Cable Installation Reference Methods A–F – Practical Guide and Feedback Welcome

It's a nice start. Some details need some more work though I think.

Just from a quick 1st reading...

• Method A doesn't cover cables "surrounded" by thermal insulation - but rather it presumes heat can escape on at least one side (e.g.through the surface of the wall) - where a cable is entirely surrounded by thermal insulation (a practice to be avoided were possible) its rating could be as low as 50% of its method C rating (see reg 623.9 and Appendix 4 section 2.6).
• Ib ≤ In ≤ Iz is only appropriate where the protective device is intended to provide overload protection and where the circuit consists of only a single cable. With UK practice that's quite often not the case - from industrial motor circuits to the domestic ring final circuit.
• Ib = P ÷ V is only true for purely resistive loads (i.e. where the power factor is 1.0) - in many other cases the overall current can contain an element of "wattless" (or reactive) current - i.e. current that flows alternatively towards the load and then returns to the source without providing any nett power (like a capacitor alternatively charging and then discharging on each cycle). Cables need to be sized to carry the overall current, not just the power delivering bit. The more general formula additionally divides by the power factor (cos φ) (and in some cases, e.g. when working from the tube power rating of fluorescent lights, may includes an additional factor for things like control gear losses).
• Method D has recently been split into two separate reference methods in BS 7671 - D1 (cables in buried ducts) and D2 (directly buried).
• In the trefoil arrangement the cables touch, rather than overlap.(The red/black/green colouring also seems odd - it's reminiscent of old (pre 1970s) single phase cables - and seems especially inappropriate for trefoil as that's mostly used for 3-phase arrangements where there are advantages to be had from the effects of the three currents cancelling each other out.)

But a good effort I think - you've certainly seem to have grasped the important principles.

    - Andy.

Agree a nice style - I assume you are preparing  a student hand out or a book chapter, and the regs alone are a bit too dry on descriptions.

there are a few extra things that the more alert reader may find interesting.

I might note that it is the heating over a time of minutes to hours that is an issue, so very short events ( capacitor inrush, motor starts etc) may exceed the rating by many times with no ill effect, while grouping cables together or a warm external temperature (roof voids in factories perhaps) this all eats into the safe temperature rise, and reduces the current rating.  

Running cables a few degrees hotter than intended does not immediately cause a failure unless the overload is quite dramatic - usually its more of an early ageing
  A Commentary_2D00_UpdateApr04.pdf
Shows the effect on lifespan of overheating by a modest amount. this implication is that  short life installations in cold places may be wired in thinner stuff than you might expect (and anything in cars seems to be using a different rule book altogether, but realise that even quite an old car has probably only seen a few thousand hours of run-time)

Also note that the annexes in the regs are not able to cover all possible cases, and are informative , not nominative, so  where the designer and or installer knows different, - perhaps the environment, or a special cable, or a low duty cycle, then that information may be used to choose  different cable size if it is more  appropriate

Mike.

Agree a nice style - I assume you are preparing  a student hand out or a book chapter, and the regs alone are a bit too dry on descriptions.

there are a few extra things that the more alert reader may find interesting.

I might note that it is the heating over a time of minutes to hours that is an issue, so very short events ( capacitor inrush, motor starts etc) may exceed the rating by many times with no ill effect, while grouping cables together or a warm external temperature (roof voids in factories perhaps) this all eats into the safe temperature rise, and reduces the current rating.  

Running cables a few degrees hotter than intended does not immediately cause a failure unless the overload is quite dramatic - usually its more of an early ageing
  A Commentary_2D00_UpdateApr04.pdf
Shows the effect on lifespan of overheating by a modest amount. this implication is that  short life installations in cold places may be wired in thinner stuff than you might expect (and anything in cars seems to be using a different rule book altogether, but realise that even quite an old car has probably only seen a few thousand hours of run-time)

Also note that the annexes in the regs are not able to cover all possible cases, and are informative , not nominative, so  where the designer and or installer knows different, - perhaps the environment, or a special cable, or a low duty cycle, then that information may be used to choose  different cable size if it is more  appropriate

Mike.

mapj1 said:

while grouping cables together

Good point - I missed that. I should have added that the "reference methods" are just a first approximation to the cable rating - there are a whole stack of correction factors that can increase or decrease a cable's actual rating according to more detailed circumstances. These cover a myriad of variables that give you Iz rather than just It - everything from the ambient temperature to cable grouping to different types of protective devices or even if you need to provide overload protection to an underground cable and depth of burial or soil type.

   - Andy.