Another LV landscape lighting question, this time transformer/current/AWG

[TT_Vert]

I have a paradise 120W transformer. I am using 14/2 direct burial wire. I have a run which is ~180F. I am using strictly LEDS. I was only able to measure DC current since I only have a DC power supply so I'm unsure if AC current calcs are much different and I didn't feel like unwiring and putting my meter inline. I suppose I could use my clamp meter but I've not done that yet. When I only had 6 spotlights (GU5.3 Each drew 3.8W on the bench, manuf. spec'd at 5W) and 6 pathway lights (T3, each drew 2.4W on the bench, manuf. spec'd at 3W) at 12VDC everything was fine. I added 6 more LEDS and the transformer just turns off and the bar goes to max load. I pulled some bulbs and I can get it to come on and gradually see LEDS dim as I plug in more so it's definitely an under power issue. I checked voltage at the end of the run and I'm at around 10.80VAC w/ all but one of the additional 6 GU5.3s plugged in. With 3 of them unplugged I was at 11.45VAC so i'm seeing about a .200mVAC drop per LED. I still have another 5 or 6 of the T3s that I've yet to plug in also.

I cannot see how I could even close to maxing out this thing. Even if I use the manuf. specs which are higher than my bench testing I'm at 5W*12=60W and 3W*6-18W total of 78W on a 120W transformer. I tried two of the same exact transformers but had the same result. I am using waterproof type twist connectors so I did cut the main run each time to splice in the lights but I cannot see that and the resistance in the wire causing this. I'm going to go grab a 300W transformer at HD to see if it makes any difference. Any opinions on this?

Thanks
Dave

 

[Max]

Your nomenclature is confusing so it’s hard to understand what you are measuring. A transformer is AC to AC only, so if you use a transformer you need to measure things with an AC voltmeter or ammeter. Similarly, if you are using a DC power supply, you have to measure with a DC voltmeter and ammeter.

If you’re running the LEDs off a DC power supply (not a transformer) then the power supply converts the AC from the line to DC for the LEDs. Note that though LEDs are diodes, they have poor reverse voltage ratings, so if you run them off of AC there needs to be a series diode to protect them from the reverse voltage from AC. If the LEDs specify that they can run off of AC directly then they likely have a series diode built in.

14 AWG copper is 2.5 ohms per 1000 feet. You have 180 feet of wire, so assuming that you have copper wire (and not copper coated Al) then your wire resistance 0.45 ohms. At 12V, 120W implies a maximum current of 10 A. 10 A through .45 ohm will drop 4.5 Volts. If you’re at 5A then you’ll see 1/2 of that or ~2.3 V drop. So it could be the wire dropping too much voltage, and if so, the solution is heavier gauge wire, less LEDs on each wire, or run another wire for the other LEDs.

It is also possible that you have a poor connection at the lights or the transformer/power supply, or both. But since I can’t tell exactly what you are measuring it is hard to say. If you can clear up what exactly you are measuring with what then I can help more.

Max

 

[TT_Vert]

Sorry I should have been more clear. The LEDS are 12V AC/DC. I only have an adjustable DC power supply which display current draw directly from the supply display w/o putting my meter inline or using my clamp meter to test AC while in circuit outside.

The transformer is a 120VAC to 12VDC/120W. I did play w/ voltage to the LEDS w/ my PS and I was able to retain the same LUX output at 9VDC as I was seeing at 12VDC so I don't think the lower voltage was the issue.

Oddly in contrast to that I went out and picked up a Home depot 300W transformer and while it did light them on the 12V lug I was seeing quite a bit of flickering. I checked my voltage at the end of the line and it was near 10VAC which was more than enough to light them fully on my bench (Although I was testing w/ DC not AC). I then checked my LEDS to see if I could power then w/ 15VDC (Again I don't have an adjustable AC power supply) and was able to w/ any more current draw so I went ahead and connected to the 15V lug on this new transformer. I've read that people use this to combat voltage drop on longer runs. This seemed to work. Problem is this is not an indoor rated transformer nor does it have the feature the other did which used a time zone rather than a light sensor. Since it's in my garage if i turn the lights on w/ a light sensor the lights outside will turn off which I cannot have. I also don't want to run that sensor and associated wiring outside for cosmetic purposes. The wire is pure copper, NOT CCA.

I should be nowhere need 120W w/ my current setup. In fact my calcs put me in closer to 80W w/ ALL LEDS In place which I still have not done, so ~6.7A. I'm pretty confident in all my connections as well.

Dave

 

[Max]

12V AC does not have the same power as 12V DC. Try hooking up the lights directly at the transformer and I suspect it will work.

Max

 

[TT_Vert]

12V AC does not have the same power as 12V DC. Try hooking up the lights directly at the transformer and I suspect it will work.

Max

This issue is through the 120w transformer, hence the question.. Based on my math him well within the limits of it but clearly not.

Dave

 

[Max]

It could be the transformer or the resistive drop. If you connect the LEDs to the transformer with a short wire then you can isolate the wire length from the transformer.

Max

 

[TT_Vert]

The wiring is already in place and the lights are in so i cannot run a short wire to all the figures in series to see if that is the case. I'm going to check the resistance of a foot of the wire I'm using to get an idea of the total resistance of the 180' ish of it.

Dave

 

[Max]

One foot of wire is only 2.5 mOhm so unless you have a milli-ohmmeter you won’t be able to measure it. If you have more wire you can parallel the existing wires to either measure the resistance or halve the resistance under power. Alternately you can try a larger transformer.

Max

 

[TT_Vert]

What do you mean halve the resistance under power? There is twice the resistance when unpowered?

 

[Max]

If you have another 180’ of wire, there are two options;
1. Connect the wires at the LED end to the new wires. Take the other end of the new wires back to the transformer and the other wires. Use you ohmmeter to measure the resistance - it should be around 5 ohms.
2. This is what I meant by “under power”: connect one end of the new wires to the LEDs and the other end of the wires to the transformer. You now have two wires in parallel to the remote LEDs which will cut the wire resistance in half. Plug in the transformer and see what happens.

Max

 

[TT_Vert]

Ah create a loop. I don't have enough wire and I don't really want to trench again. With this 300W testing unit from HD I've been ok although on the 15V lug. I "hope" the 300W indoor unit that I ordered which uses time zones to activate rather than a photocell works ok. I'm not too confident as this other 300W test unit i have didn't work on the 12V lug and this one coming only has a 12Vac output.

Dave

 

[TT_Vert]

Well sadly the 300W version of the transformer that doesn't need a photosensor didn't work either so I'm stuck w/ the 300W one from home depot which has a 15V lug. I don't know that this one is indoor rated (Doubt it as it doesn't explicitly say) but i'll be using it indoors. Wanted to do everything on the up and up but c'est la vie.

Dave

 

[Darryl2]

I probably have about 50 LED light fixtures (so far) on my ever expanding Landscape lighting. I think I am only running three circuits so far on a 300W Transformer. All is well with good voltage at the end of the runs using 12 ga wire. Mostly 3W fixtures. I am using the 12v terminals and have not had to step up to the 15v terminals at all. I have multiple 12v terminals and multiple 15v terminals so each circuits is terminated to it's own screws.

 

[TT_Vert]

Yeah I feel my issue may be my length and the fact I opted for 14AWG as I figured it'd be fine w/ LEDS. I have one 180' 14AWG run. You have 3 separate 12AWG runs. How long are your 3 runs?

Dave

 

[dogdog]

wires have voltage drops per feet... treat it as resistance in series... I don't know what c'est la vie is... probably something tasty...

easiest is just use a voltage drop calculator to estimate.. and assuming all your load total.. on the line...

https://www.calculator.net/voltage-drop-calculator.html

manual calculation would be something like this... even though the loads are at various point of the line... on paper that is

if you look up the chart 14ga wire have 2.525 ohm per 1000 feet... that is ? ****0.002525‬ ohm per foot...

****0.002525‬ x 180 = ****0.4545‬ at the end of the 180' line..

Vtotal = Vdrop +V2(voltage at the end of the line)

Vtotal - Vdrop= V2(voltage at the end of the line)

since your circuits are in series... amp is the same flowing through the whole circuit...
and ohms law

Amp = watt total / Volt total

V2 = Vtotal - ((watt total / volt total) * Reisitance of the wire at 180' )

so basically the higher the load on the line, the bigger the amp flows through it. The bigger the amp flowing through the longer line the more voltage drop.


1) maybe change the wire size?
2) increase the line voltage ?

 

[TT_Vert]

Thanks for that. That voltage drop calculator is definitely not accurate for me but it does show me that there is significant voltage drop. I measured about 10.6V vs. this 5.91 shown.

Voltage drop: 6.09
Voltage drop percentage: 50.76%
Voltage at the end: 5.91 \

I did increase to the 15V lug on a different transformer from Home Depot and it works. It would not work on the 12V lug as the lights flicker, which has to be due to the voltage drop. What i find odd is that the voltage at the end of the line was a bit over 10V when they were flickering yet when I bench powered each of these bulbs they would light down to 8V individually. I tested w/ DC and not AC if that would matter. The bulbs are AC/DC.

Dave

 

[dogdog]

what parameter did you use for AMP ?

as an example...
from calculation perspective lets say you have from your statement... 6x of 3.8Watt LEDs that would be totaling Watt 22.8Watt and 6x of 2.4Watt that would give you 14.4Watt totaling 37.2 watt between those 12 LEDs... ... Watt = Ampxvolt... so Watt/Volt= 37.2watt/12V = 3.1 AMP...is the amp you are suppose to plugin and not sure what you plugin as numbers...



The result matches pretty close to what you measured...

https://www.calculator.net/voltage-...e=180&distanceunit=feet&amperes=3.1&x=67&y=14

Voltage drop: 2.82
Voltage drop percentage: 23.49%
Voltage at the end: 9.18



V2 = Vtotal - ((watt total / volt total) * Reisitance of the wire at 180' )

V2 =12 - ((37.2 / 12) * 0.4545‬ )

V2 =12 - (3.1* 0.4545‬ )

V2 at the end of the line shows .. 10.59105‬V Give or take... hmmmm

the differences in number seems to be the formula used...

https://www.mikeholt.com/technnical-voltage-drop-calculations-part-one.php

Ohm’s Law Method – Single-Phase Only

Voltage drop of the circuit conductors can be determined by multiplying the current of the circuit by the total resistance of the circuit conductors: VD = I x R. “I” is equal to the load in amperes and ”R” is equal to the resistance of the conductor as listed in Chapter 9, Table 8 for direct current circuit, or in Chapter 9, Table 9 for alternating current circuits. The Ohm’s law method cannot be used for three-phase circuits.[/



Voltage Drop Using the Formula Method

When the circuit conductors have already been installed, the voltage drop of the conductors can be determined by using one of the following formulas:

VD = 2 x K x Q x I x D/CM - Single Phase

VD = 1.732 x K x Q x I x D/CM - Three Phase

QUOTE]

 

[TT_Vert]

I have 80w of lights at 12V so 6.67A.

 

[1mikeg]

Using the 15V lug on the transformer is one way to overcome the voltage loss in the wires. I would suggest checking the voltage at the first light (light closest to the transformer). Voltages that are considerably higher than 12V will decrease the life of the 12V bulb.

 

[TT_Vert]

Using the 15V lug on the transformer is one way to overcome the voltage loss in the wires. I would suggest checking the voltage at the first light (light closest to the transformer). Voltages that are considerably higher than 12V will decrease the life of the 12V bulb.

Good call, i had planned to do that. I know most LEDS have voltage regulators in them so i'd assume that'd be the failure point over time. I did check current draw at 15V from my bench and it was actually the same so I'm not too worried about that.

Dave

 

[rlitman]

...14 AWG copper is 2.5 ohms per 1000 feet. You have 180 feet of wire, so assuming that you have copper wire (and not copper coated Al) then your wire resistance 0.45 ohms. At 12V, 120W implies a maximum current of 10 A. 10 A through .45 ohm will drop 4.5 Volts. If you’re at 5A then you’ll see 1/2 of that or ~2.3 V drop. So it could be the wire dropping too much voltage, and if so, the solution is heavier gauge wire, less LEDs on each wire, or run another wire for the other LEDs...

Close, but not quite right. That resistance is the resistance of 180 feet of wire. But the CIRCUIT has 360 feet of wire, so the resistance is double that when the run is 180 feet. Which means that the voltage drop is also double what you think.

 

[Darryl2]

Best solution here I believe is to re-wire using 12 gauge wiring and create zones.

Water your yard or wait until a good soaking rain and take a flat shovel to slice the ground and pry open a slit, put your wire in and step on it to close it up. Minimal turf disruption and you won't even see it after a short while.

Also, you don't have to wire everything in one LONG meandering run. If you can parallel a branch circuit off here and there to different areas it will decrease the overall length and each path will be supplied with the same voltage.

 

[TT_Vert]

My furthest bulb is 180' from the transformer so the run cannot be any shorter. I could run a parallel circuit but I'm just going to stick with the 15v leg on this other 300W transformer. All bulbs are inline also, this isn't a snaking run.