Voltage drop on lighting circuit in warehouse

[s100cky]

Hello All!

I was hoping to have some help from some of the savvy bunch if at all possible? I am currently pricing a largish job for a client of mine. It involves changing some warehouse lighting (currently fluourcesents to 400W metal halide low bays. The warehouse is 75 meters long, and 50meters wide (remember this it will be useful later!)
So I thought id blow the dust of my IEE electrical installation design guide (as I havnt really looked at this since college), and started playing around with some calculations…
8 lights x (400w x 1.8) = 5760W/230 = 25.04 (brilliant that’s my design current sorted!)
Now to find a cable size! Im running in some lighting trunking, we will say less then 40% full! Ill start with a 4mm PVC single. (why not?!) and start playing with volt drop..
Table 4D1B tells me volt drop = 11 so…
11x100(75m warehouse plus 25m to the centre of the warehouse) x 25A / 1000 = 27V volt drop!!

Ok a 4mm is no good, lets try a 10mm..

4.4x100x25 / 1000 = 11v (219V end of circuit)
Well that seems to just about comply, but do I really have to use a 10mm single core for this circuit? I know all my calculations are at face value, but am I missing something vital, just doesn’t seem to make sense.

Like I said this is a largish job for me, and I want to make sure im covered on all angles, any useful input would be really helpfully.

 

[Guest123]

Try more than one circuit.:wink_smile:

 

[Guest123]

Also, VD on lighting circuits should be a maximum of 3% (6.9V).

VD on a lighting circuit is calculated differently to say a standard radial power circuit as the FLC isn't at the end of the circuit, loading decreases the closer to the end of the circuit you get.

 

[s100cky]

cheers lenny, but could you please give me a handy little hint where this would come into play? cheers bud (then if you could just pop off and do the job, ill invoice and do the rest!)

 

[s100cky]

yes this is what i was thinking, clearly the load down the circuit is dropping all the time after the first light fitting, but i have been looking for a way to factor this into the calculation without much luck. surely you would have to break the circuit down into 8 bits and carry out voltdrop on each segment of the circuit?

Also in reference to the 3% on lighting I was under the impression that was only guidance, and metal halides so fire at 218V and above, whilst in a reasonable ambient temp. thanks for the reply mate!

 

[Risteard]

Use a three-phase circuit with alternating luminaires on different phases and a common neutral conductor and make sure that you have a device to break all three line conductors simultaneously.

 

[Guest123]

surely you would have to break the circuit down into 8 bits and carry out voltdrop on each segment of the circuit?

Correct.

The cable from the DB to the fiirst light, Cable (A), would carry the full circuit (Ib), the cable from light 1 to light 2 ,cable (B), would carry 400W less and so on. Your last cable would only carry 400W in total.

 

[s100cky]

Ok thanks, both are brilliant suggestions! now i just need to find the most cost effective solution. I like the 3 phase per row idea, the only short fall i can see is space on an existing DB. Really i need a MCB per row, and i dont really want to end up having 10x 3P MCB's in a panel. But i will look into both those suggestions.

also lenny ive noticed that you put 400w, instead of (400w x 1.8), was that intentional? do you still apply a correrction factor?

Again MANY thanks

 

[telectrix]

the 1.8 allows for control gear draw and start up surge. your actual running current is more like 1.3 x 400 IMO.

 

[Guest123]

also lenny ive noticed that you put 400w, instead of (400w x 1.8), was that intentional? do you still apply a correrction factor?

Again MANY thanks

The factor applies, I just didn't bother writing it in.:disguise:

 

[s100cky]

i would agree its more like 1.3, 1.8 seems high? i would just fit the appropriate MCB type. Im working all that process out now lenny, fun time! arty3:

Thanks matey

 

[s100cky]

Following lennys guidance i have produced a small and very unprofessional (yet helpful) spreadsheet. If anyone could take there time to have a look that would be much appreciated!

http://dl.dropbox.com/u/60909676/Warehouse lighting volt drop.xlsx

Cheers

 

[Darkwood]

When designing inductive lighting circuits your first port of call is tech' data sheets of mcb ratings to no of specified fittings allowed, then you know what you can put on a said mcb.

Example a 16amp mcb type (c) can only supply 2 no' 400watt metal hallides to allow for inrush currents when energising, you are not talking 58w chokes here, i have tech data regarding mcb lamp allowance and you will design you whole install wrong if you dont use it.
http://www.neweysonline.co.uk/neweys/pdf/Square_D_LoadCentre_KQII_Distribution_Boards_Technical.pdf
This is a link to the data im using ... i using Square D but its similar throughout mcb ranges but double check their specific data first.

Page 31 gives you the 400 w metal hallides on a c type mcb but read the section head first to understand its implementation.

This is the main reason i hate modern college training because they dont bother explaining any of this but its crucial to correct design.

 

[s100cky]

When designing inductive lighting circuits your first port of call is tech' data sheets of mcb ratings to no of specified fittings allowed, then you know what you can put on a said mcb.

Example a 16amp mcb type (c) can only supply 2 no' 400watt metal hallides to allow for inrush currents when energising, you are not talking 58w chokes here, i have tech data regarding mcb lamp allowance and you will design you whole install wrong if you dont use it.

Oh right...
Any chance of having a peek then
We all know you can have more than 2 x 400W MH on a 16A C MCB though...surely you all knew that...? :S

 

[Darkwood]

NO YOU CANT!!!! ive edited my last post with a link to one of many tech sections i have in file, re-read post 13 and use the manufacturers guides but make sure you read the section head too as its based on a 20m run with 7m between fitting so you will have to allow for your design/ distances etc. Consideration too has to be designed for a possible fault that you dont lose large areas of lighting, i would be designing said circuit to go through contactor banks too with timed systematic switching so you dont take out front end devices.