Low voltage outdoor wiring

[ldavidal]

I have a problem with outdoor lighting triggering a 20 amp breaker when the lights turn on. The problem seems to be with surge current when the timer turns the lights on. Once the timer turns the lights on, the circuit breaker can handle the load. I have tried different timers with the same result. Would a capacitor in the low voltage side solve my problem? If so, what size capacitor would I need. If the capacitor should be in the high voltage side, what size? I have looked for a slow blow breaker without success.

 

[RCinFLA]

What is size (wattage) of transformer. You might just have a bad breaker.

There is a surge when transformer is first energized but to pop a 20 amp breaker I would expect that the transformer would have to be much larger then your normal low voltage lighting system.

 

[Reloadron]

I am going to venture a guess. You don't mention your location or mains voltage but since you mention a 20 Amp breaker I'll assume 120 Volt mains? Since you do mention "outside" I may assume the circuit is on a 20 Amp GFI breaker? Only because most locations require GFI outside. Now if it is a GFI breaker is there any possibility of a small ground fault current leakage?

Just Guessing
Ron

 

[EternityForest]

How old is the cable? did it always act like this?


Here is what i think is wrong, correct me if i got the facts less than straight:

If i understand you right:
1. the timer feeds power to the transformer
2. the timer is not reset by the power going out(mechanical or battery backed up or something)
3. it works fine after the timer turns on

so i think:

1.when the breaker trips the power goes down, de energizing the transformer
2.when you reset the breaker and it works fine, the transformer re energizes
3.also if it was a ground fault in the cable it would most likely just trip again

so:

1.the transformer is fine
2.the cable is fine
3.the timer has a problem? like a Ground fault in the timer or a brief short while switching on?

which means:
You can test my theory by bypassing the timer.

Another question:
what kind of lighting is it and how much power?

 

[ldavidal]

Still puzzled.

The transformer has been replaced. I have moved to a different circuit breaker in the box. The circuit is on a GFCI, but it doesn't trip. As for bypassing the timer, that is essentially being done when I turn on the circuit directly from the electrical panel. I have used a mechanical timer and an electronic timer - same result. The breaker tripping does not happen every night - maybe 2 times each week. When it happens, I reset the breaker and the lights come on and the timer works the next night. Really puzzled.

 

[audioguru]

There is a surge when transformer is first energized.

No.
A transformer is inductive which causes the current to ramp up slowly, not surge.
An incandescent light bulb is a very low resistance when cold so it draws a surge current that is 10 times its rating.

 

[madsoundman]

Leakage somewhere?

The problem would have to either one of two things-

a) a surge during turn on (I'm assuming filament type low voltage lighting here, which would have a potentially large turn on surge if you don't account for this when sizing your breaker). It could be that the surge during switching on the lights is borderline, which would explain why it's only tripping sometimes. Need to look at how many lamps, total wattage, turn on surge per fixture verses running current.

b) Some form of intermittent leakage, possibly due to humidity or some other factor, is the cableing buried? Age of wiring?

 

[madsoundman]

No.
A transformer is inductive which causes the current to ramp up slowly, not surge.
An incandescent light bulb is a very low resistance when cold so it draws a surge current that is 10 times its rating.

While this is true of transformers under no load situations, there is interaction with the secondary, which if feeding a large number of incandescents would essentially be looking into a short circuit untill they heat up.

I'm leaning towards my first option (see previous post) and getting the feeling that the breaker simply is not sized for thetotal surge current in this situation. You may have to split this feed up betweeen two transformers and two breakers, using a contactor fed from the timer to switch on both feeds.

 

[RCinFLA]

Its a transformer, not and inductor.

Did not tell us what type of bulbs and their total wattage.

 

[madsoundman]

Its a transformer, not and inductor.

My point exactly....
There is interaction with the secondary on a transformer...

 

[madsoundman]

...As for bypassing the timer, that is essentially being done when I turn on the circuit directly from the electrical panel.

Hmnnn, if this is so, your telling me that the timer is getting it's power from BEFORE the breaker? How is this so? You couldn't be bypassing the timer by simply turning the breaker back on, such an installation wouldn't pass code. None the less, I'm holding to my guess that the total turn on surge current is borderline to what the breaker can handle...

How many fixtures are we talking about here?

 

[ldavidal]

An idea.

I am "bypassing" the timer because the timer has already switched on when I reset the breaker. I know the surge is somehow related to the timer because the circuit never trips when the breaker is switched to "on". Tomorrow I am going to buy another timer and another transformer. I will hook up half of the lights to each timer and transformer and have the timers turn on lights with a two minute delay on the second set. I am sure that will solve the problem. I thought a capacitor would smooth out the current surge, but no one seems to agree.

 

[Diver300]

No.
A transformer is inductive which causes the current to ramp up slowly, not surge.

You can get a big inrush current on transformers. Depending on where in the mains cycle the transformer is turned on, the core can saturate. That reduces the effective inductance of the transformer, reduces the back-EMF, and leads to a big current surge.

You can reduce the current surge with a thermistor in series. Look up "inrush current limiter"

Larger transformers may need resistors in series to limit the inrush. A separate switch or relay shorts out the resistors after a second or so.