Why do we fit a 10mm cable for showers?

[HappyHippyDad]

I'm reading a thread at the moment about a 9.8kW shower that has been installed on a 6mm cable. My natural instinct was to think that's wrong, it should be on a 10mm. Most of us would fit a 10mm cable and believe that to be to the right choice but if the reference method is C then the 6mm has a CCC of 47A and is fine isn't it? I suppose you could argue that you should fit a 50A breaker but we all know a 40A breaker will be absolutely fine. You would have to have a constant shower for perhaps a year for a 40A MCB to trip with 42.6A running through it. In fact I doubt it would even trip then.

We don't seem to question a similar scenario, i.e a 4mm radial on a 32A MCB where virtually all reference methods bar C (in a domestic situation) give a CCC <32A.

 

[davesparks]

I agree it should be on a 45A or 50A mcb, a 40A does not comply in my opinion. The regulations state we should not design a circuit with known small overloads on it, and a 42Aish load on a 40A mcb is a small overload.

 

[Andy78]

I agree it should be on a 45A or 50A mcb, a 40A does not comply in my opinion. The regulations state we should not design a circuit with known small overloads on it, and a 42Aish load on a 40A mcb is a small overload.

I think that reg is worded as small overloads for a long duration, although it does not define what sort of timescale a long duration is. I've had it suggested before that over an hour would fit that description.

Nevertheless a 42A load on a 40A MCB would not comply with 433.1.1, In to be not less than Ib.

 

[Wilko]

Re the cable - I think the concern with domestic installations is that it’s difficult to achieve clipped direct 100% of the way.

 

[HappyHippyDad]

I agree it should be on a 45A or 50A mcb, a 40A does not comply in my opinion. The regulations state we should not design a circuit with known small overloads on it, and a 42Aish load on a 40A mcb is a small overload.

I feel that's a perfectly acceptable answer Dave although you have suggested a few times over the years that citing the regs without giving justification is a bit like painting by numbers.

Do you think a 40A MCB would trip with 42.6A running through it? I feel it would never trip as the line in Fig3A4 is pretty much vertical by the time it reaches the top of the graph. Also do you think it could be dangerous in any way assuming ref method C?

I think that reg is worded as small overloads for a long duration, although it does not define what sort of timescale a long duration is. I've had it suggested before that over an hour would fit that description.

Nevertheless a 42A load on a 40A MCB would not comply with 433.1.1, In to be not less than Ib.

Same question as above Andy. It will never trip on 40A and will also be safe assuming ref method C. What if we installed a 45A MCB with a 6mm, then it would even meet the regs. Would you class that as acceptable?

Re the cable - I think the concern with domestic installations is that it’s difficult to achieve clipped direct 100% of the way.

If you have installed it yourself then you could guarantee it's clipped direct. Although I think my question is more hypothetical as you are right in what you say, it would be very difficult to ascertain clipped direct if you hadn't installed it.

Basically I did my back in last night boxing against people 20 years younger than me and I'm now bored at home and thinking of all the electrical questions I've pondered but not yet asked.

There may be more coming throughout the day, and maybe the next week or 2 !

 

[Andy78]

Do you think a 40A MCB would trip with 42.6A running through it? I feel it would never trip as the line in Fig3A4 is pretty much vertical by the time it reaches the top of the graph. Also do you think it could be dangerous in any way assuming ref method C?

Same question as above Andy. It will never trip on 40A and will also be safe assuming ref method C. What if we installed a 45A MCB with a 6mm, then it would even meet the regs. Would you class that as acceptable?

Honestly if it was an existing situation exactly as you describe then I would leave it as it was rather than change the MCB without knowing the installation method of the cable.
I'd like to check all connections for tightness and thermal effects though.

If it was a new circuit I'd design it as per regs.

If coming to put on a higher powered shower on an existing circuit, I'd not exceed the capabilities of what was already there, obviously there would be limitations on what I could inspect though.

 

[Risteard]

I feel that's a perfectly acceptable answer Dave although you have suggested a few times over the years that citing the regs without giving justification is a bit like painting by numbers.

Do you think a 40A MCB would trip with 42.6A running through it? I feel it would never trip as the line in Fig3A4 is pretty much vertical by the time it reaches the top of the graph. Also do you think it could be dangerous in any way assuming ref method C?

Same question as above Andy. It will never trip on 40A and will also be safe assuming ref method C. What if we installed a 45A MCB with a 6mm, then it would even meet the regs. Would you class that as acceptable?

If you have installed it yourself then you could guarantee it's clipped direct. Although I think my question is more hypothetical as you are right in what you say, it would be very difficult to ascertain clipped direct if you hadn't installed it.

Basically I did my back in last night boxing against people 20 years younger than me and I'm now bored at home and thinking of all the electrical questions I've pondered but not yet asked.

There may be more coming throughout the day, and maybe the next week or 2 !

Whilst it shouldn't operate the fact is that the circuit breaker is likely to be running hotter than it was designed to if the load is sustained for a lengthy period.

 

[Lucien Nunes]

A 40A MCB should never trip at 43A, nor is it even likely to trip at 50A. But at 50A the device will run with (50/40)^2 = 56% excess temperature rise compared to In. You would not sanction that overtemp with a motor or transformer but one tends to disregard it with OCPD because there's an assumption that it will protect itself. Which it will against catching fire, but the temperature cycles or prolonged operation may fatigue and degrade it prematurely.

 

[HappyHippyDad]

Whilst it shouldn't operate the fact is that the circuit breaker is likely to be running hotter than it was designed to if the load is sustained for a lengthy period.

A 40A MCB should never trip at 43A, nor is it even likely to trip at 50A. But at 50A the device will run with (50/40)^2 = 56% excess temperature rise compared to In. You would not sanction that overtemp with a motor or transformer but one tends to disregard it with OCPD because there's an assumption that it will protect itself. Which it will against catching fire, but the temperature cycles or prolonged operation may fatigue and degrade it prematurely.

Thankyou both, that makes a lot of sense.
Is there an acceptable level of overtemp for an MCB or should it always be 0% max?
In this scenario the shower is using 42.6A, so the over temp would be 13.4%.

 

[davesparks]

I feel that's a perfectly acceptable answer Dave although you have suggested a few times over the years that citing the regs without giving justification is a bit like painting by numbers.

Do you think a 40A MCB would trip with 42.6A running through it? I feel it would never trip as the line in Fig3A4 is pretty much vertical by the time it reaches the top of the graph. Also do you think it could be dangerous in any way assuming ref method C?

No I dont think a 40A MCB will trip initially but after a period of time (months or years probably) the thermal element may weaken and the MCB start to trip occasionally at increasingly lower currents.
I don't know exactly how an MCB will behave in this small overload situation over a period of time, but I doubt it will not suffer in some way because of it.
More worrying would be if the extra heat causes damage to the MCB which prevents it detecting and breaking a fault in the required time. Obviously it's a big 'what if' question and I don't have evidence either way, but I wouldn't risk it.

Whatever will actually happen in reality that MCB is a protective device being used outside of its rated operating conditions.

 

[Lucien Nunes]

Is there an acceptable level of overtemp for an MCB

No excuses, Ib<=In<=Iz. The whole scheme of circuit protection has been carefully crafted around this paradigm. All sorts of things will function in reality (my house will run on a 2.5 T+E, Green Ginger Micropacks used a 7A BS1362 fuse for a 2.5kW load etc) but a century of R & D has given us the present ratings and they should be adhered to unless special circumstances apply.

The grass is often greener and the cable is often cheaper on the other side, but when one is exploring the hole in the middle (In<Ib<1.45In) there is always a risk of tripping and falling In. The MCB toggle is greener on the other side too.

 

[Strima]

Even a slight overcurrent is bad practice as has been pointed out. Over a protracted period of time the OCPD can be damaged and start to operate outside of its designer perimeters which at the least can cause inconvenience to the customer, at worse death or serious injury.

What about a few years of use, the MCB is tired and starts tripping out regularly at low current thresholds, the customer is a DIY guru and wrongly assumes the shower is at fault. Mrs is pestering him for a better show so he throws in a 12Kw beast for good measure.

He then realises the MCB isn't big enough when it trips a couple of times so outs in a 50 amp MCB and everything is good, Mrs is enjoy hot and powerful showers.

But he's back to square one as this shower draws over 52 amps which he doesn't realise and the 6mm cable is starting to get a little warmer.

And so the cycle continues.

 

[Pete999]

I feel that's a perfectly acceptable answer Dave although you have suggested a few times over the years that citing the regs without giving justification is a bit like painting by numbers.

Do you think a 40A MCB would trip with 42.6A running through it? I feel it would never trip as the line in Fig3A4 is pretty much vertical by the time it reaches the top of the graph. Also do you think it could be dangerous in any way assuming ref method C?

Same question as above Andy. It will never trip on 40A and will also be safe assuming ref method C. What if we installed a 45A MCB with a 6mm, then it would even meet the regs. Would you class that as acceptable?

If you have installed it yourself then you could guarantee it's clipped direct. Although I think my question is more hypothetical as you are right in what you say, it would be very difficult to ascertain clipped direct if you hadn't installed it.

Basically I did my back in last night boxing against people 20 years younger than me and I'm now bored at home and thinking of all the electrical questions I've pondered but not yet asked.

There may be more coming throughout the day, and maybe the next week or 2 !

Or just an "idiots guide", for HHD Quote from Davesarks on a previous thread

 

[davesparks]

(my house will run on a 2.5 T+E, Green Ginger Micropacks used a 7A BS1362 fuse for a 2.5kW load etc) but a century of R & D has given us the present ratings and they should be adhered to unless special circumstances apply.

Are green ginger still around? I carefully removed 12 of their install racks into a skip last year.

 

[Lucien Nunes]

No, not for years. I happen to have a few dozen Micropacks in circulation. Like Mini2 they are robust and simple enough to never really wear out.

 

[freddo]

Just a note but the 9.8Kw rating is almost certainly the rating for a supply of 240V - 40.8A (It certainly was for several manufacturers). If the supply voltage is 230V the current drawn would be about 39A

 

[MJPD29]

I'm reading a thread at the moment about a 9.8kW shower that has been installed on a 6mm cable. My natural instinct was to think that's wrong, it should be on a 10mm. Most of us would fit a 10mm cable and believe that to be to the right choice but if the reference method is C then the 6mm has a CCC of 47A and is fine isn't it? I suppose you could argue that you should fit a 50A breaker but we all know a 40A breaker will be absolutely fine. You would have to have a constant shower for perhaps a year for a 40A MCB to trip with 42.6A running through it. In fact I doubt it would even trip then.

We don't seem to question a similar scenario, i.e a 4mm radial on a 32A MCB where virtually all reference methods bar C (in a domestic situation) give a CCC <32A.

A little of topic, but youd be hard pushed for the shower to be pulling full current at any time. Installed a 9.5kw last week, left it on full for 5 mins and clamped it out of interest and it never passed 36.8a when I worked it out around 40

 

[HappyHippyDad]

Just a note but the 9.8Kw rating is almost certainly the rating for a supply of 240V - 40.8A (It certainly was for several manufacturers). If the supply voltage is 230V the current drawn would be about 39A

I new the 40A MCB would be fine ??

 

[Lucien Nunes]

I agree that in practice many resistive heating loads will run noticeably under nominal rating in typical installations. I would be hesitant to rely on that observation to show that Ib<In, unless the manufacturer's data explicitly stated it.

 

[ipf]

Just a note but the 9.8Kw rating is almost certainly the rating for a supply of 240V - 40.8A (It certainly was for several manufacturers). If the supply voltage is 230V the current drawn would be about 39A

But remember, we only work to 230v, most supplies I come across are still nearer 240...…...I'll bet them Brexit burgers aren't too bothered about our small discrepancies, though.