Working out Amps?

[JohnW2]

Even better . when designing the safety critical things like fuse ratings and maximum safe current use the highest voltage in the calculation of currents that will flow.

In terms of BS7671, which seems to allow everything to assume 'nominal' voltage, that is part of a much wider discussion. BS7671 seems to be happy for a load which draws, say, 13A at 230 to have a cable whose CCC is only just 13A, and protected by a 13A fuse, even though (if a straightforward resistive load) it would draw 14.3A at 253V.

Kind Regards, John.

 

[EFLImpudence]

Even better . when designing the safety critical things like fuse ratings and maximum safe current use the highest voltage in the calculation of currents that will flow.

I thought we had already deduced that this must have been allowed for when the nominal voltage was reduced.

 

[JohnW2]

Even better . when designing the safety critical things like fuse ratings and maximum safe current use the highest voltage in the calculation of currents that will flow.

I thought we had already deduced that this must have been allowed for when the nominal voltage was reduced.

As far as I am aware, there was no change in, for example, cable CCCs at the time of the change in nominal voltage.

However, there would only be a (very small) issue if/when manufacturers started re-designing their products to consume the rated power at 230V (which I can't see happening, so long as the average supply remains around 240V). If they did that, then current at maximum permissible voltage would (assuming straightforward resistive load) be 10%, rather than the old 6%, greater than that specified at nominal voltage - so if there were previoulsy a 6% 'allowance' for that, it would theoretically no longer be quite enough.

I think we are agreed that there are some pretty massive 'margins' built into most of the figures we work with - but I suppose that would not really be a good reason to knowingly allow things to routinely 'eat into' the margins.

Kind Regards, John.

 

[EFLImpudence]

As far as I am aware, there was no change in, for example, cable CCCs at the time of the change in nominal voltage.
margins.

That was the point.
Changing the factor for all calculations would surely not have been sanctioned had the resulting 4%+ difference not been within the tolerance of the conductor; not to mention the fusing factor as previously discussed.

 

[JohnW2]

That was the point. Changing the factor for all calculations would surely not have been sanctioned had the resulting 4%+ difference not been within the tolerance of the conductor; not to mention the fusing factor as previously discussed.

Ah, I see. When you wrote "...this must have been allowed for when the nominal voltage was reduced", I thought you were suggesting that an additional allowance was added when the nominal voltage changed.

So, yes, I agree with you - that they were using some of the existing 'margin' to accommodate this change. However, as I said, this effectively means that (if when appliances/equipment starts being rated at 230V), they will have 'eaten up' some of that previous leeway (i.e. reduced the 'margin' available for other things).

Kind Regards, John.

 

[ban-all-sheds]

Surely the margin which will have been reduced will be the one before the OPD operates on overload, not a safety one?

In terms of BS7671, which seems to allow everything to assume 'nominal' voltage, that is part of a much wider discussion. BS7671 seems to be happy for a load which draws, say, 13A at 230 to have a cable whose CCC is only just 13A, and protected by a 13A fuse, even though (if a straightforward resistive load) it would draw 14.3A at 253V.

What would have been the imperial flexible cord size used on 15A radials with 15A BS546 sockets?

 

[JohnW2]

Surely the margin which will have been reduced will be the one before the OPD operates on overload, not a safety one?

Yes, but we are talking about degrees of (persistent) potential 'overload' which would be below the level at which the OPD would operate.

What would have been the imperial flexible cord size used on 15A radials with 15A BS546 sockets?

I haven't any idea - but safety considerations were different in those days, anyway.

Kind Regards, John.

 

[EFLImpudence]

BS7671 seems to be happy for a load which draws, say, 13A at 230 to have a cable whose CCC is only just 13A, and protected by a 13A fuse, even though (if a straightforward resistive load) it would draw 14.3A at 253V.

We seem to be back to square one.

You again seem to be regarding the 'maximum allowed CCC' of a conductor (as per the regs.) as the 'actual physical maximum CCC' .

As discovered in a recent thread no one appears to know the current limit before damage of any particular conductor.

Although, as we realised, the sizes of a 2.5mm² conductor and an 80A fuse wire are virtually the same and the fuse wire has a fusing factor of 2.

A CCC of 27A for 2.5mm² (as per the regs, method C) would seem to be extremely conservative to say the least.

Even if, as I put forward, we derate the CCC of the conductor by 50% (i.e down to 40A) because of its insulation and sheath (seemed logical) or, conversely, if the 2.5mm² conductor were in free air presumably its CCC could be 80A it would be hard to imagine how it could ever be damaged in normal usage.
.

 

[JohnW2]

We seem to be back to square one.

No, I don't think so ....

You again seem to be regarding the 'maximum allowed CCC' of a conductor (as per the regs.) as the 'actual physical maximum CCC' .

Not at all - that's the whole point ...

We As discovered in a recent thread no one appears to know the current limit before damage of any particular conductor.

Exactly. We are agreed that there are some massive 'margins' built into the tabulated figures of 'CCC' we work with.

The only (very trivial) point that has come under discussion here is the fact that, if equipment were to be rediseigned so as to be 'rated' at the current nominal supply voltage, under maximum permitted supply coltage that would (only slighly) 'eat into' that margin slightly more than when (as remains largely the case) appliances/equipment are 'rated' at 240V. However, the existing margins are clearly so large that this little bit of 'eating into them' is of little importance.

Kind Regards, John.

 

[securespark]

What would have been the imperial flexible cord size used on 15A radials with 15A BS546 sockets?

Table 22 has it as 70/.0076 (18A). Or are you suggesting 40/ .0076 (13A)?

It also says VD between consumer's terminals and any other point must not exceed 2.5% of the nominal voltage when the conductors are carrying flc, not including start-up, not including wiring fed from the secondary of an ELV transformer.

Nominal voltage under the 14th?

Low Voltage - Normally exceeding 50V but not exceeding 250V.

 

[Paul_C]

What would have been the imperial flexible cord size used on 15A radials with 15A BS546 sockets?

Table 22 has it as 70/.0076 (18A). Or are you suggesting 40/ .0076 (13A)?

40/.0076 was permitted on a 15A circuit, subject to the actual load not exceeding 13A.

Regulation A.10 gave three specific exemptions for:

14/.0076 (rated 3A) to be protected by a 5A fuse;
23/.0076 (rated 6A) to be protected by a 13A fuse; and
40/.0076 (rated 13A) to be protected by a 15A fuse.

The equivalent regulation 108 in the 13th edition provided only for 14/.0076 used for pendants to be protected at 5A.

The first metric set of regulations (revised 14th edition, 1970) provided similar exemptions for:

0.5 sq. mm (rated 3A) protected by 5A fuse;
0.75 (rated 6A) or 1.0 sq. mm (rated 10A) protected by 13A fuse; and
1.00 sq. mm protected by 20A fuse, subject to it being close protection (i.e. not a BS3036 rewireable fuse).

Nominal voltage under the 14th?

It depended upon the area, since this was before standardization at 240V. An idea of the leeway taken into account then can be seen in table D.1 for maximum earth-loop impedances for different fuses. The tabulated figures are shown as being for use with systems with a nominal declared voltage between 230 and 250V, with a correction factor of Eo/240 to be applied for lower nominal voltages.

 

[Space cat]

no one appears to know the current limit before damage of any particular conductor

The rate of degradation of conductors or their insulation depends upon temperature. This in turn depends on the conductor size, the current, and the thermal properties of whatever is around the cable. Given all those factors, you could work out the running temperature but that in itself won't give you a definitive current limit.

The rate of damage increases with temperature so you have to decide what kind of failure rate you're prepared to accept. The actual dependence on temperature will look something like:

Failure rate is proportional to exp(-something/kT)

where k is Boltzmann's constant (1.38 x10exp(-23)) and T is the absolute temperature. To get a low failure rate, you want to make that 'something' a lot bigger than kT.

 

[JohnW2]

The rate of degradation of conductors or their insulation depends upon temperature. This in turn depends on the conductor size, the current, and the thermal properties of whatever is around the cable. Given all those factors, you could work out the running temperature but that in itself won't give you a definitive current limit.

Quite - but one can certainly establish 'absolute ceilings'. In that previous discussion, we got to talking about fusing current of the conductor - which, as EFLI recently wrote, suggested that, say, the fusing (effectively melting) current of a bare 2.5mm² conductor may be around 160A. However, a more sensible ceiling would be based on the melting point of the insulation, obvioulsy far lower than the melting point of copper.

The rate of damage increases with temperature so you have to decide what kind of failure rate you're prepared to accept.

Indeed so - and one has to consider both immediate and progressive effects. I doubt that degredation of the conductor is going to be a significant issue, but getting any handle on this issue would require a detailed understanding (which undoubtedly exists) of the short- and long-term behaviour of insulation materials (essentially variants on the theme of PVC) in response to thermal stress.

You go on to talk about 'failure rate'. Whilst one could certainly do that in terms of 'immediate catastrophe', I'm not sure that traditional failure models are necessarily applicable to cables in practice. In other words, I'm not sure that moderate degrees of 'excessive temperature', well short of that which produces immediate catastrophe, is necessarily going to have a long-term accumulative effect of 'shortening cable life' - which is what would matter to us.

Kind Regards, John.

 

[scousespark]

The original post asked the simplest question. Step 1 in a cable calc shows the calculation. See first answer.

It's not rocket science.

 

[JohnW2]

The original post asked the simplest question. Step 1 in a cable calc shows the calculation. See first answer. It's not rocket science.

I think we all know that

Kind Regards, John.