Can I join 2 radials to make a ring?

[Sparkingmad]

Just to throw another curve ball 2.5mm t&e will carry 27A, how many sparks can honestly say they have checked at what overload current their protective devices operate at?
If you haven’t I suggest you try it for a giggle when you have time and inclination. 16A mcb’s operating after extended time with loads reaching 24+Amps. 32A mcb taking almost 60A before operating on overload.

 

[Richard Burns]

Just to throw another curve ball 2.5mm t&e will carry 27A, how many sparks can honestly say they have checked at what overload current their protective devices operate at?
If you haven’t I suggest you try it for a giggle when you have time and inclination. 16A mcb’s operating after extended time with loads reaching 24+Amps. 32A mcb taking almost 60A before operating on overload.

All this is accounted for in the cable rating values as it is a design characteristic of the MCB.
The bad situation is loading a cable at less than 1.45xIn for extended periods (small overload of long duration).
The 27A cable on a 32A MCB would have to be carefully overloaded to have a short term problem but the cable would have a reduced life in the long term as it would be running closer to its maximum temperature.
All the loading examples above will have little effect as they are short term loads, less than an hour. Having 7kW of lighting running 24 hours a day would be the killer. Heaters all have thermostats and switch off after a while, so the cable would warm then cool; possibly bad for thermal cycling but not too bad overall.

 

[Sparkingmad]

All this is accounted for in the cable rating values as it is a design characteristic of the MCB.
The bad situation is loading a cable at less than 1.45xIn for extended periods (small overload of long duration).
The 27A cable on a 32A MCB would have to be carefully overloaded to have a short term problem but the cable would have a reduced life in the long term as it would be running closer to its maximum temperature.
All the loading examples above will have little effect as they are short term loads, less than an hour. Having 7kW of lighting running 24 hours a day would be the killer. Heaters all have thermostats and switch off after a while, so the cable would warm then cool; possibly bad for thermal cycling but not too bad overall.

I agree Richard but the point I was making is that even though we may do the calculations for Design current and taking into account correction factors to determine our protective device rating to protect our cables and the circuit, the protective device can still allow continued overloading for more than just a short duration. The example I gave above would allow that circuit to operate at currents higher than it was intended to for longer periods than it should be possible too. It would not be unreasonable IMO to expect that mcb to allow that circuit to be overloaded by an extra 10A for quite a long time before it operated and continued overloading like this is possible until the mcb gets tired or weakened what is the unseen damage to the cable and detrimental effects could or would happen?

 

[Richard Burns]

I agree Richard but the point I was making is that even though we may do the calculations for Design current and taking into account correction factors to determine our protective device rating to protect our cables and the circuit, the protective device can still allow continued overloading for more than just a short duration. The example I gave above would allow that circuit to operate at currents higher than it was intended to for longer periods than it should be possible too. It would not be unreasonable IMO to expect that mcb to allow that circuit to be overloaded by an extra 10A for quite a long time before it operated and continued overloading like this is possible until the mcb gets tired or weakened what is the unseen damage to the cable and detrimental effects could or would happen?

This problem is most likely to occur on socket circuits where the load cannot be determined in advance and where say a standard 32A ring is installed in a commercial kitchen, a situation I have seen, then the circuit breaker could be routinely overloaded, if the load is over 46A then the circuit breaker will trip in less than 1 hour but during that hour the cable will be getting warm, but because it is only for one hour it should not suffer much (remembering that the cable can take its rated current 24hrs a day 365 days per year without serious degradation), it is the constant resetting of overloaded breakers that will take its toll on the cables and breaker and start to degrade them through thermal damage.