Number of discharge lights on one circuit

[Jonny66]

Hi all.....first post, here goes!

Appendix 1 of OSG states the demand of a discharge/fluorescent circuit is taken as the rated lamp watts multiplied by 1.8. I was taught this and thought that I =(P x 1.8)/230 would indicate the amount of lights you could put on say 10A MCB protected circuit,
eg. 15No fluorescent switch start lights (each 4x18w)
I = (15 x 72w x 1.8)/230
I=8.5A
Therefore possible to install 15No lights on 10A MCB type C protected circuit
(assuming cable size, length of run, volt drop etc are ok.)

I'm now starting to think that maybe I'm missing something, Im just starting out and want to do things correctly. A new job requires 16No Thorn Chalice 190v discharge recessed lights (each with a 70w HIT-CE lamp) for a reception area. The ballasts are Mag Tridonic Ombis 70w, connected load 82.2w lambda 0.99.

My calculations using I =(P x 1.8)/230

I = (16 x 70 x 1.8)/230
I = 8.76A
therefore possible to put 16 on 10A typeC MCB

However, as Ive been doubting myself, though I'd check with Thorn Tech help line....big mistake - blood out of a stone comes to mind! Eventually got starting current of 0.58A which seemed good and related to my calculations I = (70x1.8)230 =0.55A x16 =8.8A.

I ask for clarification on number of lights eventually was told max of 7No on 10A typeC, 12No with PF correction capacitor fitted. He got this from a tridonic chart but couldnt give me any other details.

so 7x 0.58A = 4A

So can someone with a better grasp of this tell me what Im doing wrong, I thought the 1.8 multiplier accounted for losses/harmonics etc??? Is it the difference between starting current/steady state, or the 190v, Im lost??
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Sorry bit long winded but Im completely confussed, so any help appreciated

Thanks Jon
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[Phil Mumford]

I thought the 1.8 was for starting current of switch start fluorescent and discharge lights.

These should run happily on a 10A MCB - the start up might trip a b curve. I would put in a better switch - running current of 8A is asking to burn out a 10A switch sooner or later.

I once tested a light like this (70W CDMT style) and it took 1.5A for about a second then settled to 0.7 A.

Personally I would use a 16A B type (C type if necessary) with 20A switch and 1.5mm cable if the installation method allows.

 

[Darkwood]

The 1.8 factor is for cable calcs but when loading up an mcb its a different kettle of fish as inrush of several lamps must be taken into account and to make matters more complicated different size and kind of discharge lighting gives different allowances, i have hagar, sq D, merlin etc product books as they are a must when working out discharge lights to mcb rating, they all have large tech' section relating to this as well as motors transformers, contactors etc.
If im following a new lighting install on discharge lights its the most common mistake other sparkies make at their cost as the circuits either need dividing or cable size increasing.
Along the same note switches need to be double rated with inductive loads as this is another point often overlooked.

The reason for double rated switches is due to the fact that a emf is generated and flash over occurs in the switch contacts especially when switching off inductive loads, this arc damages the contact surface and eventually switch failure is inevitable, by doubling the switch rating it can withstand this to some extent with a larger contact surface area.
Small start-up overloads wouldnt be a concern as the switches are reasonably durarable if overload is of a short duration.