Cable Calculations on Ring Circuit

[iDragonBox]

Hello,

I am a trainee electrician just starting to get into doing cable calculations. I understand them when it comes to a radial circuit but get lost when it comes to ring circuits.

If I take my Ib and In as being 32A for example how would I go about selecting a cable size and proving it is acceptable for the Iz? I know rings are typically wired in 2.5MM but installed in refence method B for example that would only be 23A is there a formula to show that the multiple paths of the ring allow for the cable to carry more or do I just need to state this.

Thanks for your help.

 

[Lucien Nunes]

The RFC is a special case, that has been proven by 75 years of use to be safe and reliable. The intended application is for general distributed loads spread randomly around the circuit, leading to reasonable sharing between the cables. If the load is spread evenly along the length of the ring, it will be shared equally and the highest current the cable needs to handle is In/2 = 16A. I am not near my regs but could someone please confirm (with reg no. if poss) that any cable with an installed Iz of 20A is permitted? This allows for the load to be somewhat unevenly distributed, e.g. if a large appliance is near one end, or one leg of the home run is longer than the other, resulting in up to 20A in one leg and 12A in the other.

In theory you can contrive a situation where In or Ib > Iz and the cable is technically overloaded. E.g. put four fanheaters at the first few sockets on one leg and nothing anywhere else. However the overload will be modest and these situations are rare and usually of short duration. A cable that will withstand Iz for say 50 years, might have its life reduced a few percent this way, but seeing as no-one is going to run the ring at 32A non-stop for 50 years, let alone have four fanheaters at one end for the entire time, the cable will probably outlast its spec anyway.

 

[Pretty Mouth]

I am not near my regs but could someone please confirm (with reg no. if poss) that any cable with an installed Iz of 20A is permitted?

433.1.204, minimum 2.5mm2, unless mineral insulated cable used, in which case min. 1.5mm2

 

[Lucien Nunes]

Thank you, couldn't remember whether it actually stated Iz>=20A or just the CSA. For no obvious reason, the regs were sat on the piano for months, but now I actually want to look something up at home they're in the office. Because for many years I've only really been involved with machinery / theatre / studio technical installations, it's a very long time, decades probably, since I actually installed a ring.

Also, welcome to the forum, @iDragonBox. Stick around, there is lots of knowledge available here, along with good banter / humour / nonsense.

 

[iDragonBox]

433.1.204, minimum 2.5mm2, unless mineral insulated cable used, in which case min. 1.5mm2

Thank you so much for your help really cleared it up for me!

 

[iDragonBox]

Thank you, couldn't remember whether it actually stated Iz>=20A or just the CSA. For no obvious reason, the regs were sat on the piano for months, but now I actually want to look something up at home they're in the office. Because for many years I've only really been involved with machinery / theatre / studio technical installations, it's a very long time, decades probably, since I actually installed a ring.

Also, welcome to the forum, @iDragonBox. Stick around, there is lots of knowledge available here, along with good banter / humour / nonsense.

Thanks a lot for the information was looking in my regs book trying to find something and looking for the reg online to no avail! Really helped me out so thanks a lot.