Current carrying capacity of a ring final circuit - grouping factor?

happysteve · Jun 18, 2015

Eh up

I'm trying to challenge myself with scenarios where I'm not sure if my interpretation of the calculation rules + regs is correct or not.

RFCs are doing my head in.

I think I understand most of the theory:

- 433.1.204 lists a specific case where a RFC protected by a 30/32A protective device may be employed. In there, it says, "Such circuits are deemed to meet the requirements of 433.1.1 if the current carrying capacity (Iz) of the cable is not less than 20A..." (and some other stuff about high single point loads).

So that seems pretty clear - when determining the appropriate cable size (taking into account all the rating factors and the reference method), It should be 20A or more.

Without any rating factors, then for either singles (70C thermoplastic insulated, table 4D1A) or T&E (table 4D5), 2.5mm is ok for most common rating factors listed in the tables, apart from 101 (It=17A) and 103 (It=13.5A). For instance, reference method A (enclosed in conduit in an insulated wall) is 20A.

My confusion relates to grouping factor. If you run the two legs of the RFC in separate conduit, then you're ok. But if you run the two legs of the ring in the same conduit, does that mean you need to apply Cg=0.80 (table 4C1)... in which case 2.5mm cable will no longer be adequate?

I wondered if note 9 below that table would help; but in the worst case scenario, where 20A is in one leg, and 12A in the other, then 12 is greater than 0.3x20, so can't be discounted.

Even reference method B ("enclosed in conduit on a wall") would not be suitable; It=24A and 0.8x24 = 19.2A, which is less than the stated miniumum Iz of 20A.

So, do we simplify all that to: "Don't run your two legs of your RFC down the same conduit, as it won't meet the CCC, and 433.1.204"?

Or is there another take on this?

telectrix · Jun 18, 2015

grouping factors don't come into it as it's a single circuit. you would only need to apply grouping factors if there were more than 1 circuit in the containment.

happysteve · Jun 18, 2015

Thanks telectrix.

That was my gut instinct at first... but then, reading the title of the grouping factor table (4C1) it says, "... for a group of circuits, or a group of multicore cables..." So the first bit isn't true, but the second bit is.

I guess, thinking of it another way: if you're running a distribution circuit, splitting your load over several identical conductors (complying with 523.7); if it's the same circuit, do you need to apply the grouping rating factor, or not? I would argue that you would... let's say each pair of conductors takes 50A (100A total, same circuit, split over 2 2-core cables grouped together). They're not going to behave thermally any differently than if you have two separate 50A circuits next to each other.

I'm not disagreeing with you as far as the RFC is concerned - like I say, it was my gut instinct at the start - but there's still a bit of niggling doubt and lack of understanding in my mind.

Cheers!

telectrix · Jun 18, 2015

let's say each pair of conductors takes 50A (100A total, same circuit, split over 2 2-core cables grouped together). They're not going to behave thermally any differently than if you have two separate 50A circuits next to each other.

. in that situation, you have 2 conductors, each carrying the same current. they will both warm up equally. you'd only have possiblity of over heating if the containment was over filled without sufficient airspace.