OSG Ref B installation limits

Referring to BS 7671, sheathed cables in conduit embedded in masonry is method 60, which maps to reference method B.

Sheathed cable under capping in the same situation would be method 58 and reference method C, as would a plain cable without any added mechanical protection.

   - Andy.

Just a minor point with your wording "in any of the allowed installation methods" sort of implies that all wiring systems you encounter will fall  neatly into one of these example categories for calculating the current ratings and so forth.

In reality this is far from the case, and it is not really reasonable to expect the authors of the regs to have measured cable temperature rises in all possible combinations of mounting methods and location. As such the tables are a guide, and generally a very good one, that tend to err on the side of giving you ratings with a little slack. Even so, from time to time you will find yourself facing a situation that  is not really neatly in one category or another. Then you have to decide which is it most like, and perhaps if there is a choice decide that the current rating is somewhere between the two 'best fit' cases, and confirm the ratings so that is not a problem. Or the cable route will include more than one method, so you have to rate the whole circuit for the part that will run hottest.

Also note that even really simple things like a cable clipped direct, but exposed to fresh air, cools a bit differently vertically to horizontally, due to convection, but there is only one figure given, the worst one, so sometimes a cable will run quite a lot cooler than expected from the tabulated ratings. In fact if you fit peak temperature indicator stickers to cables in cases that look marginal, quite often you come back 3 months later and find the stickers have not changed colour at all. And just to be fair, on other occasions they can show up a really bad unexpected overheating ....

This is also the reason that DNO cables often seem to be a bit undersized when seen though the lens of the BS7671 ratings, but they have access to a lot of real world data using the exact cable, and not some generalised figure, and can deduce more accurately when it is safe to sail closer to the wind as it were.

Mike.

Thanks Andy, good confrmation that, as expected, plain cable in chase and plaster is Ref C. But it doesn't explain why B isn't in table 7.1(i).

Thanks Mike. It's fair that the tables can't cover all possible scenarios, but in walls in the domestic setting aren't the most common scenarios:

• Studwork walls (covered by Table 7.1(i))
• Blockwork/Brickwork walls in a plaster with/without chase and with/without oval conduit or capping (not covered by Table 7.1(i), particularly the oval conduit)
• Blockwork/Brickwork walls with dot and dab or batten plasterboard (kind of covered by Table 7.1(i)).

Indeed, I was only pointing out that sometimes a bit of interpretation may be  needed.  - it is not a problem if something else is done.

I have heard of cases where  some folk doing inspections are a bit funny and decide 'not a valid installation method' is a reason to fail a length of SWA in a cavity wall or something. There may be other reasons, but not being in the tables is not really one.

Mike.

I think that I see the problem here, which is the use of the phrase, "Allowed installation methods".

Remember that OSG is only a guide and in many ways table 7.1(i) is a ready-reckoner.

I don't think for one moment think that it means that other installation methods are prohibited: it is simply that if you want to know the maximum length of a circuit, columns 5 to 8 work only for the installation methods given in column 4.

Completely agree, but what maximum lengths do people use in the case of T&E in oval conduit and plaster on a blockwork wall? As this is a pretty common scenario I am surprised it isn't covered by the information. Surely allowed as it is so common and 'makes sense'.

But it doesn't explain why B isn't in table 7.1(i).

I don't have the latest OSG to hand, but If it's for flat T&E cable and only covers methods 100, 101, 102, 103, C & A - I'd hazard a guess that it's a copy of BS 7671 table 4D5. That's a special for UK flat cables in certain conditions - for other installation methods you'd to refer back to  the general purpose table for any shaped multicore sheathed cables - 4D2A (which covers installation methods A, B, C & E). Hopefully that's reproduced in the OSG somewhere.

4D5 is a bit of an oddity - we didn't have it until someone spotted that 2.5mm² T&E wasn't always compliant on a 32A ring circuit where there was thermal insulation involved - despite there being no actual problems being noticed in reality. Noticing that flat T&E were a little better at loosing heat than the internationally recognised round cables (by virtue having a thinner sheath and larger surface area) some experiments where done and new ratings for T&E cables in certain situations were drawn up. For all the other situations, we go back to the original table still.

   - Andy.

Once you are on site, isn't it a bit late to worry about the length of a circuit?

length of circuit will be defined by the size of the building, and will feed into a check of voltage drop and far point Zs or R1+R2. If the resistance is excessive then a larger  cable will be needed.

Cable resistance  is only weakly affected by mounting method - unless you actually stretch the copper, the resistance is un affected by what is outside the insulation   Cable cooling and therefore the thermal current rating is however dramatically affected by where and how the cable is routed and what else it is grouped with.
It is quite possible to rock up on site, realize that the thermal insulation or grouping conditions are far worse than originally expected, and have to either step up a cable size or re-route something to avoid a potential hotspot.

Mike.