Cable Sizing for 2 nos x 400W Floodlight

yes you are absolutely correct, our national regulation also using the same thing shown in your appendix, that 1.8 multiplier and 66% current demand (allowance for diversity) for domestic.

Let say I include that in my previous calculation,

Ib = [ Power / ( Voltage x  Power factor) ] x 1.8 x 0.66

Ib = [ 2x400W / (230V x 0.9) ] x 1.8 x 0.66

Ib = [ 800W / 207 ] x 1.8 x 0.66

Ib = 3.86 A x 1.8 x 0.66

Ib = 4.6A

so theoretically Ib 4.6A < In 10A < Iz 17.5A (for 1.5sqmm). With this result and my previous understanding, most likely I will still use the 1.5sqmm cable. But after further explanation from Mr. AJJewsbury, it seems like I need to check back and consider the inrush current as well.

Thank you for your input. 

the voltage drop limitation is less than 5%. (voltage drop calculated is 1.147%)

for derating factor, Cg = 0.7 based on Table 4C1 | Item 1. Bunched in air, on a surface, embedded or enclosed | number of circuits or multicore cable is 3 | ..... assuming my cable laid from the DB to the load through cable trunking then conduit , so other coefficient = 1 / not included

cable current carrying capacity , 17.5A for 1.5sqmm based on Table 4D1 Single core non-armoured cable, PVC, reference method B (enclosed in conduit on a wall or in trunking, 2 cables, -single phase)

voltage drop, (mV/A/M)=29  for 1.5sqmm based on Table 4D2 Single core non-armoured cable, PVC, reference method B (enclosed in conduit on a wall or in trunking, 2 cables, -single phase)

I did ask them on the justification why 1.5sqmm is not suitable, but it seems like most of the answer is through their experiences, hence there is no computation provided. But I did not say that they are wrong and I'm correct, since my previous project also used 2.5sqmm for floodlight. But I believe as a design engineer / electricians there must be a reason / fundamental behind the design either through calculation or assumption (which I might have overlooked).

Could I ask your about your thought process for the 10A MCB rather than a 5A RCBO?  I assume the flood lights will be outdoors?

Yes, it is outdoor equipment and supposed to have IP65 rating, and in my circuit actually I already have RCCB with 100mA sensitivity .

This may sound silly but what does the manufacturer recommend for the flood light?  Also what cable will you use, what is that manufacturers recommendation?

however the cable (in this case 1.5sqmm, current carrying capacity of 17.5A) won't be able to carry the inrush current and resulting in cable melting/damage etc.

Nooooo . How long can 1.5mm take  30-40A ? Well probably getting on for an hour, but certainly far longer than the any inrush - cable does not spontaneously combust just because you have passed two or three times the current through  it than its steady state rating. The steady state assumes a temperature rise then needs minutes, perhaps tens of minutes to occur. Keep the  overload short relative to that timescale and let it cool afterwards, and all will be well.

As an example I have jump started a car in the past - drawing probably quarter of a kA of peak cranking current for a few seconds, - using the cores of a length of 2.5mm2 , & yes it was a bit hot afterwards, but it was fine for the few seconds needed and apart from the fact I had no use for a 6 foot length with the ends where contact had been made slightly cooked, the middle of it would have been perfectly serviceable.

(Rather amusingly much to the horror of the alarm installer whose car battery was serving as host- he clearly knew enough about current ratings to be very worried, but not enough about thermal inertia to be reassured.)

The problem is that the volt drop during inrush can be so much that a discharge type lamp flickers and pops but never actually strikes..

Mike

cable does not spontaneously combust just because you have passed two or three times the current through  it than its steady state rating

The problem is that the volt drop during inrush can be so much that a discharge type lamp flickers and pops but never actually strikes..

okay, i got it.

As i said earlier, this design is for learning purpose, hence I haven't consider the brand. But i don't think the brand confirmation is really needed for this kind of general equipment in the early stage. Usually we have to design it first based on our experience and assumption, in order for us to determine the maximum demand and for regulatory application submission etc., since awaiting brand confirming from clients tends to be time-consuming . Our local regulation however has mandate the minimum cable for lighting is 1.5sqmm and should have SIRIM (Malaysia) and IEC standard, but again that is a minimum size (we can increase the size of the cable due to several factor such as derating factor and voltage drop).

You do not specify in your plan which type of floodlight you are using. Will it be LED? Why not 30ma for your residual current device? Especially if the installation is outdoors.

I have fitted 4 x 40W LED floodlights in a large hall supplied via 1.5mm NYY-J cable clipped direct to 4" tray and connected to a B6 RCBO with no problems whatsoever. Can't recall the length of each run but the longest two were at least 20 metres each.

In England we require 30ma (RCD) Residual Current Device for outside lighting that could be accessed by the public, I take this to mean all outdoor lighting .