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Hi , looking for some advice or if my question is correct please from you pros.

so my worcestor combo boiler (for few years) is fed of a 0.5mm flex from a spur, which is around 4 metres from my boiler, and has a 5amp fuse in the pull out of the spur.
ive researched and it should have a 3amp in it ideally, so my question is do cables pull more than the maximum load the experts say? Or is it because I’m guessing the tolerance of the 3amp fuse and the 5amp fuse is minimal? I know it would be silly to have a 13amp fuse put in the spur, but it seems all well and good and never any issues with the 5amp.

I know now minimal flex is 0.75mm, I researched this.
should I change to a 0.75mm flex?

thanks guys
 
Hi , looking for some advice or if my question is correct please from you pros.

so my worcestor combo boiler (for few years) is fed of a 0.5mm flex from a spur, which is around 4 metres from my boiler, and has a 5amp fuse in the pull out of the spur.
ive researched and it should have a 3amp in it ideally, so my question is do cables pull more than the maximum load the experts say? Or is it because I’m guessing the tolerance of the 3amp fuse and the 5amp fuse is minimal? I know it would be silly to have a 13amp fuse put in the spur, but it seems all well and good and never any issues with the 5amp.

I know now minimal flex is 0.75mm, I researched this.
should I change to a 0.75mm flex?

thanks guys
Can I also add sorry, the circuit it is on with is the upstairs sockets on a 32amp MCb in the fuseboard, so it’s got a 2.5mm cable feeding my boiler spur.
‘thanks
 
The two concepts are
1) how much current is the appliance drawing in normal use
2) in fault conditions will the fuse blow before the flex melts or the appliance becomes damaged.

0.5mm flex is rated for 3 amps. Your current fuse is 5 amps. So irrespective of the load (the boiler) that isn't great as the fuse doesn't protect the flex.

As main priority I'd stick a 3 amp fuse in.
 
As above, under overload conditions (less likely here) a 5A fuse is too large, but under a hard fault condition it would clear fast enough so the cable is not overheated.
 
Thanks guys, may as well change it To a 3amp then, il get one.
is it true that cables can actually take more than is stated in books (although obvs you stick to the guidelines, but are these guidelines worst case? so They can be slightly understated so people don’t abuse rules?

and if the boiler permanently works fine say forever there would never be an issue with that cable?
ive been trying to research it all and I don’t think 0.5mm has often been used in boilers for years standard practice would be 0.75mm and upwards, would that be correct?

thanks again
 
Cable ratings are not simple as you have three limits:
  • Current carrying capacity, based on maximum safe insulation temperature
  • Voltage drop, a factor for long lengths
  • Fault clearing time, so safe disconnection in under (typically) 0.4s
The first of these depends on the cable (resistance and type of insulation) but also its environment, so the ambient temperature and the level of thermal insulation around it. For critical applications you can look up the various factors and apply them, but for most cases there are simple safe guide lines.

Check the boiler spec, if it is no more than 3A then no problems!
 
Cable ratings are not simple as you have three limits:
  • Current carrying capacity, based on maximum safe insulation temperature
  • Voltage drop, a factor for long lengths
  • Fault clearing time, so safe disconnection in under (typically) 0.4s
The first of these depends on the cable (resistance and type of insulation) but also its environment, so the ambient temperature and the level of thermal insulation around it. For critical applications you can look up the various factors and apply them, but for most cases there are simple safe guide lines.

Check the boiler spec, if it is no more than 3A then no problems!
The actual glass fuse in the boiler is 1amp, so I’m guessing it’s never going to pull anything more than that anyway
 
Cable ratings are not simple as you have three limits:
  • Current carrying capacity, based on maximum safe insulation temperature
  • Voltage drop, a factor for long lengths
  • Fault clearing time, so safe disconnection in under (typically) 0.4s
The first of these depends on the cable (resistance and type of insulation) but also its environment, so the ambient temperature and the level of thermal insulation around it. For critical applications you can look up the various factors and apply them, but for most cases there are simple safe guide lines.

Check the boiler spec, if it is no more than 3A then no problems!
I’m thinking the flex is 0.75mm anyway as 0.5mm seems a bit small
 
The two concepts are
1) how much current is the appliance drawing in normal use
2) in fault conditions will the fuse blow before the flex melts or the appliance becomes damaged.

0.5mm flex is rated for 3 amps. Your current fuse is 5 amps. So irrespective of the load (the boiler) that isn't great as the fuse doesn't protect the flex.

As main priority I'd stick a 3 amp fuse in.
1amp fuse inside the boiler, so I’m guessing it will never be more than that? But what about because it’s on with the upstairs sockets? Which is fed by 32amp MCb. The fuse will protect anything after the it doesn’t it? So as long as my boiler doesn’t drag more than 5amps or 3amps as the case should be, I’m fine?
 
1amp fuse inside the boiler, so I’m guessing it will never be more than that?
Maybe, but not your problem! In any case the fuse should protect the flex to cover highly unlikely circumstances like it getting sliced/drilled through.
But what about because it’s on with the upstairs sockets? Which is fed by 32amp MCb. The fuse will protect anything after the it doesn’t it?
As the overcurrent protective device for the circuit is larger, the supply to the spur isn't really relevant. It meets the regs. The job of the spur is to "fuse down" and protect the smaller flex.
So as long as my boiler doesn’t drag more than 5amps or 3amps as the case should be, I’m fine?
Yes. Being honest I wouldn't lose lots of sleep about how it is right now, but to comply you should stick a 3A fuse in really.
 

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