I can’t get my Iz greater than In

[Mike2themoon]

Hi all, this is driving me mental, if I have a Ca of 1.03 (25degrees ambient) & a In of 6A or any number you like then Iz will never come out greater than In unless I add another grouping factor just to make it work, which shouldn’t be necessary.

There has to be an explanation to this but I can’t figure out how to make it work.

Please help

 

[westward10]

You aren't ignoring it as such, your calculation suggests Iz is lower than In but you can still use the equation and select a larger Iz it will still follow that Iz is greater or equal to In.

 

[davesparks]

My understanding is that his calculation ends up with a number which is the minimum he could use, taking into consideration the protective device and the rating factors. (Appendix 4 section 5.1.1)

The number has has is the number you use to select a cable size from the tables. It is just a number which exists within the calculation, nothing else, it doesn't exist in the real world, it doesn't represent any actual current.

My answer would be that you can't stick a cable that can carry 15.53 amps (if such a cable actually existed with that as it's maximum CCC) in front of a 16 amp protective device.

That is not what is happening here, the cable can carry the 16A at 25 degrees. What you are doing is selecting a cable from its rating at 30 degrees for an application which is at 25 degrees.

 

[timhoward]

That is not what is happening here, the cable can carry the 16A at 25 degrees. What you are doing is selecting a cable from its rating at 30 degrees for an application which is at 25 degrees.

I think the light bulb has finally come on, thank you for your patience!
So this method essentially comes up with a number based on the ln so that the standard (30 degree in this case) tables can be used.

That also explains why the number drops when the actual CCC of the cable increases.
I think I'll stick to looking up the cable and derating it.

 

[davesparks]

I think I'll stick to looking up the cable and derating it.

The trouble with that is you could end up doing multiple calculations before getting to the size of cable you need.

Not so much of a problem now we all have a calculator to hand on our phones, but when these methods were developed it would have been a long-hand process

 

[pc1966]

I think I'll stick to looking up the cable and derating it.

In most cases a glance at the "standard circuits" in the OSG Table 7.1(ii) will answer most cable-selecting needs, both the cable CCC needed (via combination of size in column 3 and acceptable installation methods in column 4) as well ad VD limits (via columns 5-8). But there will always be the odd case that is not covered there and then you need to compute it (for example if more grouping that mentioned in section 7.2.1).

Also, obviously enough, for exam questions to establish that you can compute it as needed!

The OSG tables are quicker and less error prone for most domestic circuits, but they are not the regs as such.