IEE Circuit Design-Over Engineered

Quote Ryan "16 Amp MCB and utilizing method 1 installation it would yield a 3 x 4mm2 cable which is somewhat unrealistic"

1. Why do you need 3x4 mm^2 for lighting. Is that what we call a multiwire ckt, 2 wires sharing a common neutral?

2. 4mm^ is approximately equal to #12 ampacity of 20 amps and is used quite often for receptacles and lighting. Why do you feel it is unrealistic?

3. 1.5 mm^2 is approximately equal to #14 ampacity of 15 amps and is the minimum conductor used for any installation. Often used for ckts to lighting and Rec. What size would you suggest using?

Dear wareagle

Thanks for your comment. Well for a 16 Amp load circuit I would normally use the 3 x 1.5 mm2 cable although to be quite honest I would not load the circuit to 16 Amps. One thing that I keep in mind is that the MCB does not "fault protect" at its rated current (in this case 16 Amps) but a (factor) x its rated current for a designated amount of time. here is where you verify IEEs second condition.

Atleast thats what I think...(which is not necessary correct :))

Apologize for the late reply.

Ok lets say that I have a lightly loaded circuit of about 500 W, single phase thus terminal voltage would be around 230 V (Europe). I(n) MCB rating is 16 Amps and we are going to use for arguments sake method 3 installation. The circuit is along side other 2 circuits.

From the above Load current I (b) is approx 2.18 Amps. From IEE tables

Ca taken to be 1.0 (temp 30 deg. Cel) and Cg to be 0.7 (3 circuits)

from the above therefore I(z) is 16/1/0.7*1.0 is 22 Amps

from table 4D1A fourth column for 22Amps we have 3 x 4mm2.

Not considering the conditions that need to be satisfied we have the following conclusion. a 2.18 Amp circuit is to be wired with a 3 x 4mm2 cable which in my oppinion is too excessive.

When you design you need to make sure that such design is financially practicable and when your out in the industry such considerations matter a lot!!. Its not a question of saftey its a question of good design and judgmnet without jeopardizing saftey.

What do you think?

I yet to understand your posting. Here in UAE, the maximum lighting circuit allowed upto 1200 W. You could use 16A MCB (Minature Circuit Breaker) and 3X1CX2.5mm2 pvc. Three wires, one for phase, one for neutral and one for earthing. Some people use flex cable but .....In 1200 W circuit, you could have 12 lighting points, even when applying the standard like considering each normal light point of 100 W (regardless the light being of 18W). Chandlier is of 500W and even 1000W.