.6A).

Burned neutral connection almost certainly down to loose connection. I've repaired many like this by crimping in a new piece of conductor (if there's no slack to pull through). I've also taken to putting cord end ferrules on the conductors to stop the wires spreading under the connection screws.

Thanks for the useful info. As the burning was on the load side it could be a case of constant turning on and off the switch as I know he does. Loose connection + the constant arching from a loose termination would burn it. Hhmm that makes sense. I'll try replacing the mcb with a new 32amp and see how that goes.

 

[SPECIAL LOCATION]

Went on a callout yesterday, burning smell from shower switch. On inspection I discovered burnt neutral on load side. Checked the cable sizing which is 6mm and shower rating is 9.5kw. Am I right in saying there is no diversity on an electric shower? And if not why has the 32amp not been tripping since it was installed? (faulty breaker)? I told the customer that he needs a 10mm cable in or replace the shower to a lower wattage unit. I'm due back there to test the circuit later but I'm not sure if I'm missing anything.

Missing a few bits off me thinks....

9.5KW Will be the manufactures 240v rating

9500/240 = 39.5amps approx..

39.5A - 32A = 7.5A not a vast amount of extra currant running for a short duration not too much of a problem..

Using R1+R2 you can calc the cable length..

But TBH not many average domestic shower circuits are much more than 15m or 20m long tops!

Page 52 OSG..

clipped direct 6.0mm can take 47 amps

In conduit 6.00 can take 38 amps

If buried under loadsa insulation obviously drops down..

BUT as you already say it has been running on a 32A breaker..

not tripping..

which 32A is the rating for method A enclosed in conduit AND buried in an insulated wall!

so back to reality...

Your fault "Overheating at switch"

Is a LOSE connections when the switch was fitted..

Is it a cord pull?

If yes.. is it a square switch?

If Yes again put a circular Crabtree shower switch in.

e.g.

http://www.screwfix.com/prods/90597/Electrical-Supplies/Switches-Sockets/White-Moulded/Crabtree-Range/Crabtree-50A-Pullcord-Sw-Neon

Put a decent switch on ..

And tighten up ALL connections...

Double check connections at CU and Shower as well..

Double check insulation resistance to verify cable has NOT been damaged!

Stick a clamp meter on just to see what it is actually drawing.

all will be fine..

If you want to you could move any insulation from the cable in the loft

also you could stick a 40A breaker in IMHO..

Is it RCD protected?

UNLESS of course the customer is considering rewiring.. in which case stick in a new 10mm circuit...

and obviously if you are doing new installs or re-wire you'd be sticking in a 10mm

but if the installation is existing... and it will be lots of disruption to the decoration.. The 6.0mm will still work..

Balance of Best ideal regs solution -vs- Best safe feasible solution in customers budget.

:coffee

---------- AUTO MERGE Post added at 11:41 ---------- Previous post was at 11:39 ----------

AH...

PC's been replying while ole slow paws has been typing.

 

[meady]

All rcd protected yes, the switch is wall mounted but it's not the best tbh. It looks like a cheap b&q make.

 

[SPECIAL LOCATION]

All rcd protected yes, the switch is wall mounted but it's not the best tbh. It looks like a cheap b&q make.

Right...

Get a decent quality Brand Name switch then...

Is it a single gang size box or double gang size?

maybe one of..

http://www.tlc-direct.co.uk/Main_Index/Wiring_Accessories_Menu_Index/White_Mk_Accessories/Wall_Switches_45_4/index.html

or

http://www.screwfix.com/prods/46279/Electrical-Supplies/Switches-Sockets/White-Moulded/Crabtree-Range/Crabtree-45A-2G-Sw-Neon

or

http://www.screwfix.com/prods/98632/Electrical-Supplies/Switches-Sockets/White-Moulded/Crabtree-Range/Crabtree-45A-DP-Sw-Neon

etc...

 

[meady]

Right...Get a decent quality Brand Name switch then...

Is it a single gang size box or double gang size?

maybe one of..

http://www.tlc-direct.co.uk/Main_Index/Wiring_Accessories_Menu_Index/White_Mk_Accessories/Wall_Switches_45_4/index.html

or

http://www.screwfix.com/prods/46279/Electrical-Supplies/Switches-Sockets/White-Moulded/Crabtree-Range/Crabtree-45A-2G-Sw-Neon

or

http://www.screwfix.com/prods/98632/Electrical-Supplies/Switches-Sockets/White-Moulded/Crabtree-Range/Crabtree-45A-DP-Sw-Neon

etc...

It's a 2 gang switch.......thanks for the quick replies. In general what switch gear do u use? I always steer clear of the cheap stuff and mainly go for Mk.

 

[Doc Hudson]

Notwithstanding the guidance that no diversity is used for the first instantaneous water heater in a domestic property.

Doc H.

 

[PaulSC]

Went on a callout yesterday, burning smell from shower switch. On inspection I discovered burnt neutral on load side. Checked the cable sizing which is 6mm and shower rating is 9.5kw. Am I right in saying there is no diversity on an electric shower? And if not why has the 32amp not been tripping since it was installed? (faulty breaker)? I told the customer that he needs a 10mm cable in or replace the shower to a lower wattage unit. I'm due back there to test the circuit later but I'm not sure if I'm missing anything.

+1 to whats already been said.

With regards to your original question, it might not be a faulty breaker.

If its a type B, then your going to need a lot of current (3 to 5 times) flowing through the MCB to cause it to trip.

See OSG Table 7.5 p58

 

[Badger]

dont know if is of any use to anyone but bs7671 has a table table number FIG3.4 page 249, and says Type B circuit breakers to bs60898 or bs61009, current for the tripping time 0.1-5 seconds

6A = 30A

10A = 50A

16A = 80A

20A = 100A

25A = 125A

32A = 160A

40A = 200A

so from reading that and the graph for a trip of bs60898 or 61009, in 0.1 seconds needs 160A worth of overcurrent to flow through that breaker, in theory then a 40 amp circuit breaker 60898 or 61009 will quite happily run a 9.5kw shower for at least 30 minutes, now if most people spend unlike us guys on here about on average 20 minutes then a 40 amp breaker in my opinion will sit quite happily with that current loading.

 

[Andy™]

Am I right in saying there is no diversity on an electric shower? And if not why has the 32amp not been tripping since it was installed? (faulty breaker)?

a shower is either on or its off. so no, you cannot apply diversity

and a 32A MCB will not trip at 32.0000001A

 

[Evans Electric]

And don't forget the voltage, we all have to pretend that it,s 230v, measured one before Christmas at 250v .

 

[Andy™]

And don't forget the voltage, we all have to pretend that it,s 230v, measured one before Christmas at 250v .

i was shocked at a job the other day - plugged in tester and 219V

im not used to the display saying anything below 235

 

[Badger]

[quote name='Andy