UK How to ascertain amperage from cable thickness?

[James1985]

When coming across the average common cable in UK residential/commercial setups, is there a way via cable diameter/thickness that we can accurately determine the cable specs?

Some cables may have thicker sleeving etc, so I assume its not a simple case of just measuring thickness alone?

Some example cases

a) Residential - Assuming we have a 60A service fuse installed, Determine(through cable thickness) if the main service cable is capable for 100A upgrade

b) Commercial - Assuming we have a 300A 3ph(100A per phase) installed, Determine if the main service cable is suitable to upgrade to 600A 3ph(200a each phase)

 

[Taylortwocities]

Cable size may not be the only consideration, other loads on the phase may limit the max current available. Also, if you have a linked supply you will not be permitted to add more large load (eg EV charging) without the supply cabling being upgraded.
Also there are several types of cable, some have earthed serving, with the earth being supplied. Some are split concentric where the earth is derived from a combination Neutral/earth. All of these affect the current carrying capability.

So it ain't as easy as you think. Call the DNO and ask them.

 

[James1985]

Cable size may not be the only consideration, other loads on the phase may limit the max current available. Also, if you have a linked supply you will not be permitted to add more large load (eg EV charging) without the supply cabling being upgraded.
Also there are several types of cable, some have earthed serving, with the earth being supplied. Some are split concentric where the earth is derived from a combination Neutral/earth. All of these affect the current carrying capability.

So it ain't as easy as you think. Call the DNO and ask them.

Hey taylor, thanks for your input.

However, my examples where just that, and not actual requirements.

The "earthed serving" example you gave, would there be writing specification on the outer-most sleeving of the cable indicating such inside?

As for other loads limiting the current available, in this case we can omit this because I am trying to ascertain if we can calculate amp rating of cable(how its then distributed is not relevant)

 

[Taylortwocities]

Again, it isn’t easy. Take the example of a supply cable that has an earthed outer and two internal conductors (L&N). Typically used on TN-S supplies. The outer of the cable may be further wrapped perhaps with fabric, perhaps with all sorts of other covering (plastic, pitch, all sorts). The inner conductors in a new cable will be metric. Older cable will be imperial. None of these cables will have any external markings. The outer diameter will vary depending on these internal factors.

Some cables have standard round conductors. Some have “waveform” conductors where the conductor is a sort of triangular shape - this allows a bigger load capability in a thinner cable. There are different types of concentric cable.
Some cables are aluminium, some are copper. This has effects on current capabilities.
Have browse HERE. These are some. There are many more that are installed now and a myriad from the past.

Again, the only people who really know are the DNO.

 

[DPG]

Again, it isn’t easy. Take the example of a supply cable that has an earthed outer and two internal conductors (L&N). Typically used on TN-S supplies. The outer of the cable may be further wrapped perhaps with fabric, perhaps with all sorts of other covering (plastic, pitch, all sorts). The inner conductors in a new cable will be metric. Older cable will be imperial. None of these cables will have any external markings. The outer diameter will vary depending on these internal factors.

Some cables have standard round conductors. Some have “waveform” conductors where the conductor is a sort of triangular shape - this allows a bigger load capability in a thinner cable. There are different types of concentric cable.
Some cables are aluminium, some are copper. This has effects on current capabilities.
Have browse HERE. These are some. There are many more that are installed now and a myriad from the past.

Again, the only people who really know are the DNO.

Don't forget the 'waveform' bit refers to the way the outer strands are shaped (ie. not spiralled around) to ease jointing.

 

[James1985]

Double post..............sorry

 

[James1985]

Again, the only people who really know are the DNO.

Out of curiosity I rang the dno, they couldn't answer the question, only saying a single phase would be 23kva.

I've heard from numerous sparkies that despite the fuse being 100a, the main service cable can actually take all the way upto 400a in some cases.

However we are going off topic slightly. The main question being how to determine cable rating/capacity via diameter of sleeving... which is seeming to be not possible unless knowing the internal specification of the cable... such as cores, Insulation etc

 

[Lucien Nunes]

As above, there are so many parameters, different British standards, manufacturers, cable constructions and finishes etc that it takes a series of educated guesses, experience, a dozen books of data tables and manufacturers' specs going back 50 or 75 years (some service cables are even older than that) and you might still have to make do with an answer along the lines of 'probably either 0.0225 sq. ins. if it's round or 0.04 if it's sector, but maybe chunky 0.015, which is useless for determining the current rating.

One of the problems is that with certain types such as small sizes of PILCSTA, the conductor makes up only a small fraction of the total cross-sectional area of the finished cable. The difference in outside diameter between two conductor sizes might be less than the difference between one production batch and another. Also it is the cross section of the conductor that is important, which varies as the square of the diameter. Small changes in diameter lead to significant changes in CCC.

Imagine a pencil wrapped in 50 turns of toilet roll and covered with duct tape, and then having to decide from the outer diameter whether the pencil is 6 or 7mm diameter. Bearing in mind you can't tell whether it's wartime economy bog roll or luxury 3-ply.

 

[pc1966]

Out of curiosity I rang the dno, they couldn't answer the question, only saying a single phase would be 23kva.

UK domestic supplies are up to 100A = 23kVA for the nominal 230V. Actual voltage is usually different and often closer to 240V which was the UK standard until harmonisation (where UK's 240V and EU's 220V became 230V with wider tolerances so neither system had to change anything in practice).

However, many DNO will have the fuse at 80A or 60A, not always because the supply cable is unable to cope with more, but usually as the total load on the substation is limited and they need some means to put a sane limit on what homes might take for short periods.

For long periods the cost usually limits peoples use!

I've heard from numerous sparkies that despite the fuse being 100a, the main service cable can actually take all the way upto 400a in some cases.

Not the usual 16mm or 25mm concentric that feeds the typical cut-out! However, it is common for the cable they come off to be protected by 400A fuses.

However we are going off topic slightly. The main question being how to determine cable rating/capacity via diameter of sleeving... which is seeming to be not possible unless knowing the internal specification of the cable... such as cores, Insulation etc

It is all but impossible unless the cable as its type and specification printed on it, which some new cables do but most older cables do not.

As mentioned above, there is more to max capacity then the cable size as you usually have the following to consider:

• Current carrying capacity. This limit is based on how hot the cable can safely run, which is usually determined by its type of insulation, how hot the environment is, and any other thermal insulation factors from how it is run such as in duct/open air/under loft insulation/etc and of course the I^R heating for a given conductor resistance.
• Voltage drop, based on various factors but mainly operating current, impedance per unit length (cable property) and the length.
• Fault current. In the DNO case to supply an installation they do not have to meet the max fault disconnection times for shock protection, but when you are doing a sub-main inside an installation covered by BS7671 you do have to meet the disconnection times (typically 0.4s final circuit, 5s sub-main) and that can sometimes force a smaller fuse or circuit breaker than the supply is capabile of, or you look at more expensive and complicated solutions like delay RCDs, etc

It really is the electrical equivalent of "how long is a piece of string?"!