How to verify the size (and therefore current carrying capacity) of an old mineral insulated submain cable?

Question

I have a client who's 1 bedroom flat is now fully electric (shower, heating, hot water, etc.) and wants to install further appliances such as a largeish (5 ring) induction hob. The flat is supplied via an old mineral insulated cable. It is 2 core with the sheath as the main earth - glanded into an old rusty Ryefield board. From previous cables Ive seen and verified it looks bigger than a heavy duty 2 core 10mm.. however the only markings i can see on the pots (or glands?) are what looks like the numbers 590. The inner cores where exposed are marked on the over sleeving with 0225. 
 Not sure if this is an imperial size, or if the 590 is not relevant to the size. Basic concern is if it is only a 10mm then I believe the flat could potentially overload the submain cable especially with the additional induction hob. The Ryefield cutout fuse is marked 100A on the carrier, but I haven't opened this up yet to confirm the fuse size. 
 Any advice or pointers in the right direction really appreciated. As a newish electrician still very inexperienced with mineral insulated cable work. as a side note would be good actually to find somewhere to gain more experience with it. 
 Jonathan

AJJewsbury · Accepted Answer

You could try some manufacturer's data sheets - e.g. https://www.aeicables.co.uk/uploads/6/2021-01/7b5785642eac225b6c775e870dadbd14.pdf - you might have to interpolate a bit if it is imperial size of course. Or just measure the actual conductor diameter and work it out (A=&pi;(d/2)&sup2;) (I've got a plastic vernier caliper similar to this that seems to works well enough for me) Or someone else here might recognise those part numbers... 
 - Andy.

AJJewsbury · Answer

Is the new induction hob a completely new load, or is it replacing an exiting electric hob? If the latter, it might not represent much of an increase in load at all (induction tend to be more efficient that traditional electric hobs). 
 - Andy.

mapj1 · Answer

Numbered pots like that are a pyrotenax thing. 
 Look on page 3 of this 6 page PDF https://www.pyrosales.com.au/media/blfa_files/IMPERIAL_CABLE_DATA.pdf slide down to the pot size 590 (which is a catalog number that means nothing about the cable type, but defines a combination of the gland body size and the diameter of the cable it fits. Sometimes there is more than one gland body size for the same cable type ) 
 Now, the 2 core cable that a 590 takes is 15mm OD - so it should look about the size of the old half inch copper central heating pipe so confirm that as a sanity check against having the wrong no. Then reading across, the cores are 14.2 square mm or so, and rated for 90A when the cable is exposed to touch - and more when it is not. 
 Note that the current rating of pyro wire is a many splendoured thing, as the insulation and the copper are very happy even to reach a few hundred degrees - you don't damage the cable by overload, but depending what it is pushed up against, you may damage that instead!Modern practice is to assume that something like the XPLE ratings apply or that the outer jacket must not exceed 90C unless it's on fireproof supports. 
 Indeed if it fails the insulation test, the normal wheeze is to heat it with a blow lamp and move the flames along to stroke the damp out of the ends. If the pot is left on and is too steam tight this can be fun, don't stand in the firing line if you have to do this the pot can leave the cable at speed.... far safer to loosen it before lighting up the torch. Back in the day, there were brave attempts to wire rings in 1.5mm2 pyro in about the time of high copper prices and aluminium twin and earth, - and from a current handling point of view it is all fine, but in the end voltage drop, and incompatibility with plastic furnishings/fittings put paid to that and it has rather fallen by the wayside. 
 In any case, check the diameter just in case it has the wrong gland fitted (especially if it has been soldered to the cable - it should be a compression fit, but the old sweats sometimes did odd things with bits in the toolbox ), but if it is a proper fit, then I think you are looking at a current rating of 90 amps - or more if out of reach, and that is not so far out of whack when covered with a 100 A fuse to be any cause for alarm. Numbers on rubber sleeves could be almost anything, as a larger sleeve is an easy fit over a thin wire so I'd not trust that. Mike.

Chris Pearson · Answer

AJJewsbury said: I've got a plastic vernier caliper similar to this 
 Andy, shame on you! Where is the Vernier scale please? 
 I admit that I find a pair of dial callipers easier to use, and electronic ones even better. 
 That said, measuring the conductors would be helpful. If the circuit is isolated, why not use a good quality metal device?

mapj1 · Answer

happy to be of assistance where I can - the old knowledge seems to be a dying art these days, once upon at time this was standard stuff, but instead folk need to understand programming instead. 
 There used to be loads of application notes for pyro wire, and they may be online somewhere, telling you things like bend radii, how to strip it and so on, 
 Its fascinating to watch mineral insulated cable be made - in effect it starts of very short and fat with chunky cylinders of metal and bricks of insulation with holes in threaded onto the cores, all a rattling fit, and then gets rolled and heated and stretched to the desired cross section/current rating and in the process the ceramic beads that were threaded onto the wires before they got poked inside the outer get crushed to dust, but like the writing in seaside 'rock' the geometry is beautifully. preserved. 
 Mike.

How to verify the size (and therefore current carrying capacity) of an old mineral insulated submain cable?

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