Please can someone tell me, is comparing 4mm and 2.5mm cable the same as comparing a chainsaw and a blender?

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PotatoSoup

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Hiya, thank you for reading this :-)

I'm at the point of needing to confirm with my electrician if we want a 2.5mm ring or 4mm radial circuits putting in. I'm leaning towards the 4mm radials. The only thing I'm not sure about is whether (where there is the option to have either) opting to have a greater amount of 4mm cable running through the house is more of a safety risk than 2.5mm in terms of fire risk and shock hazards? Or is it a bit like comparing holding on to a running chainsaw blade vs putting your hand in a blender ie both situations are going to end the same way if things go wrong so it's pretty much a moot point? Thank you in advance for any help!
 
More like comparing Kartoffelsuppe or Soupe de pomme de terre

But seriously.....

Radials were my choice for the majority of my rewire of my cottage with a ring final for the small ( movable ) appliances in the kitchen.
 
PotatoSoup said:
I'm at the point of needing to confirm with my electrician if we want a 2.5mm ring or 4mm radial circuits putting in. I'm leaning towards the 4mm radials. The only thing I'm not sure about is whether (where there is the option to have either)
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I would say if the route is a ring then have a ring.
If it is a straight(ish) route without the need to return to the CU then have a radial.

PotatoSoup said:
opting to have a greater amount of 4mm cable running through the house is more of a safety risk than 2.5mm in terms of fire risk and shock hazards?
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But it won't be a greater amount, will it? 2 x 2.5 = 5

PotatoSoup said:
Or is it a bit like comparing holding on to a running chainsaw blade vs putting your hand in a blender ie both situations are going to end the same way if things go wrong so it's pretty much a moot point? Thank you in advance for any help!
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I would say it is more like comparing a ring to a line.

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A 4 mm² radial can have up to approx 40 meters of cable, where a 2.5 mm² can have 106 meters of cable approx, so much depends on the size of the property, but in general you need more radials than ring finals to wire most properties.

A MCB costs around £4 and to have an extra MCB is not expensive, but having a load of MCB's on one RCD can cause problems, so we now lean towards the RCBO which cost around £18 still not very expensive, but with 10 circuits per RCD looking at 180 - 40 + 40 (for the RCD) so approx £100 extra for 10 RCBO's compared with MCB's so being able to use half the circuits gives the ring final a huge cost advantage. Plus 4 mm² cable cost more, and in many cases can't use 4 mm² you need 6 mm² and with 6 mm² most makes of socket will not take a third cable, so no spurs possible.

Using 20 amp and 2.5 mm² is also a possibility, but then looking at 24 meters of cable to comply with volt drop. In the main people who think radials are best don't worry about volt drop, and often install non compliant circuits, and there is a case for ignoring volt drop, and the line - neutral loop impedance or prospective short circuit current is often not recorded so they get away with it.

The point is the 13 amp socket was designed to be used with a ring final, so in the main using a ring final works out best, and to calculate volt drop including corrected mV/A/m is not easy, we tend to use rule of thumb, one role a cable for one ring final, but to measure the cable to be used with 24 or 40 meters is not easy, and too easy to exceed the limit.
 
Don't forget the option of a 2.5 radial @ 20A.

There is somewhere an article by the IET discussing the pros and cons of 20A radials.

IIRC, they concluded that in many instances, you could install such a circuit as the usage in many cases (like a downstairs circuit excluding the kitchen or a first floor circuit) would be well under 20A.
 
I remember being called to a job as the welder would not work. Pre-inverter welders, it seems the welder had been used many times before with no problem, but on this job the MCB was tripping all the time. It was a 16 amp radial so measured the loop impedance and too high to fit a C type, in fact could not even for a 20 amp B type.
 
bernardgreen said:
More like comparing Kartoffelsuppe or Soupe de pomme de terre

But seriously.....

Radials were my choice for the majority of my rewire of my cottage with a ring final for the small ( movable ) appliances in the kitchen.
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That made me laugh once I got Google translate on the case!!

Please can I ask, did you put a ring in the kitchen because of them being movable?
 
ericmark said:
A 4 mm² radial can have up to approx 40 meters of cable, where a 2.5 mm² can have 106 meters of cable approx, so much depends on the size of the property, but in general you need more radials than ring finals to wire most properties.

A MCB costs around £4 and to have an extra MCB is not expensive, but having a load of MCB's on one RCD can cause problems, so we now lean towards the RCBO which cost around £18 still not very expensive, but with 10 circuits per RCD looking at 180 - 40 + 40 (for the RCD) so approx £100 extra for 10 RCBO's compared with MCB's so being able to use half the circuits gives the ring final a huge cost advantage. Plus 4 mm² cable cost more, and in many cases can't use 4 mm² you need 6 mm² and with 6 mm² most makes of socket will not take a third cable, so no spurs possible.

Using 20 amp and 2.5 mm² is also a possibility, but then looking at 24 meters of cable to comply with volt drop. In the main people who think radials are best don't worry about volt drop, and often install non compliant circuits, and there is a case for ignoring volt drop, and the line - neutral loop impedance or prospective short circuit current is often not recorded so they get away with it.

The point is the 13 amp socket was designed to be used with a ring final, so in the main using a ring final works out best, and to calculate volt drop including corrected mV/A/m is not easy, we tend to use rule of thumb, one role a cable for one ring final, but to measure the cable to be used with 24 or 40 meters is not easy, and too easy to exceed the limit.
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Thank you for the info!!

And this is why I could never be an electrician!! I don't know how people get their heads around these sorts of calculations involving voltage drops etc and can just trot them out - most impressive when you see people do it!!

My electrician is excellent so I'll obviously just leave it to him to calculate voltage drops and the like and he won't put in anything non-compliant, I'm sure

It's a really interesting point re the 13 amp socket being designed for ring finals - hadn't heard that but yeah, totally makes sense. Just out of interest what happens in other countries where radials are the norm- does it mean they have different standard sockets? I could Google but I'm being lazy ahem...

And thank you for outlining the cost implications - I appreciate it :-)
 
securespark said:
Don't forget the option of a 2.5 radial @ 20A.

There is somewhere an article by the IET discussing the pros and cons of 20A radials.

IIRC, they concluded that in many instances, you could install such a circuit as the usage in many cases (like a downstairs circuit excluding the kitchen or a first floor circuit) would be well under 20A.
Click to expand...
Thank you - I shall hunt the article down :-) My only question about that (which would obviously be the best of both worlds) is that every wall and ceiling in the house is packed with insulation (for sound reasons) so I wasn't sure if this would rule out 2.5mm radials?
 
flameport said:
Do you really expect to need multiple circuits with 7kW or more connected to them?
The vast majority of domestic electrical items have tiny power requirements.
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Nope, I'm honestly not great at applied mathematics so I'm not confident about what I need. From reading around, my (basic) understanding is that insulation causes issues and the house is packed with the stuff so I thought (quite possibly incorrectly) that we would need the bigger sized cables? This is not my strong suit at all - hence hoping to pick the brains of you clever folk!!
 
Thank you so much, All!! Reading everyone's responses, I think maybe I've started by asking the wrong question. My ideal would be to do as securespark said and use 2.5 radials. Please could someone tell me what... I don't even really know what question to ask... It's something like - if you walked into a house and every wall and ceiling had insulation in it, what would you say, please, if the customer said they were after 2.5mm radials? Would I realistically need to build service walls for the electrician? Would upsizing the cable help at all? There's nothing weird or wonderful in the house that requires an above average power supply, that I know of - just phones, computers, laptops, consoles, hairdryers, white goods, gas boiler, hairdryers, battery operated drills, a mains operated lawnmower, etc.
 
... although... BernardGreen and EFLImpudence have said there isn't really a safety issue with choosing 4mm over 2.5mm cabling in terms of shock or fire hazards, so is there a benefit to having bigger cables that would make the initial outlay worth it? This stuff makes my head spin so I didn't want to be trying to get my head round it with my electrician standing in front of me waiting for an answer - I really do appreciate your help, guys!!
 
PotatoSoup said:
Please can I ask, did you put a ring in the kitchen because of them being movable
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The maximum foreseeable load was the reason a ring final could be used for all the sockets in the kitchen. Fixed loads ( oven, induction hob and fitted microwave ) are on a radial.

The reason the rest of the cottage is on radials is due to constraints that ancient building impose on cable routings.
 
PotatoSoup said:
Just out of interest what happens in other countries where radials are the norm- does it mean they have different standard sockets?
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We have 16A circuits and 16A sockets.
No need for fuses everywhere nor polarity in the sockets, plugs and appliances because the overcurrent protection device is always in the line conductor.

That might seem horrible to you but it is only the UK ring circuit and its 32A capacity that brought about the need for the plug fuses and polarity restrictions in order to bring everything back to, in effect, the same as the 16A (for some reason 13A) circuit.
 
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