32A ring, broken into 20A radial

[endecotp]

Dear Experts,

I have a 32A ring circuit. If I break it at some point, turning it into a radial that forks at the CU, and replace the MCB with a 20A one, is that always OK?

I think it's always true that the CSA of the cable will be sufficient, but I wonder if there could be voltage drop or earth impedence issues in some circumstances.

(I'm considering doing this temporarily while I do some building work.)

 

[EFLImpudence]

I have a 32A ring circuit. If I break it at some point, turning it into a radial that forks at the CU, and replace the MCB with a 20A one, is that always OK?

You seem to know.

Subject to:

I think it's always true that the CSA of the cable will be sufficient, but I wonder if there could be voltage drop or earth impedence issues in some circumstances.

Csa will be alright.
Volt drop doesn't matter unless you have any equipment which would be affected.

EFLI may be a problem.
You could measure the resistance of the cable, as a ring where you are going to split it (mid way?) end to end and work it out or give us the readings.

If there is a problem, you can always go with 16A or even lower - unless the kitchen.

 

[PrenticeBoyofDerry]

Should really be no problem splitting a ring final, and fusing down to 20A or 16A.
Providing it is done correctly

 

[ericmark]

With a ring final you can have 106 meters of cable and stay within the volt drop, with a 20 amp radial your down to 32 meters, so it all depends on how much cable is in the ring final and where it is split. With 16A radials it goes to 42 meters, so more likely to be within limits.

However I tried to calculate what loop impedance would be expected if the cable was to have the maximum 11.5 volt drop, looking at around 0.98Ω with a 16A supply so plus incoming that is around 1.33Ω, this is fair enough in theory but in practice measure a cable three times and you will get three different results. So if you measured 1.4Ω you could not say the cable was too long when the ring was split as the measurement is not that accurate.

If of course some one measures it and it's well out then they could say cable too long, if the result was 2Ω then that is a bit OTT. But question is then does the volt drop really matter that much, I have supplied a shed with LED lights where the loop impedance is well over the limit, the RCD protects from earth faults, and what is used in the shed is not really that worried about volt drop, this is OK for DIY but as a professional job you would not exceed the 6.9 volt drop to lights.

So the comment "Providing it is done correctly" does mean actually measure, and do a inspection and test of the two circuits, have you made a new circuit? I really don't know how you stand with part P.