Educate me...

[EFLImpudence]

Very little. 13A on a 27A cable.

 

[JohnD]

And what if you put 32A of load on the first one-metre?

Or 39A - a tumble drier, a washing machine and a dishwasher, all during their heating cycle?

 

[EFLImpudence]

I would think you are asking silly pointless questions.

 

[JohnD]

but I think I am trying to understand the assertion that an unbalanced ring is undesirable.

 

[AndyPRK]

And what if you put 32A of load on the first one-metre?

Or 39A - a tumble drier, a washing machine and a dishwasher, all during their heating cycle?

1. I think that adds up closer to 33A (10A each for WC DW)
2. Most kitchens these days have their own ring (ok not the origional intention of a ring), so unlikely to load up one end like that)
3. well some current will use the other "return" leg

 

[BS3036]

I think we are forgetting the original purpose of the ring final circuit. It was designed between 1942 and 1947 to provide interminable pointless arguments on the internet.

 

[AndyPRK]

It's been very sucessful, and annoyed the hell out of electricans.

 

[JohnW2]

if the 1-meter leg warms up due to the current passing through it, how much does its resistance increase?

Relatively little - but, in any event, any such rise in resistance would reduce the imbalance between legs of the ring, so that the current in that 1-metre bit of cable would reduce.

Kind Regards, John

 

[JohnW2]

but I think I am trying to understand the assertion that an unbalanced ring is undesirable.

In reality, it is nearly always far more of a "theoretical undesirability" than one of practical importance.

In the (pretty common) case of Method C 2.5mm² cable (CCC=27A) the 'concern' is almost silly. Such cable is deemed to be able to carry at least 30.5A indefinitely, and at least 39A for at least an hour without coming to harm (and even those figures appear to include quite generous 'safety margins') - so, even if a 'full' 32A load is connected very close to one end of the ring, the cable will not be at risk.

Even if the installation method is such that the CCC is only 20A (the lowest permitted for ring finals), that cable is deemed to be able to to carry at least 22.6A indefinitely, and at least 29A for at least an hour without coming to harm - so, even then, it is extremely unlikely that a large enough load would be connected very close to one end of the circuit for long enough to do any harm.

Bear in mind that very few loads in domestic premises draw high currents continuously for long periods of time, and that it is the average of significant periods (e.g. 30 or 60 minutes) that matters.

Kind Regards, John

 

[JohnD]

Relatively little - but, in any event, any such rise in resistance would reduce the imbalance between legs of the ring, so that the current in that 1-metre bit of cable would reduce.

Kind Regards, John

yes, that is what I had in mind.

I don't know how to calculate resistance change by temperature of the hot leg.

 

[JohnW2]

yes, that is what I had in mind. I don't know how to calculate resistance change by temperature of the hot leg.

That's easy enough if one knows what change in temperature one is talking about - the resistance of a copper conductor will increase by a factor of approximately 0.004 for each °C of temperature rise. For example, if the resistance is, say, 10Ω at a certain temp, then the resistance will increase to about 12Ω if the temperature rises by 50 °C [10 + (10 x 0.004 x 50) ].

Kind Regards, John

 

[SUNRAY]

I think we are forgetting the original purpose of the ring final circuit. It was designed between 1942 and 1947 to provide interminable pointless arguments on the internet.

NO IT WASN'T

 

[SUNRAY]

I think we are forgetting the original purpose of the ring final circuit. It was designed between 1942 and 1947

NO IT WASN'T designed between 1942 and 1947

 

[BS3036]

Rittenhouse strikes again...

 

[mattylad]

If I was working in the shed (if I could have one - but I cannot) and I tripped a breaker etc. then I would be really pi$$ed off having to go back into the house in order to reset it.

The best reason to have a CU in the shed/garage/workshop IMO.