32A mcb on a 2.5mm cable

[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