Unbalanced final ring

[Steve-S]

I have seen a final ring that had the heavy appliances on one leg (L1). Drawing 19.5 amps in total, one leg (L1) was drawing 82% of the 19.5 amps of current. If the ring went up to 32 amps the 2.5mm cable would in theory be just safe enough under the 27 amp maximum.

My question is, if 32 amps worth of appliances were drawing current on L1 would the 82% be reduced as there would be more resistance on L1, so more current would be sent around L2, reducing the 82%?

I have not seen any tests on this. It would be interesting to see some.

 

[littlespark]

Is this a practical situation?

The general norm for 2.5mm rfc is a 32A OCPD, what is on the circuit just now?

If it’s 20A, I’d be worried it wasn’t an ring at all




For your question though….. if you’re pulling the full load of 32A, is the ratio of what percentage of the load goes back on which leg different?

That would be down to how far each leg is from the board, not the load or cable size.

A high load (say 32A) midpoint on a ring, you would expect it to be 50/50 (16 on each leg)
The closer to the board on one leg the further from the other…. But even if your load was only 1m from the board, some of that load would still return on the longer side of the ring. It would never be 100% one way.

The 82% you measured is relative to your particular installation, with its particular lengths of cable.
That percentage will increase as the load moves away from the board, and increase as it gets nearer.

 

[PS Electrics]

I have seen a final ring that had the heavy appliances on one leg (L1). Drawing 19.5 amps in total, one leg (L1) was drawing 82% of the 19.5 amps of current. If the ring went up to 32 amps the 2.5mm cable would in theory be just safe enough under the 27 amp maximum.

My question is, if 32 amps worth of appliances were drawing current on L1 would the 82% be reduced as there would be more resistance on L1, so more current would be sent around L2, reducing the 82%?

In theory yes because resistance is dependant on cable temperature and cable temperature is effected by load current.

I have not seen any tests on this. It would be interesting to see some.

There are calculations for this that would be interesting I guess to play around with.

I would imagine the effect would be very tiny though to be of no practical use.

 

[Steve-S]

In theory yes because resistance is dependant on cable temperature and cable temperature is effected by load current.


There are calculations for this that would be interesting I guess to play around with.

I would imagine the effect would be very tiny though to be of no practical use.

This is an actual installation with a complete ring with no bridges or breaks. 50 square metres of floor area with 21 sockets, all double except two singles. It is a finished flat, so no simple opportunity to run say 4mm on leg one (L1) from the breaker in the CU to the first two or three sockets, to ensure that section of the ring does not overheat.

Calcs are fine but I thought someone on Youtube or whatever may have done a demo of this sort of thing, the sort if thing John Ward does, or real live written test data in an actual house. Of the 19.5A drawn on the full ring 15.99A was going through one leg. It is borderline as 82% of 32A is 26.24A, if the ring is on max load, just inside the 2.5mm cable's maximum rating.

 

[DefyG]

I'm confused as to what you are referring to as one leg (L1)? If it is a ring (RFC) there is no 'one leg'!
Ideally the loading should be equally spread around the ring but in practice if part of that ring goes by way of a kitchen its possible it may take more of the load, but not all of the current flow as some would still flow around the ring.
As per #3 there are calculations but if you stick to the OSG recommendations, should be fine as they take into account the calculations.

 

[Steve-S]

I'm confused as to what you are referring to as one leg (L1)? If it is a ring (RFC) there is no 'one leg'!
Ideally the loading should be equally spread around the ring but in practice if part of that ring goes by way of a kitchen its possible it may take more of the load, but not all of the current flow as some would still flow around the ring.
As per #3 there are calculations but if you stick to the OSG recommendations, should be fine as they take into account the calculations.

There are two conductors off the breaker L1 and L2 they meet in the middle forming a ring.

I am not after advice on how to install a ring which you are giving. I explained that the ring is already there with no opportunity to easily amend it. Where the ring snakes around the place is a mystery at present. I was after some real life test data that proves or disproves the 82% of the ring's current on one leg may shift one way or the other when the ring is on full load - 32A from 19.5A. Many here may know pointers to such data or vids, or give some real life experiences of such situations.

 

[Avo Mk8]

Calcs are fine but I thought someone on Youtube or whatever may have done a demo of this sort of thing, the sort if thing John Ward does, or real live written test data in an actual house. Of the 19.5A drawn on the full ring 15.99A was going through one leg. It is borderline as 82% of 32A is 26.24A, if the ring is on max load, just inside the 2.5mm cable's maximum rating.

Here's sort of a simple demo, but not in a house, of what you were asking:

View: https://youtu.be/FNgTnFs7f5o?feature=shared

I note the word 'leg' is also used in the video!

My take is that many appliances don't constantly run at maximum demand, they generally have thermostats that turn on & off, or the appliance is used for a short time (eg kettle, hob, food processor, toaster, etc). Even dishwashers, washing machines or tumble dryers only draw full load while their heating elements get stuff up to temperature, and then the thermostat manages consumption. So the heating effect on ring final cables would not perhaps be as great as instantaneous measurements might suggest. I guess you would need to monitor over time to find out.
Also worth mentioning that fixed loads over 2kW, such as electric radiators, should be on a dedicated radial circuit, not the ring.

 

[Steve-S]

Here's sort of a simple demo, but not in a house, of what you were asking:

View: https://youtu.be/FNgTnFs7f5o?feature=shared

I note the word 'leg' is also used in the video!

My take is that many appliances don't constantly run at maximum demand, they generally have thermostats that turn on & off, or the appliance is used for a short time (eg kettle, hob, food processor, toaster, etc). Even dishwashers, washing machines or tumble dryers only draw full load while their heating elements get stuff up to temperature, and then the thermostat manages consumption. So the heating effect on ring final cables would not perhaps be as great as instantaneous measurements might suggest. I guess you would need to monitor over time to find out.
Also worth mentioning that fixed loads over 2kW, such as electric radiators, should be on a dedicated radial circuit, not the ring.

Thanks v much. I had a look.

a) He had a total load of 8.56A with one heater on the socket nearest to the CU. The leg the heater was on drew 7.1 A, which is 82.94% of the total load.

b) At the end he had a ~5kW load which is 21.73A at 230v which is 2.53 times more than the first test in a). He measured 16.43A on the loaded leg which is 75.61% of the total load - down 7.33%. So the percentage draw dropped. At 75% at 32A full load leg 1 would draw 24.2A, within the max current limit of the 2.5mm cable - that is if it stayed at 75.61% of course.

So the percentage of the load on the loaded leg, leg 1, did go down the greater the total load. But to drop only 7.33% the total load was increased 2.53 times.

In my situation, it is: washing machine, kettle, dishwasher, microwave, tumble dryer, all clustered in the kitchen on the first leg from the CU. On a total load of 19.5A 82% is coming from one leg, on not too off a total load to the final test in the Efixx vid. In my situation there is obviously more resistance on leg 2 than in the Efixx test. A full test with all appliances on has not been done, as some were not there to test.

The final conclusion is that the test by Efixx indicates that the higher the load the less the percentage draw on leg 1 with more current coming around the back from leg 2. So in my situation if the total ring load went to near 32A the percentage of the load on leg 1 would drop just below what the cable is safely capable of holding.

Anyone agree?

 

[Steve-S]

Here's sort of a simple demo, but not in a house, of what you were asking:

View: https://youtu.be/FNgTnFs7f5o?feature=shared

I note the word 'leg' is also used in the video!

My take is that many appliances don't constantly run at maximum demand, they generally have thermostats that turn on & off, or the appliance is used for a short time (eg kettle, hob, food processor, toaster, etc). Even dishwashers, washing machines or tumble dryers only draw full load while their heating elements get stuff up to temperature, and then the thermostat manages consumption. So the heating effect on ring final cables would not perhaps be as great as instantaneous measurements might suggest. I guess you would need to monitor over time to find out.
Also worth mentioning that fixed loads over 2kW, such as electric radiators, should be on a dedicated radial circuit, not the ring.

Yes, the likelihood of all appliance being on and all drawing max current simultaneously for long periods is slim - diversity. But Murphy's Law says if it can happen it will happen.

 

[Avo Mk8]

There are learned articles on the web showing load profiles of kitchen appliances, that show many of them do not consume their maximum rated current all the time - in fact some don't consume that current most of the time.
In your example of washing machine, kettle, dishwasher, microwave, tumble dryer, you would need a pretty frenetic family to be operating all of them at once, and if they were, those such as the kettle or microwave would be off again (at full power) within a few minutes, otherwise the food in the microwave would be on fire, or the kitchen full of steam!

Try applying diversity to loads for a more realistic estimate.
And if you have the time, here's a comparison of calculations vs. actual:

View: https://youtu.be/PnagUPuFlak?feature=shared

 

[DefyG]

Where the ring snakes around the place is a mystery at present.

So in my situation if the total ring load went to near 32A the percentage of the load on leg 1 would drop just below what the cable is safely capable of holding.

As the installation is a mystery at present you dont know what the installation method is, you wont know what derating factor to apply and therefore current the cable is safely capable of carrying before overheating.

 

[PS Electrics]

I have seen a final ring that had the heavy appliances on one leg (L1). Drawing 19.5 amps in total, one leg (L1) was drawing 82% of the 19.5 amps of current. If the ring went up to 32 amps the 2.5mm cable would in theory be just safe enough under the 27 amp maximum.

My question is, if 32 amps worth of appliances were drawing current on L1 would the 82% be reduced as there would be more resistance on L1, so more current would be sent around L2, reducing the 82%?

I have not seen any tests on this. It would be interesting to see some.

I assume you are investigating this because of some issues?

My only concern regarding the installation would be if the ring was not installed to best practice. I was always taught to try as best as possible to run the cables so sockets are spaced out evenly along the ring rather than wire them in a straight line with one long dead return leg so that loads are balanced along the ring as best as possible, obviously though with bad luck the ring can still be skewed with loads on one side of the ring.

As Avo Mk8 said though I cannot see a scenario where this should be an issue but since you are investigating this something must be awry?

 

[Pretty Mouth]

This question piqued my interest, so I played around with the numbers, and concluded that the heating effect on the cables doesn't make much difference to the proportion of current that will flow down the short leg vs the long leg.

Example ring final:

30m long
Cable rated at 20A
30A load all at one point exactly 10m along one leg.

Initially, the current will flow through the legs in the proportion 20:10

(10/30) X 30 = 10A flowing through the long leg 20m
(20/30) X 30 = 20A flowing through the short leg 10m

As the short leg of the ring is loaded to its full rating, it will theoretically heat to its full 70 degree operating temperature, and its resistance would increase by approx 1.2 times. Some heating should also occur in the long leg, but a lot less, so for simplicity, lets assume no heating of the long leg.

Now the current should flow in the proportion 20:12

(12/32) X 30 = 11.25A flowing through the long leg
(20/32) X 30 = 18.75A flowing through the short leg

So in this example, the extra resistance caused by the heating of the short leg, caused 1.25A to 'redirect' down the long leg. Which is very little, about 4% of the total 30A.

 

[Steve-S]

I assume you are investigating this because of some issues?

My only concern regarding the installation would be if the ring was not installed to best practice. I was always taught to try as best as possible to run the cables so sockets are spaced out evenly along the ring rather than wire them in a straight line with one long dead return leg so that loads are balanced along the ring as best as possible, obviously though with bad luck the ring can still be skewed with loads on one side of the ring.

As Avo Mk8 said though I cannot see a scenario where this should be an issue but since you are investigating this something must be awry?

No issues. I saw the ring could be unbalance by the proximity of the CU to the kitchen cluster. As I have said the ring is in, in a fished place. My quick test was as I reported also with a total ring resistance of 1.2 ohms on L & N with a multimeter. The Efixx vid appears to indicate that the 2.5mm cable will not draw more than the max permitted current when just under 32A. With diversity the likihood of drawing a full load for an extended period is slim.

When there is no option to round robin the sockets to balance with an unbalanced ring looking likely, it is best to run on leg 1 4mm cable to the cluster of high current drawing sockets and then 2.5mm for the rest.

This question piqued my interest, so I played around with the numbers, and concluded that the heating effect on the cables doesn't make much difference to the proportion of current that will flow down the short leg vs the long leg.

Example ring final:

30m long
Cable rated at 20A
30A load all at one point exactly 10m along one leg.

Initially, the current will flow through the legs in the proportion 20:10

(10/30) X 30 = 10A flowing through the long leg 20m
(20/30) X 30 = 20A flowing through the short leg 10m

As the short leg of the ring is loaded to its full rating, it will theoretically heat to its full 70 degree operating temperature, and its resistance would increase by approx 1.2 times. Some heating should also occur in the long leg, but a lot less, so for simplicity, lets assume no heating of the long leg.

Now the current should flow in the proportion 20:12

(12/32) X 30 = 11.25A flowing through the long leg
(20/32) X 30 = 18.75A flowing through the short leg

So in this example, the extra resistance caused by the heating of the short leg, caused 1.25A to 'redirect' down the long leg. Which is very little, about 4% of the total 30A.

Nice. That 4% seems to fall in line with the Efixx test, which dropped by 7.33%.

 

[Steve-S]

As the installation is a mystery at present you dont know what the installation method is, you wont know what derating factor to apply and therefore current the cable is safely capable of carrying before overheating.

Looking at the construction, a lot of hollow partition walls, it looks like 22-23A for the 2.5mm. Borderline. Relying on diversity for sure.

In my own place all heavy appliances are on their own radials: w/machine, oven, hob, dishwasher, boiler (so it is not dropped out by other circuits faults). I only use a toaster, rice cooker, TV, laptop and small fridge and vacuum on the final ring, which IS also unbalanced on an AFDD, but nothing there to overload it if I tried.