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Voltmeter fails to detect voltage at low voltage (12V) outdoor lights

Voltmeter fails to detect voltage at low voltage (12V) outdoor lights

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  #1  
Old 07-25-12, 09:00 PM
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Voltmeter fails to detect voltage at low voltage (12V) outdoor lights

Greetings. I'm from San Diego and am planning to expand the low voltage system around the house. The transformer is fairly powerful (900 Watts on 3 lines, 12V, 13V and 14V outputs). There are 26 lamps on the system - 22 X 10V path lights and 4 X 20V downlights. Unfortunately the transformer was placed right at the back of the 150 ft lot and some of the lights are 100-150 ft away, in the front yard. It looks like 10G wire was used on the longer runs, with 12G going to the lamps.

Everything has been working well since the installation 8 yrs ago but I'd like to add a few lights at the front. I'm sure the transformer has the capacity but I'm concerned about voltage drop down the longer lines and want to measure voltages at the lamps. When I try to use my old analog multitester (Micronta) to try to measure the secondary voltage at the transformer or the voltage at one of the lights I get nothing - just a tiny deflection. The meter accurately measures the AC voltage at the primary and AA battery voltage - no problem there.

Is there some reason why an old analog meter won't measure DC voltage in a transformer secondary? Something to do with rectified current verses DC from a battery? If so, can I buy any digital meter to do the job or do I need something specific?

Thanks in advance.
 
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  #2  
Old 07-25-12, 09:45 PM
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Is there some reason why an old analog meter won't measure DC voltage in a transformer secondary?
Yes, because a transformer secondary is AC not DC. I assume these are Malaboo lights and they are AC not DC. The meter should be on the lowest AC scale.
 

Last edited by ray2047; 07-26-12 at 06:59 AM.
  #3  
Old 07-26-12, 12:36 AM
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Thank you Ray. Don't ask me how I got the idea that it was a DC transformer .. .. but I'm now getting accurate readings at the 12, 13 and 14 volt taps.

I've taken readings at some of the lights in the front yard (farthest from the transformer) and they vary quite a bit. One reads 11.0V, another 11.5V and one - an obviously dimmer light - only reads about 10V.

The light reading 11.5V is at the end of about 130 ft of 10G cable. Would it be OK to splice in a 20V down light on the end of about 12 ft of added 12G cable and about 15 ft in front of that 11.5V light? I have no idea what other lights might be further up that particular 10G cable, if any.

There appear to be 4 or 5 cables attached to each of the 12 and 13 V taps, with nothing at the 14 V tap. Is it normal to have so many cables in the system?
 
  #4  
Old 07-26-12, 08:58 AM
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You would have fewer voltage drop problems with several cables connected to the transformer and going off in different directions (or even remaining close together for awhile and then branching off).

Compared with one cable of the same size leaving the transformer and branching off in different directions further on down the line.

Or use heavier wire to combat voltage drop problems.

Voltage drop depends on the number of amperes. It's a little more complicated to compute the voltage drop along a line with lights spaced out along it since the number of amperes is greater in the section of wire nearest the transformer and less in the section going to the last light.

A "DC transformer" takes AC input and has other things (rectifier, etc.) inside the box to yield DC output.
 
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Old 07-26-12, 09:38 AM
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A "DC transformer" takes AC....
I respectfully disagree with this statement because it can confuse a newcomer. I would suggest always using the term DC Power Supply where the output voltage is DC because the output of a transformer is always AC so there is really no such thing as a DC Transformer.
 
  #6  
Old 07-26-12, 11:25 AM
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Got you Ray. The term "DC transformer" is a bit of a misnomer I suppose. At any rate, "AC transformer with DC output" is what I thought I was dealing with (i.e. rectified output).

Thanks Allan, that was very clear - and makes sense, even to me.
I had indeed figured that the many cables out of the box must have been to deal with voltage drop on the longer runs. There are 3 commons in the box (FX Luminaire, 3 X 300W) so these must determine which transformer is running which cables. But since each transformer is running multiple cables I can't associate lights with individual cables (something I'd really like to do) just using the 3 switches. All I know is which lights are on the same transformer.

The only ways I can think of to know which lamps are on which cables are to either 1) disconnect the cables at the taps one by one and see which lights are affected [not something I'm willing to mess with since access is so difficult and I may not be able to reconnect them properly], or 2) pull all of the cables out of the ground and observe [no way am I doing that ....]. Is there some other way to map out the system while leaving it physically intact? I wish the contractor had left me with a diagram of the whole installation but he didn't - and he's out of business now.

As to adding lights now, correct me I'm wrong but I suppose I can splice in a length of 12G, add another light to it and see what voltage I get on the new light. If the voltage is too low (<11.0V, say) I can just undo the splice and put the cable back where it was. Tedious but effective, no? I know I could step up the voltage on that line by moving to a higher tap if it's too low, but only if I could identify the cable in the box and do the work without messing up.
 

Last edited by Stanford; 07-26-12 at 12:04 PM.
  #7  
Old 07-28-12, 05:06 AM
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Unfortunately the only way to reliably identify the cables is to disconnect them at the transformer end and short them one at a time at the far end. Use your meter set to ohms to find the shorted cable.

I would add the new downlight using 10g, not 12. It's at the end of the run so you'll want to give it the biggest pipe you can.

Also, you're using 10v and 20v to describe the lights. "V" is voltage. "W" is wattage. To calculate the total current draw on the transformer, add the wattage of all of the lights and divide by the voltage. In your case, 22 x 10 watts = 220 plus 4 x 20 watts = 80 for a total of 300 watts. At 12 volts the lights are presenting a total load of 25 amps to the transformer. That's why you have multiple feeders. Use the same formula to calculate each cable's current.
 
  #8  
Old 08-05-12, 02:45 PM
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Thanks for your patience Rick, I'm not quite as electro-ignorant as I seem - but very nearly

I meant to say 10W and 20W when referring to the lights used - fingers faster than brain. I mostly have 10W bulbs but some of the down lights are 20W. The new downlights that I recently bought came with 20W halogens, as I discovered when observing the light intensity after I installed one. So I replaced it with a 10 to match another downlight that it pairs with in the walkway. Very nice.

Thanks for letting me know about the line check, that makes sense. It would be too risky for me to try to disconnect the lines at the taps one at a time as you suggest. There are so many of them stuffed into a small space in there: it's difficult to get down low enough to see what you're doing and my eyesight isn't what it was. If I were 30 years younger ...... I guess I don't care as much as all that.

I just installed a downlight in a tree using about 15ft of 12G after splicing into 10G near the end of the line. I thought about using 10 and even bought 30ft of it just in case but decided that the effect on voltage wouldn't be very much. The terminal light is now at 10.1V rather than 10.4, which is a tad low but acceptable - hardly perceptibly dimmer and the bulbs should last a bit longer no? [I never do anything now without checking voltage before and after ...]

May I try your patience one more time? In the front yard (and about 140 ft from the 3-transformer box in back) there are three exposed cables that are connected to each other with 2 large Winged Cable Connectors. One cable is 10G, the other 2 are 12G as it happens. What is going on here? I assume that these are the ends of 3 separate lines but if so, why are they connected together rather than just ending separately (in a weatherproof seal of course)? This will have created electrical loops would it not? Not a bad thing I suppose, but why do it?

Also, does this mean that all 3 of these cables are attached to the same tap in the transformer box. Pardon my ignorance again but it doesn't seem like a good idea to have any 2 of these lines connected to different taps (which all have different voltages) so I'm assuming that they share a common tap.

As it happens these triple connectors are sitting above ground right near a tree that I want to put a down light in. So I suppose I could go ahead and splice in some 10G at this connection point (10G rather than 12G to go with your previous suggestion) and send it up the tree to the light. OK? I'd rather not use a massive Wing Connector to do it - wouldn't it be better to use some sort of little box that will more reliably accept the 2 X 4 wires involved. Something with little screws that allowed you to securely attach all of the cables. Also, do I have to preserve the original "polarity" with these cables, i.e. make sure that the orientations of the wires now sitting on the ground are preserved when I splice in the new downlight cable?

Many thanks in advance,
Charles.
 

Last edited by Stanford; 08-05-12 at 03:03 PM.
  #9  
Old 08-07-12, 07:16 AM
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12.0 volts at the 3-cable connection: nice! Must have come from the 13V tap (no wires on the 14 as I recall).

I added the downlight from there (15 ft of cable) and all is well. I used a "bolt connector" (getting a nice collection of connectors in my toolbox) rather than a twist due to the no. of cables joined at this one place. Might not bury the thing but rather use an above-ground plastic "box" of some kind as a cover. Very professional looking ....

Great thing about low voltage - it's hard to seriously screw up even if you're learning as you go along.
 
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