LV XLR Cables for Birdies

[Stu]

Hi Peeps

 

What does everyone out there use cable wise for their Birdies, and the Low Voltage XLR cable to go with them? Got to make some up, and would just like to know if there is anything I should stay well clear off, or anything which works a treat.

 

Cheers

Stu

[TomLyall]

could you not just use H07 in one of its smaller (0.75mm?) flavours? I imagine this would be good, its nice to handle, fairly easy to wire, and readilly avalible

[gareth]

Just bear in mind that voltage drop is a significant factor with Birdies. Get your transformers as close as possible to the fixtures, and use the chunkiest cable possible for any transformer-to-fixture extensions that you need to make.

[Stu]

From my research on good old Google it seems 2m is about the max recommended distance between Birdie and Transformer.

 

BTW, as a complete aside, does anyone know if Mic cable (i.e. 2core and screen) has a max amount of amps you can put down it? I can't imagine it being very much if there is, but I've always wondered nevertheless.

 

Stu

[robloxley]

As far as I can work out - late at night:

Neutrik standard 3-pin XLRs are rated upto 16 A and should take 2.5 mm² cable - so fine, but have a max cable OD of 8mm - probably ruling out 2 core 2.5mm². The voltage drops of 2-core HO7 in 1, 1.5 & 2.5 mm² are (roughly) 44, 29 & 18 mV/A/m. As for sensible voltage drops, 4% is referred to in the Regs (however correct me if I'm wrong but they don't cover temporary installations - eg. birdies on set). This would be 0.48V for a 12V 75W lamp, which would in turn be about 1.7m of 1mm² or 2.6m of 1.5mm² or 4.3m of 2.5mm² cable.

 

Our theatre has the cylindrical electronic 105VA transformers with the XLR on the 6" long flying lead, so not a problem; and my birdies have integral transformers so again not a problem. However, you probably do want to have sensibly chunky birdie cable and not to do long runs.

What is an tedious game is installing lots of 12 lights across a ceiling and trying to cable them up with 2.5mm² T&E to a smaller number of transformers!

 

As for mic cable, I'd not want to run upto 6.25A down what is probably some bits of 0.25mm² cable! As far as I remember, 3A mains cable is usually 0.5mm², for comparrison.

 

And to be pedantic, Low Voltage is anything up to 600 V AC between conductors and earth; birdies are Extra-Low Voltage which is less than 50 V AC or between and earth.

[Stu]

Rob thanks for the very detailed answer. Gives me something to go on.

 

Oh and sorry, I shall remember it's Extra Low Voltage

 

Stu

[Peter Tovey]

As far as I can work out - late at night:

Neutrik standard 3-pin XLRs are rated upto 16 A and should take 2.5 mm² cable - so fine, but have a max cable OD of 8mm - probably ruling out 2 core 2.5mm². The voltage drops of 2-core HO7 in 1, 1.5 & 2.5 mm² are (roughly) 44, 29 & 18 mV/A/m. As for sensible voltage drops, 4% is referred to in the Regs (however correct me if I'm wrong but they don't cover temporary installations - eg. birdies on set). This would be 0.25A for a 12V 75W lamp, which would in turn be 1m of 1mm² or 1.4m of 1.5mm² or 2.2m of 2.5mm² cable.

Rob, using your figures for voltage drop I get lengths of 1.7m, 2.7m and 4.3m respectively. I'm not quite sure how you've done the calculation, but giving a voltage drop as 0.25A does seem to indicate a slight (no doubt weariness induced) error! Using 4%, the allowable voltage drop is 0.48V. The current is 6.25A and so the length in each case is merely 0.48 / (6.25 * voltage drop coefficient)

 

Peter

[robloxley]

I'm not quite sure how you've done the calculation <snip>

Obviously you're right! I stand corrected but hold by my original claims of it being late at night

[TomLyall]

er, slightly off topic but... if you use robs figures for voltage drop over H07, (which im not arguing with) does that not give a maximum lenght of 19 meters at 16a 230v, which seems quite short to me... have I done my calculations wrong?

[Peter Tovey]

No, if as it looks you've used the figure for 1.5mm cable. Bear in mind of course that you should only draw 15A, not 16A through this, but it still gives you a length only just in excess of 20m.

 

In practice you'd rarely draw as much as 15A through a single bit of TRS that long - if you need to it's probably time to think about moving dimmers closer to the fixtures. 10A gives you an acceptable length of 30m, and to be honest it won't make a noticeable difference if one goes a smidgeon over the 4%, which is not strictly applicable to temporary cabling.

[TomLyall]

yes I had used the 1.5mm figures, although I had always thought it to be rated at 16a, although you learn something new every day

[Brian]

Something to watch when using electronic 'transformers' is that their output is usually high frequency AC, coming straight off the secondary of a HF transformer. With long cables there is a risk of introducing loads of crap into the surrounding ether and the 'transformer' itself may go unstable.

[Jivemaster]

Electronic transformers outputs should be via twisted pair wire of suitable size, NEVER thru parallel bar or taught wire suspensions as the RF emmission is way OTT

[ianl]

it is important to use fat wire in a slim package, I would use 2.5mm speaker cable as it is the largest cable designed to fit in XLRs

 

because you are running elv you dont need the extra insulation properties of mains cable, certainly not HO7 which is double insulated

 

you might want to use HO5 if it saves money but I would guess 2 core HO5 in 2.5mm is getting on for the price of speaker cable anyway

 

avoid 3 core mains cos to get it to fit an XLR you would need to drop to 1.5mm

 

I personally would suggest

maximum 5m lead for optimum performance

maximum 10m lead for still ok performance

much longer than that you will notice the lamp is not as bright

 

I suggest you make a 5m , a 10m and a 20m or something like that. plug them all in and compare the brightness of the lamps and decide which is the longest cable you are happy with before you feel there is unaceptable brightnes drop

[Martin]

I personally would suggest

maximum 5m lead for optimum performance

maximum 10m lead for still ok performance

much longer than that you will notice the lamp is not as bright

These lengths would be okay for a toroidal transformer, but as Brian said, when using electronic transformers you need to keep the output cable as short as possible. I would suggest an absolute max of 0.5m.