Troubleshooting techniques

Fixing non-working light sets

The following is a description of how to use those cheap bulb testers to troubleshooting light strings that won't work.

One has to first understand how MINI light sets are wired.  A quick explanation of how they are wired and then on to troubleshooting.

Mini lights are wired in series.  Thus the power comes out of the outlet on one wire, runs through all the lights and then returns to the other side of the outlet on the other wire.  If there is a break in the wiring anywhere along the light set, power won't flow and thus the lights don't come on.  What we are wanting to do is locate this break and fix it.

Some light sets are wired so the power passes through all the lights and comes out the other end of the light set.  Then you would have one solid wire going all the way through the light set back to the plug  Let's call this type "A".  Usually you'll have this type of light set if it has plugs on both ends.

Other light sets have the lights leap-frogging each other.  Using the example of a 35 light set, the power goes through the lights (1, 3, 5, 7, 9,...) and then back (29, 31, 33, 35, 34, 32, 30, 28... 6, 4, 2) to the plug.  Let's call this type "B".  This light set usually has only one plug.

Now to testing.  There is one side of every AC outlet that is HOT (or powered) and the other side is ground.  Because these cheap testers detect power they will only detect one side of the outlet (the HOT side) in a non-working light set.  This is because the power isn't returning to the outlet because of the break we are trying to locate.  If you plug in a NON-WORKING light set and place the tester near one of the wires coming out of the plug, one side will light up the LED and the other side will not because the light set doesn't work and thus power isn't making it's way back to the plug.  Make sure you pull the wires apart so you can tell which side is which (no you don't have to do pull the whole light set apart).

Next simply drag the tester down the light set until it's LED turns off.  That's where the problem is and it's usually a bad bulb.  Sometimes it can be a broken wire.

If the LED stays on all the time that means you have a type "A" set and that the single wire running the length of the light set is plugged into the HOT side of the outlet.  Simply unplug from the outlet, turn the plug over, and plug it back in.  Now the HOT side should run through the lights  Do the test again starting at the plug and at some point the LED should turn off.  There is your problem.

Continuity tests sometimes work and sometimes don't due to variance in the resistance of the filaments in the bulbs.  These cheap bulb testers have helped me fix several strings.

Of course the easiest place to start troubleshooting is checking the mini-fuses located in the plugs.

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Electrical power considerations

Here's a good formula to remember:

Watts = Volts x Amps

Usually on a package of Christmas lights you can find 2 of the values you need in order to find the missing 3rd piece.

In North America one can use the value of 115 for the voltage (i.e. 115VAC).

If you wish to figure out current (amps) a 100 watt light bulb needs, simply plug in the values you have into the formula: 100 watts / 115 volts = .87 amps.

Some people like to use 110 for the voltage value while others use 120.

Once you convert everything to amps add all the values up.  Then you can see the load you're placing on each breaker in your breaker box.

Just a reminder - my father always told me to make sure I had a safety margin when it came to working with electricity.  So if you have a 20 amp circuit, don't run it at 20 amps.  Try 15 instead.

You also have to know all the devices that are on that circuit.  For example, when I plug an extension cord into an outlet in my living room and then run it out to the yard, I have to know there is a 500 watt halogen lamp in my living room on that circuit as well as what I have out in the yard.  That lamp pulls 500 watts / 115 volts = 4.35 amps all by itself!

Lastly, remember to figure out what the extension cords you're using can handle.  I don't have a chart but looking out on the internet should turn up a chart of wiring gauge sizes and the amount of current (amps) a cord can handle.  In the past (before I knew all of the above) I had some extension cords that were warm to the touch and it was below freezing outside!

Wire sizes used in USA inside wall

For a 20 amp circuit, use 12 gauge wire. For a 15 amp circuit, you can use 14 gauge wire (in most locales). For a long run, though, you should use the next larger size wire, to avoid voltage drops.

Here's a quick table for normal situations. Go up a size for more than 100 foot runs, when the cable is in conduit, or ganged with other wires in a place where they can't dissipate heat easily:
Wire gauge (AWG) Current (in amps)
18 8
16 13
14 15
12 20
10 30
8 40
6 60
4 80
2 100
1/0 125
2/0 150
3/0 175
4/0 200

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Last modified on 02/01/01