Cabling LED Application

Russell Seddon · Jun 3, 2018

I apologise for what must seem an elementary question to most of you:

I have need to power a series of LED lamps (in total, up to 60). I am trying to understand if I must run a 220v AC circuit from which each lamp connects its own driver, or if I can use 2 or more ‘big’ drivers and connect each lamp to a DC circuit.

Each lamp comes with the following specification, printed on the box:

10 watt
12 v
50/60 hz
20 Ma

I have successfully sized 220v circuits before by applying the simple V x A = Watts equation and then calculating my maximum wattage load. But if I try this approach with my lamps at 12v 10w each I seem to need a cable the size of a tree-trunk. I am obviously misunderstanding something fundamental about DC/LED applications.

I can see availability for ‘30 amp’ 12v drivers, these take as input 220v 350watt 50/60 hz at 1.6amps, and as output 12v (+/- 0.5) 25a 300w (why a ‘30 amp’ driver outputs 25 amps is a mystery to me).

But would I be right to suppose that one of these units would power up to 30 (to be safe, 25) of my 10W lamps, if I connected them in parallel? And if that is correct, what kind of cable would I need? Would not a 25 amp cable be overly-substantial for a sting of LEDs; and why does the box say that each unit is only 20 Ma?

Any help or advice on this would be gratefully received.

Russell.

buzzlightyear · Jun 3, 2018

these leds what ceiling lights?

Russell Seddon · Jun 4, 2018

No they are in fact IP rated ‘garden’ lamps. They will be installed into the eaves of a barn to cast light onto the internal roof timbers.

HandySparks · Jun 4, 2018

The figures you've given aren't self-consistent. A pic of the lights and their packaging or data sheets might help. A 10W 12V light will draw around 0.83A so, yes, 60 lights is 50A at 12V (assuming a power factor of 1).

Have you bought the lights already? If not, then 230V LED lights might be simpler and neater to install. If you try running high current 12V any distance, you'll run into problems with volt drop unless the cable is big.

Deleted member 26818 · Jun 4, 2018

You have up to 60 x 10W lamps.
60 x 10W = 600W.
600W / 230V = 2.6A.
Because you are converting 230V ac to 12V dc, there will be losses, so you might want to allow for 20% loss.
Divide the 2.6A by 80 and multiply by 100 = 3.26A.
Hmm, they’re saying 20mA, I would have thought 50mA, though that may be 20mA at 12V, not 230V?

dmxtothemax · Jun 4, 2018

QUOTE - [ and why does the box say that each unit is only 20 Ma? ]
my guess would be that 20ma is the drive current for each individual led element.
Lights like that will have many single leds connected together in series/parallel arrangements.

Russell Seddon · Jun 4, 2018

Thank you for all of your various responses to my question.. here is an image of the box.

I am still struggling a little with this.. my response from handyspark suggests I would need a huge cable, to carry the DC amperage. But may I ask spinlondon to clarify your reply please.. do you suggest that I only need a 4 amp cable? Is that because you assume that I am creating a 220v circuit from which each lamp attaches its own separate 12v driver? I had originally wondered if I could run the whole circuit from a large 12v driver.

[ElectriciansForums.net] Cabling LED Application

HandySparks · Jun 4, 2018

Russell Seddon said:
I am still struggling a little with this.. my response from handyspark suggests I would need a huge cable, to carry the DC amperage.

Yes, you will need heavy cable if you only have one or two transformers/drivers.

Russell Seddon said:
... do you suggest that I only need a 4 amp cable? Is that because you assume that I am creating a 220v circuit from which each lamp attaches its own separate 12v driver? I had originally wondered if I could run the whole circuit from a large 12v driver.

If you have a separate driver for each light, then the cable between each driver and light will have to carry less than 1A. The mains cable supplying all the drivers will have to carry less than 3A.

No idea what the reference to 20mA is about.

You haven't said whether you've bought all the lights.

Can you post a pic of the light.

Deleted member 26818 · Jun 4, 2018

Yes one driver for 60 lamps would require a ridiculous size of cable over any real distance.
Up to 2m, you could probably get away with 4mm2 flex (would need to be calculated to be sure).
Whereas if you were to use a number of drivers ( one for every five lamps for instance) you would be able to use much smaller cables, 1mm2 perhaps (again would need to be calculated).

Russell Seddon · Jun 5, 2018

Thanks for confirming handyspark. Here is a picture of the light. I have purchased all of them having bought one to check it fitted where needed and was bright enough. It seemed well made and has a high IP rating. I could have gotten a much cheaper price by buying direct from China but decided to go thru a European distributor/retailer.

The cable I would need to carry 25 amps (assuming I ran 2 circuits) is ridiculous. I will clearly need to run a 220v circuit and buy 60 drivers. This will be another expense and will make the wiring more complex, but on the bright side, I will be less vulnerable to an expensive driver failure.

One last question from me if I may..

Please consider the second picture I posted, from EBay. This purports to be a 300w 25amp 12v driver.. but look at the output cables.. no way are they sized for 25amps? What is going on here, please?

Deleted member 26818 · Jun 5, 2018

The cables look very short.
150mm (6”) of 1.5mm2 would be fine for 25A.

HandySparks · Jun 5, 2018

Russell Seddon said:
Please consider the second picture I posted, from EBay. This purports to be a 300w 25amp 12v driver.. but look at the output cables.. no way are they sized for 25amps? What is going on here, please?

Looks like the 25A output can be split equally over the three output cables. So they only need to carry 8A each.

Deleted member 26818 · Jun 5, 2018

Or 60 lamps, you’d need 20 of those drivers.

DPG · Jun 5, 2018

At 12v you need to be careful of volt drop at those currents.

Richard Burns · Jun 5, 2018

What you would probably want to do as well is to split up the lights into several groups independently switched because there could be a very high inrush current on switch on which may cause a driver or switch to overload. This may also mean that you might need additional (or more powerful) drivers to keep within the limits of the drivers.
If you want to have a small number of drivers then it is generally best to distribute the 230V (in smaller cables) as far as possible and then radiate out from the driver to keep the 12V cables as short as possible.
Along the eaves of a long room is not usually an easy place to position drivers in an inobvious way.
If the lights were split into four groups then you should be able manage the cable size reasonably, inrush current would be dependent on the light itself, but PTC resistors may help limit this if you have a space for a hot component.