OSG Ref B installation limits

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.

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 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)).