On the breaker protecting the cable, I understand that 2.5mm cable used as a ring provides support for up to 27 amps. However, most installations have a 32 amp breaker. The cable would not be able to carry the overcurrent in fault conditions. What's the rationale for this?
Cable and Breaker Sizes
- Thread starter Jupiter01
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There are 2 cables which share the current. It is extremely difficult to overload the cable. The fault would have to be very near one end of the ring.
The cable is a ring and therefore the current carrying capacity is doubled.
I thought 27 amps was because of the ring and a 2.5mm on its own (as radial) has 20 amps?
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That would be the case with BS3036 rewireable fuses which take longer to blow so the cable must be derated by 0.725.
27 x 0.725 = 19.58A.
The regulation for ring circuits is still written for BS3036 30A fuses - which is why ring circuits are not really worth bothering with today.
No, 27 amps is the single cable's rating installed using reference method C. Installed in the wall in conduit, it would be rated 20amps. Two of them in parallel would be near doubled, but they are limited to 32amps by the MCB. The actual load they can support as a ring, depends upon where on the ring the load is applied - the entire load placed near one end is the worst case.
I think you are probably getting too concerned about voltage drop.
For a start, there is essentially no requirement regarding VD in the regs, beyond the fact that voltage drop should not be so great as to "impair the safe functioning of the equipment" - and I think you would struggle to think of any equipment whose "safe functioned" would be impaired by its supply voltage being too low. The 5% 'maximum' VD (for 'power' circuits) is merely a recommendation in an ('informative') Appendix of the regs.
In addition to there being no 'safety' issue, there is not even much of a 'functional' issue. Nearly all' high power' loads are thermostatically-controlled producers of heat (ovens, hobs, immersions, space heaters etc.). Even if their supply voltage is very low (due to large VD), they will still achieve the same desired temperature, the only difference being that the thermostat will cause them to be 'on' for longer periods, so gthey would take a little longer to initially 'heat up'. For the eco- or cost-conscious, higher VD means more energy 'wasted' in heating the cables - but even that may not always be truly 'wasted' if it results in the house being heated (at times when house heating is required).
I think most people would probably say that it would be plain silly to consider even 4mm², let alone 6mm² cable to supply a 13A load - unless, perhaps, the cable were incredibly long.
In any event, the 5% 'maximum recommended VD' can be argued to be rather silly, since it relates only to the VD within the installation, regardless of what the supply voltage is at the origin of the installation. For an installation with a supply voltage of 253V, even a 10% (of 230V) VD within the installation will only reduce the voltage at the load to 230V, which is considerably greater than the minimum permitted voltage at the origin (216.2V) and even more greater than would be the ('permitted') voltage (205.39V) at the load if the vo9ltage at origin wer 216.2 V and the VD within the installation 5% (of 230V).
As so often, I think that common sense is an important consideration.
Kind Regards, John
Since this is an educational/academic thread, I probably should point out that that statement is an over-simplification.
If a load is placed at the very middle of a ring then, yes, the cable (as a whole) can carry twice as much current as would be the case if there were only one 'leg' of the ring, since the total current is shared equally between the two 'legs' of the ring.
However, if one moves the load away from the middle of the ring, then the sharing of current between the legs becomes less equal, and as one approaches the situation at which the load is right at one end of the ring, one approaches the situation in which all of the current would go through one leg. That's why we hear all the concerns about over-loading the cable of one leg of a ruing if high loads are applied close to one end of it.
Kind Regards, John
A very helpful response
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You missed a space. Are you stupid?
The thing is though, electrical installations typically last many decades. you might find your supply is towards the upper end f the permitted range today but it's anybody's guess if it will still be at the upper end of the range in 30 years time.
The worst case is a break in the ring, forcing all of the current down one leg, which should be picked up by regular checks - even so the one leg on it's own is capable of maintaining around 65 to 90% of the full load of the ring. Not many people would have, or be able to afford
such a load for long.A fully functioning ring, would be even more difficult/ impossible to overload, even with all the load placed close to one end.
He may just spaced out......
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