Voltage drop at 12V - is there a table?

[sparkybird]

Hi all

Installing some ELV lights in a garden and one is some way from the rest. Is there a table of calcs for volt drop at 12V. I estimate the distance from the transformer to be around 7 metres and was planning on using 1.0 cable, but think I might have to go bigger??

Thanks in advance

SB

 

[ban-all-sheds]

Been a long week has it?

Voltage drop depends on current - not voltage. 7m of 1mm² will drop 308mV/A, whether the voltage is 12V, 120V or 12MV.

 

[sparkybird]

errr, yes it has Ban.

I thought that ELV dropped off more quickly (in distance) than 230V... must have been having a blonde moment.

So I can just plug that into the formula?

But is Ib the design current of that circuit or just that one light

I'm a bit confused now....

Also the max volt drop is 3% for lighting - is this 3% of 230v or 12v????

 

[westie101]

I thought that ELV dropped off more quickly (in distance) than 230V

In terms of actual volts it is the same, as Ban says, but in terms of a percentage of the original volts it is much higher.

 

[plugwash]

The number of volts dropped is proportional to the resistance of the cable and the load current.

Lower voltage systems are generally worse from a volt drop perspective for two reasons.

1: You have less voltage to start with so you can afford to drop less voltage. In a 240V system a 10V drop is generally acceptable, in a 12V system it would be crippling.
2: For a given power develivered to the load a lower voltage requires a higher current.

To illustrate suppose we have two 60W light bulbs one designed to run off 12V and one designed to run off 240V. Further we will assume the bulbs are a constant current load and that a 5% volt drop is considered acceptable (note: these numbers are just for the purpose of the exercise, I belive BS7671 uses a smaller percentage for acceptable volt drop in lighting circuits than this).

The 12V bulb will draw 5A. The maximum acceptable volt drop is 0.6V. So the maximum acceptable series resistance is 0.12 ohms.

The 240V bulb will draw 0.25A. The maximum acceptable volt drop is 12V. So the maximum acceptable series resistance is 48 ohms.

In other words there is a quadratic relationship between operating voltage and acceptable series resistance (note: remember to count the series resistance on BOTH sides of the circuit). By increasing the voltage by a factor of 20 we have increased the acceptable resistance by a factor of 400.

Finally note when designing a multi voltage system you have to consider how the voltage conversion device will operate. That is whether it's output voltage is dependent on it's input voltage or not.

 

[bernardgreen]

For an installation of several ELV ( 12 or 24 volt ) lamps that is some distance from the power source it is worth considering a power source that has remote voltage sensing. This will compensate for the voltage drop.

Wire each lamp on its own cable to a central point such that all the cables are approximately the same length from lamp to the central point. At the central point all the cables are joined at a junction box. From that junction box a supply cable is run the the power source as well as a smaller sensing cable.

The supply cable carries the large current for the lamps and will have a significant voltage drop. The sensing cable only carries a few milli-amps and will have no significant voltage drop. The power source adjusts its output voltage so that the voltage it sees on the sense cable, the voltage at the junction box, is the voltage that the lamps require.

This allows a large voltage drop along the supply cable meaning smaller cale can be used. It has the benefit that the voltage at the junction box is constant no matter how many lamps are ON.

Without remote sensing and a fixed voltage at the power source then if one lamp fails then the current in the supply cable is reduced, the voltage drop is reduced and as a result the voltage at the junction point increases and the remaining lamps are supplied with a voltage higher than they are designed for.

 

[davelx]

This allows a large voltage drop along the supply cable meaning smaller cale can be used. It has the benefit that the voltage at the junction box is constant no matter how many lamps are ON.

True, but the CCC of the cable must still be correct for the required current. Also, as I'm sure you're aware, the more voltage drop, the more power is lost (as heat) in the cable.

For example, 240W (easy figure) of 12V ELV lighting requires 20A. If you have a 5V VD in the feed cable, then you are using & losing 100W of additional power from that cable as heat. This may help to heat your house in winter but it's hardly cost effective, and you also don't want that in the summer.

 

[Stoday]

Or you could connect ELV lamps in series. Then all lamps get the same current.

Airport runways are lit using series connection to eliminate the volt drop problem.

 

[redpis]

Or you could connect ELV lamps in series. Then all lamps get the same current.

Airport runways are lit using series connection to eliminate the volt drop problem.

Off topic, but I was intrigued and went searching, hope this link save a bit of time:

http://www.amerace.com/NewFiles/intro1.html

 

[JohnW2]

Airport runways are lit using series connection to eliminate the volt drop problem.

Taking that statement literally, I have to say that surprises me, even though it makes sense in electrical terms - since it's the antithesis of 'fail safe'. Do I take it that there are at least two runs of series connected lamps, perhaps alternating?

Kind Regards, John.

 

[holmslaw]

..

 

[JohnW2]

Heres a post I made earlier. //www.diynot.com/forums/viewtopic.php?p=1155736#1155736[/QUOTE]
Thanks. I'm reassured to hear about the overlapping circuits

Each light has its own 5kv to 12volt transformer called a pot - they're about the size of a 5" flowerpot. From each pot a standard flex is taken to each light. They are wired in series via a single core cable armoured with copper tape and a 6mm core.

I'm a bit confused as to what is wired in series - if it's the primaries of the 5Kv to 12v transformers, you would need hundreds of Kv across a run of them in series. Could you perhaps clarify?

Kind Regards, John

 

[bernardgreen]

since it's the antithesis of 'fail safe'. Do I take it that there are at least two runs of series connected lamps, perhaps alternating?

The arrangement is that at each light there is a transformer. The secondary feeds the lamp. All the primaries are connected in series. The current through them is controlled by the control equipment to alter the brightness.

The transformers are designed so that if the associated lamp fails the transformer continues passing current through their primary.

 

[holmslaw]

..

 

[JohnW2]

Yes, the primaries are connected in series. I think the maximum circuit current was 6.5Amps and the transformers had very low power ratings, I would guess around 50 watt.

I'm afraid I'm still confused. If the lamps are 12v, there has to be 5kv across the primary of each of the '5kv to 12v' transformers - so, as I said, you'd need a supply of hundreds of Kv for an appreciable run of lights - which doesn't sound right or make sense.

Kind Regards, John.