Wylex 45A MCB or Compatible

[LeighSawczyn]

Hi Peeps,
Doing an EICR where a shower cable is 6mm[SUP]2 [/SUP]from the CU to the pull switch then 10mm[SUP]2[/SUP] to the shower. Assuming the 6mm cable is rated at 47A then the Wylex 50A MCB fails In<Iz.
It's a 9.5kW Shower so is pulling 41A therefore a 45A MCB would be perfect to meet the demand whilst being rated less than the cable. The only problem is Wylex don't make a 45A MCB so who else makes one that will fit?
I'm not totally happy with putting in a 40A one as overheating alarm bells are ringing in my head. I don't fancy rewiring 10mm between the CU and the switch either.
Thanks in advance. Oh and ignore the Trainee status I don't know how to change it.

 

[Octopus]

Fit a 40A - unless they have 30+ minute showers its not going to be a problem.

 

[Geoffsd]

9500W @ 240V = 39.6A is 8740W @ 230V = 38A

Disconnect the overheating alarm bells.

 

[davesparks]

For a fixed load which is not subject to overload faults you don't need to provide overload protection to the supply cable, only fault protection.
So an electric shower on a 6mm connected to a 50A MCB is almost certainly compliant with the regulations.

 

[HappyHippyDad]

I'd go with Murdoch on this one and fit a 40A MCB. I know you've said 'assume' 47A for the CCC (ref method C) so I'm guessing it could be any ref method which would lower the CCC.

Also you could clamp meter the shower then you could see how much current is actually flowing, this would give you some peace of mind that the shower wont be tripping.

I'm not sure what you meant by 'I'm not totally happy with putting in a 40A one as overheating alarm bells are ringing in my head'? If you put in a 40A MCB this will decrease the chance of the cable overheating compared to a 45 or 50A MCB.

 

[davesparks]

If you put in a 40A MCB this will decrease the chance of the cable overheating compared to a 45 or 50A MCB.

How will the 40A reduce the chance of the cable overheating? The shower won't draw any less current as a result of changing the mcb, and the fact that it is not subject to overload faults doesn't change.

 

[Simonslimline]

I'd go with Murdoch on this one and fit a 40A MCB. I know you've said 'assume' 47A for the CCC (ref method C) so I'm guessing it could be any ref method which would lower the CCC.

Also you could clamp meter the shower then you could see how much current is actually flowing, this would give you some peace of mind that the shower wont be tripping.

I'm not sure what you meant by 'I'm not totally happy with putting in a 40A one as overheating alarm bells are ringing in my head'? If you put in a 40A MCB this will decrease the chance of the cable overheating compared to a 45 or 50A MCB.

He could mean the MCB getting hot possibly. But realistically the shower is not going to be pulling the current for long when used anyway.

 

[LeighSawczyn]

I was referring to the MCB getting hot happyhippydad. If more current flows through than it is designed for then it's bound to produce quite a bit of heat before it trips.
And I have to disagree with davesparks a 6mm cable ref C is good for 47A so if there was a fault in it then what's gonna blow first the MCB or the cable? This is more about the cable than the shower. What's the.use of In<Iz if you can use a 50A overcurrent protective device for a 47A cable? I see Figure 3A4 on page 301 shows that a 40A MCB won't actually trip at 41A anyhow more like 58A and then that's after about 4000 seconds or 67 minutes

 

[Simonslimline]

I was referring to the MCB getting hot happyhippydad. If more current flows through than it is designed for then it's bound to produce quite a bit of heat before it trips.
And I have to disagree with davesparks a 6mm cable ref C is good for 47A so if there was a fault in it then what's gonna blow first the MCB or the cable? This is more about the cable than the shower. What's the.use of In<Iz if you can use a 50A overcurrent protective device for a 47A cable? I see Figure 3A4 on page 301 shows that a 40A MCB won't actually trip at 41A anyhow more like 58A and then that's after about 4000 seconds or 67 minutes

Check BS7671 for omission of devices for protection against overload . 433.3

EDIT...

 

[LeighSawczyn]

Thanks simonslimline apologies but I got sidetracked at 433.1 and 433.1.1 (i) &(ii) though

 

[davesparks]

I was referring to the MCB getting hot happyhippydad. If more current flows through than it is designed for then it's bound to produce quite a bit of heat before it trips.
And I have to disagree with davesparks a 6mm cable ref C is good for 47A so if there was a fault in it then what's gonna blow first the MCB or the cable? This is more about the cable than the shower. What's the.use of In<Iz if you can use a 50A overcurrent protective device for a 47A cable? I see Figure 3A4 on page 301 shows that a 40A MCB won't actually trip at 41A anyhow more like 58A and then that's after about 4000 seconds or 67 minutes

If more current flows through an mcb than it is designed for then it will operate and disconnect the circuit, that's the whole point!

And you have missed the point on your second point.
You are confusing overload protection with fault protection. Overload protection is not required for fixed equipment which cannot by its nature overload. An electric shower is a heating element in a tin can, heating elements either fail open circuit or short circuit/short to earth they don't overload .

In<Iz does not apply to items which are not subject to overload, in this case you use Ib<Iz.

 

[snowhead]

It's a 9.5kW Shower so is pulling 41A

Stick a clamp ammeter on it and see what it's actually drawing.

 

[HappyHippyDad]

I was referring to the MCB getting hot happyhippydad. If more current flows through than it is designed for then it's bound to produce quite a bit of heat before it trips.
And I have to disagree with davesparks a 6mm cable ref C is good for 47A so if there was a fault in it then what's gonna blow first the MCB or the cable? This is more about the cable than the shower. What's the.use of In<Iz if you can use a 50A overcurrent protective device for a 47A cable? I see Figure 3A4 on page 301 shows that a 40A MCB won't actually trip at 41A anyhow more like 58A and then that's after about 4000 seconds or 67 minutes

Sorry Leigh, misunderstood!

I'm afraid I dont know enough about the actual make up of an MCB to know if that will 'overheat' it, I wouldn't have thought so, but I dont know. If they are rated at 6000A though???

If more current flows through an mcb than it is designed for then it will operate and disconnect the circuit, that's the whole point!

And you have missed the point on your second point.
You are confusing overload protection with fault protection. Overload protection is not required for fixed equipment which cannot by its nature overload. An electric shower is a heating element in a tin can, heating elements either fail open circuit or short circuit/short to earth they don't overload .

In<Iz does not apply to items which are not subject to overload, in this case you use Ib<Iz.

I know that 433.3.1 (ii) can apply Dave but ever since I got these results I get a bit jumpy:



Also.. we're not sure about the reference method?

 

[telectrix]

the whole point of 433.1 is that the 50A MCB is there to protect against fault current up to 6000A in the case of a short circuit. a socket circuit can be overladed just by plugging too much load in, but a shower is fixed. as said before it can only go open/short/ or fail to E.

open means it won't heat. short means a fault current which will trip any MCB. fail to E is handled by the RCD. QED.

 

[HappyHippyDad]

My apologies about the example above with the 800W microwave with a current of 5.95A. I see now that a microwave has 2 ratings

The cooker power rating (which is 800W)
and the operating power requirement or rating input (which shows 1200W on the back of the microwave),

so 5.95A is 'about' right.. I will now trust 433.3.1 !