4.2Kw Oven install

[JohnW2]

.... Everytime I have come across something like this the thing that goes through my mind is: "If only this circuit had been designed correctly, this damage wouln't have happened".

In different circumstances, the thing which went through your mind could have been "If only I had not got into the car /crossed the road / climbed the ladder / used the power tool / whatever ....".

Life id absolutely full of extremely small risks. The only rational way in which one can live with that is to make judgements about whether particularly extremely small risks are 'likely to happen'. Such judgements will obviously varying according to personal experiences, and personal views of risk, but it would seem that Your judgements about what is 'likely' (and/or maybe your personal view of what 'likely' means) probably differ from those of many others.

Kind Regards, John

 

[SUNRAY]

Replied in a new thread: Underated cable.

 

[333rocky333]

because people used to have to use 6mm²

May i ask, why was that

 

[JohnW2]

May i ask, why was that

BS3036 fuses, perhaps?

Kind Regards, John

 

[333rocky333]

BS3036 fuses, perhaps?

Kind Regards, John

Oh, I thought maybe 4mm was not around in the old days

 

[JohnW2]

Oh, I thought maybe 4mm was not around in the old days

I'm sure that it (and/or the Imperial equivalent) was - but even Method C 4mm² has a CCC of only just 32A when protected by an MCB, so far less than than 30A if protected by a 3036.

Kind Regards, John

 

[SUNRAY]

Oh, I thought maybe 4mm was not around in the old days

certainly 4.6mm² was available in imperial days but imperial cables were lower rated than metric.

IIRC 7/0.029 (2.9mm²) = 21A & 7/0.044 (6.8mm²) = 34A
So I'd estimate 7/0.036 (4.6mm²) ~ 27A

Cooker circuits were always 7/0.044 on 30A fuse so I understand the current use of 6mm²

 

[JohnW2]

Cooker circuits were always 7/0.044 on 30A fuse so I understand the current use of 6mm²

As I implied, 6mm² would be understandable if we were still using BS3036 fuses, but 4mm² protected by an MCB has a similar CCC.

Kind Regards, John

 

[winston1]

certainly 4.6mm² was available in imperial days but imperial cables were lower rated than metric.

Why? Are metric cables over rated? Were imperial cables under rated? Perhaps a bit of both! Are/were the regs wrong again?

 

[JohnW2]

Why? Are metric cables over rated? Were imperial cables under rated? Perhaps a bit of both! Are/were the regs wrong again?

Indeed. I suspect that he may be muddling the metric/Imperial difference with the BS3036/MCB one. In the heyday of Imperial cables, there were no MCBs and a lot of BS3036 fuses - and the effective CCC of a cable with a particular CSA (regardless of what units it's measured in) is considerably lower when it is protected by a BS3036 fuse.

 

[winston1]

Indeed. I suspect that he may be muddling the metric/Imperial difference with the BS3036/MCB one. In the heyday of Imperial cables, there were no MCBs and a lot of BS3036 fuses - and the effective CCC of a cable with a particular CSA (regardless of what units it's measured in) is considerably lower when it is protected by a BS3036 fuse.

OK. I seem to recall metric cables appeared around 1970, quite a bit before MCBs.

 

[JohnW2]

OK. I seem to recall metric cables appeared around 1970, quite a bit before MCBs.

That's probably true but, as I said, in the 'heyday' of Imperial cables, many/most were protected by BS3036 fuses, so people's ideas as to how CSA related to CCC will have been influenced by that (and, in many cases, those ideas 'stuck' when MCBs came to be more prevalent).

 

[SUNRAY]

My recollection of the introduction of imperial cables and current ratings:
1/0.044 (0.98mm²) = 5A (usually 5A fuse)/ 1.0mm² = 10A
3/0.029 (1.28mm²) = 10A (usually 5A fuse)
3/0.036 (1.97mm²) = 15A (usually 10A fuse) /1.5mm² = 15A
7/0.029 (2.9mm²) = 21A (usually 15A fuse) / 2.5mm² = 25A
7/0.036 (4.6mm²) = 28A (usually 20/25A fuse) /4mm² = 35A
7/0.044 (6.8mm²) = 34A (usually 30A fuse) / 6mm² = 40A
7/0.052 (9.6mm²) = 45A (usually 40A fuse)/ 10mm² = 60A
7/0.064 (14.5mm²) = 65A (usually 60A fuse)/ 16mm² = 80A
My new school building built 1966 (same year Istarted there) was all wired imperial, 3rd/4th year I did a fair bit of additional stage lighting wiring which was all metric and flex was brown/blue, So I agree 1970 seems bang on.

Bearing in mind I was about 15 when this happened and MCB's were available but only used in very expensive installations, the above metric current ratings are based on the max size of the fuse used and not actual ratings. In those days my recollection is none of the derating factors were used, In the mid 70's I did a fair bit of domestic new build work and circuits were pretty much installed as above regardless.

 

[JohnW2]

My recollection of the introduction of imperial cables and current ratings:
1/0.044 (0.98mm²) = 5A (usually 5A fuse)/ 1.0mm² = 10A
3/0.029 (1.28mm²) = 10A (usually 5A fuse)
3/0.036 (1.97mm²) = 15A (usually 10A fuse) /1.5mm² = 15A
7/0.029 (2.9mm²) = 21A (usually 15A fuse) / 2.5mm² = 25A
7/0.036 (4.6mm²) = 28A (usually 20/25A fuse) /4mm² = 35A
7/0.044 (6.8mm²) = 34A (usually 30A fuse) / 6mm² = 40A
7/0.052 (9.6mm²) = 45A (usually 40A fuse)/ 10mm² = 60A
7/0.064 (14.5mm²) = 65A (usually 60A fuse)/ 16mm² = 80A

I don't have any figures for CCCs (max fuse/MCB ratings) for Imperial cables, and I'm not sure where your 'current ratings' for metric cables actually come from, but the actual situation as regards metric cables is (per BS7671) as per the Table below. As you can seen the effective CCC of 6mm² cable (highest permissible available fuse rating) when protected by a BS3036 fuse is, at 30A, lower than that of 4mm² cable protected by an MCB (37A - hence, in practice a 32A MCB).

Kind Regards, John

 

[SUNRAY]

I don't have any figures for CCCs (max fuse/MCB ratings) for Imperial cables, and I'm not sure where your 'current ratings' for metric cables actually come from, but the actual situation as regards metric cables is (per BS7671) as per the Table below. As you can seen the effective CCC of 6mm² cable (highest permissible available fuse rating) when protected by a BS3036 fuse is, at 30A, lower than that of 4mm² cable protected by an MCB (37A - hence, in practice a 32A MCB).

View attachment 254361

Kind Regards, John

Thanks for the info, which I'm comfortable with other than 1.5mm² & 4mm² which were regularly used with 10A & 25A respectively.
I'll repeat my recollections

the above metric current ratings are based on the max size of the fuse used and not actual ratings. ... In the mid 70's I did a fair bit of domestic new build work and circuits were pretty much installed as above regardless.

Meter tails back then seemed to be only 2 sizes metric sizes; 6mm² up to 30A or 16mm² including those rare installations with 100A fuses.