32A mcb on a 2.5mm cable

[cambria]

Just wanted to see if I'm being a bit too 'picky' but I've come across a few installations recently which have a 32A mcb for a radial circuit using only 2.5mm cable for which the current carrying capacity is approx 26A. I've been picking it up but it seems to be quite common. What do others do in these situations

 

[acat]

32A for ring 20A for radial


Chris

 

[nickblake]

AS chris has said 32amp for a ring 20 / 16 amp for a radial 2.5 is rated at 26 amps clipped direct so when protected by a 32 amp mcb its 6 amps more than carrying current capacity , and if its a rewireable fuse 3036 , even more as you have a correction factor of .725 to add to the equasion , and also if you have insulation covering it not exceeding 100 mm method 100 ,its rated at 21 amps more than 100mm its rated at 17 amps method 101 so max mcb rating would be 16 amps ,in my view your not being picky you are being correct so ,coding its a risk of overload so Code 1 it

 

[Graeme Harrold]

Defo 'code 1' as it aint safe, dosent matter how common it is.

 

[cambria]

Cheers for the replies guys it confirms I was being correct its just that I've come across this so often recently it makes you wonder

 

[nickblake]

yep alover the place especially in rewireable boards 3036 say 30 amp fuse 3036 / 0.725 = 41.37 amps

in the case of other factors that may be required : In / Ca x Cg x Ci x .725 = Amps

Correction factors :
Ca ambiant temperature
Cg Grouping
Ci surrounded by thermal insulation
0.725 correction factor for BS3036 rewireable fuses

 

[fixit4u]

code 1 needs replacing with 20a mcb

 

[Spudnik]

Im not convinced its a code 1, as it doesnt present an immediate danger.

What about spurs?

Edit: Once again i didnt read the op properly. Been a looooooooong day.

 

[nickblake]

It could if you stick large loads on it , understand what your saying about spurs but these normally are short lengths,however from the CDU it could be right through the loft and have more than one socket on it , when i do a PIR if i find these i test them indipendantly and some time you can have anything up to 10 sockets on it hence my reason for Code 1, jason i know how you feel get a beer and stick you feet up

 

[andekoch]

did a cu change the other day and found the immersion heater on a 30 amp rewirable with a 2.5mm cable. cable in conduit in a brick wall (equivalent derating of method B (if that is conduit on a wall) I believe, memory fades as the night wears on).

was tempted to put the new mcb in as a 32A.

the load (based on kw rating of immersion elements and other 'stuff' being fed from the circuit was around 20A.

good IR and very short run so low R1+R2 on the cable. about 15years old installation.

what would you guys have installed as a breaker?

 

[Spudnik]

20A all the way.

 

[andekoch]

my sentiments exactly, no point in continuing a bad practice.

 

[gaz2212]

been testing in a hotel most of the week... about 10 radials on 2.5 protected by 30amp mcbs!!!

 

[davelerave]

of course it's an overload risk

 

[lamb]

If the 2.5 supplies a fixed load less than the CCC of the cable, then the 32 amp breaker is providing short-circuit protection only (and obv. earth fault). If it can be shown by calculation that this breaker will operate under short circuit conditions in a time sufficient that the conductors will not rise above their final limiting temperature (from their normal operating temp.) then the circuit complies with BS 7671 and is safe.

 

[Graeme Harrold]

If the 2.5 supplies a fixed load less than the CCC of the cable, then the 32 amp breaker is providing short-circuit protection only (and obv. earth fault). If it can be shown by calculation that this breaker will operate under short circuit conditions in a time sufficient that the conductors will not rise above their final limiting temperature (from their normal operating temp.) then the circuit complies with BS 7671 and is safe.

EH NO.......The current rating of the MCB CAN NOT BE HIGHER than that of the cable. Absolutely nothing to do with the connected load on this one. The MCB is there to protect the cable not what is connected to it.

 

[lamb]

Have a quick read through 'Electrical Installation Design Guide' published by the IEE, and 'Guidance Note 1 - Selection and Erection'. These will point you to the relevant regs in BS7671 that allow for the ommission of overload protection in circuits with fixed load.
Don't have my books with me at the mo.

 

[davelerave]

hardly good practice on final circuits-someone may up the load later

better to match cable to protective device

 

[lamb]

Agree, not good practice.
But PIR codes are for deviations from BS7671, not a judgement on good or bad practice.

In addition, you can assume that "someone upping the load later" will be an electrician, who will take account of the cable current carring capacity and protective device.

 

[Risteard]

Lamb is correct. A fixed load does not require overload protection.

 

[Graeme Harrold]

Lamb, you are technically correct see 433.3.1 (ii) provided fault current protection is in place, however OP did not mention fixed current loads, and I would still say such a circuit is lazyness on part of the designer or installer and would still prefer the rules of 433.1.1 to apply as its just safer all round.

 

[davelerave]

it's aTKO

 

[lamb]

Graeme, I agree with you in principal.
I just don't like it when people say 'a 2.5 radial on a 32A MCB is wrong', that is a simplistic view, it may or may not comply with BS7671.

I had a previous QS that would always find something wrong with my work (others on the firm suffered more than me), so I learned very quickly how to back myself up with BS7671 and OSG, which made him shut up and gave the other lads a good laugh at team meetings.

 

[Denatone]

In addition, you can assume that "someone upping the load later" will be an electrician, who will take account of the cable current carring capacity and protective device.

Call me a cynic but i don't think you can ever assume this, there's too many cowboys still about.

 

[keith915]

Where's the advantage of putting a 2.5mm with fixed load on a 32A breaker? Only one I can see is cost cutting.... if fixed load is above 20A you could use a 2.5mm instead of a 4mm.

Also does the fixed load have to be a purely resistive load...? i.e immersion