2.5mm2 ring main protected by 20amp breakers

[atm84]

So an old colleague of mine stipulates that ring main should be protected by 20 amp breakers. Why you ask?

Well he said because a piece of 2.5mm2 can only take circa 26 amps and if the ring main broke at any point it would effectively make it two radials, and it would be protected by a 32 amp breaker if done the traditional way.

I can sort of understand his reasoning but if it was such a problem surely it would be common for everyone to wire ring main with 20 amp breakers.

Any thoughts?

 

[Darkwood]

You could say that about any circuit, its why we have testing and ring mains rarely get anywhere near full load anyway. Consider a crushed or stretched cable then the ocpd may over-rate the weak spot.. its good that he thinks like this as its how to learn but we have enough regulations without the what if scenario's

 

[ImpededLoop]

If you're going to rate the protective device at 20A, you may as well run two radials rather than a ring final and save some cable.

Ring finals have been debated for years, but are fully compliant with BS7671. The same could be said for parallel cables for large submains etc.

 

[snowhead]

A piece of 2.5mm won't explode, melt down or even get hot with 32amp through it.
It's temperature may be higher than normal and may exceed recommended temps.

If we're going to be pedantic about it, the chances are the other half of the ring will have some load on so the 32 amp breaker will most likely trip before the the first half gets to 32amp load.

 

[Dave OCD]

And if you were to use 20 amp MCBs for RFCs you would undoubtedly end up with unwanted tripping out on occasions, particularly in kitchen or utility room applications. The whole point of the circuit is surely to double up on the conductor size and allow 32 amps to be drawn perfectly safely even for significant periods of time - although of course in reality this would be very rare.

 

[7029 dave]

So an old colleague of mine stipulates that ring main should be protected by 20 amp breakers. Why you ask?

Well he said because a piece of 2.5mm2 can only take circa 26 amps and if the ring main broke at any point it would effectively make it two radials, and it would be protected by a 32 amp breaker if done the traditional way.

I can sort of understand his reasoning but if it was such a problem surely it would be common for everyone to wire ring main with 20 amp breakers.

Any thoughts?

Does he now, the 30amp/32amp RFC has been around for donkeys years in this country and it has served us well.

 

[telectrix]

by the same token, ask your mate if all airlines should issue parachutes to all passengers, in case the aeroplane breaks.

 

[snowhead]

by the same token, ask your mate if all airlines should issue parachutes to all passengers, in case the aeroplane breaks.

I always take one with me and its always put in the cargo hold where I know it's safe..

 

[telectrix]

up until 2012, i'd never flown without a parachute strapped to my arse. was a wake-up call. less leg room in a 737 than in a WW2 fighter.

 

[ipf]

So an old colleague of mine stipulates that ring main should be protected by 20 amp breakers. Why you ask?

Well he said because a piece of 2.5mm2 can only take circa 26 amps and if the ring main broke at any point it would effectively make it two radials, and it would be protected by a 32 amp breaker if done the traditional way.

I can sort of understand his reasoning but if it was such a problem surely it would be common for everyone to wire ring main with 20 amp breakers.

Any thoughts?

... why do we complete a test certificate?

 

[Lucien Nunes]

The RFC cable ratings are a fudge, but one that has stood the test of time for 68 years. I wonder whether your colleague can find any documented incidents arising specifically due to overload of one leg after loss of ring continuity?

Many pilots will tell of times they flew out of envelope, which they probably couldn't do if the wings had immediately fallen off. Equally, the wings don't fall off 2.5mm at 26A, although if you are using it for advanced display aerobatics it should be inspected regularly.

 

[Hellmooth]

... why do we complete a test certificate?

Doesnt stop Mr DIY changing a socket and terminating badly though lol

 

[Nightmare1308]

Im sure the reg says that, when designing the ring final circuit each leg of the ring must be able to carry 20Amps to use a 32A breaker, as alot of un experienced electricians dont allow for themel insulation in walls and ceilings what could change its current carrying capicity, thus could lead to needing 4mm ring final circuits on 32A
Same a lighting circuits on a few of my last condition reports they throwing 1mm in, with insulated walls and ceilings on lengthy runs, when it should be 1.5.

 

[ImpededLoop]

Doesnt stop Mr DIY changing a socket and terminating badly though lol

Its no different to a radial where Mr DIY could leave a socket with no cpc or crossed polarity.

 

[Richard Burns]

Same a lighting circuits on a few of my last condition reports they throwing 1mm in, with insulated walls and ceilings on lengthy runs, when it should be 1.5.

The minimum CCC for 1mm² cable when run buried in thermal insulation is 8 A so any 6 A lighting circuit should be OK in 1mm² even if it is run (badly) buried in insulation. Admittedly a 10 or 16 A circuit would not be OK.

 

[Nightmare1308]

The minimum CCC for 1mm² cable when run buried in thermal insulation is 8 A so any 6 A lighting circuit should be OK in 1mm² even if it is run (badly) buried in insulation. Admittedly a 10 or 16 A circuit would not be OK.

I just done a cable calc to 17th Am3,
6A Design current
40m cable run
In insulation greater than 100mm
ZE 0.35
Run on its own (Not grouped, no realistic as they run with other cables but still)
= 1.5/1 Twin and Earth ??

 

[ImpededLoop]

The only thing is that the design current is likely to be lower than 6A for most domestic lighting circuits.

 

[davesparks]

I just done a cable calc to 17th Am3,
6A Design current
40m cable run
In insulation greater than 100mm
ZE 0.35
Run on its own (Not grouped, no realistic as they run with other cables but still)
= 1.5/1 Twin and Earth ??

But you were originally talking about cables in insulated walls/above ceilings. They would not be buried in insulation as they would be clipped to joists etc, so a different installation method.
The method you are using is for cables completely surrounded by insulation on all sides.

Also Ib is very unlikely to be 6A

Why do you say if is not realistic to not apply grouping? Grouping factors will almost certainly never be required for a domestic installation!

 

[baldsparkies]

I always take one with me and its always put in the cargo hold where I know it's safe..

Me to, but I also keep a cordless jigsaw in my carry on. eye iz clevur eye iz

 

[Nightmare1308]

But you were originally talking about cables in insulated walls/above ceilings. They would not be buried in insulation as they would be clipped to joists etc, so a different installation method.
The method you are using is for cables completely surrounded by insulation on all sides.

Also Ib is very unlikely to be 6A

Why do you say if is not realistic to not apply grouping? Grouping factors will almost certainly never be required for a domestic installation!

Would you say running cables in a wall which have insulation in, isn't all around the cable?
Do you not count running cables through the same route all tucked together not grouped as they would get warm and heat each other up?
If you was asked to install a 1200w heater you would run it in 1mm T&E?
My point is people change lights all the time and I always think that no one would need more than 6A and no one keeps them pendants.
Sorry but I'm the electricain who pushes that bit of cable into the ceiling for the next person.
Then again we are all differnt.....