Case 1: I have two Halo H4, 4 inch recessed cans over the kitchen sink each with an R20 light under the control of a single Lutron Maestro dimmer. The circuit is 15A and also has on it a motion switch in the pantry with 75W bulb and five under cabinet lighting fixures each with three 20 watt lights all under the control of a single Lutron Maestro dimmer. Nothing else is on the circuit. The light on the right side of the sink will burn out after about two weeks and then the light on the left will last about 6 more weeks. We rarely use the lights during the day time hours. During the rest of the day, they’ll be on 80%-100% brightness for a max of two hours and then anywhere from 10% to 40% continuously from 9PM to about 7AM. I’ve tried Phillips 50W bulbs, GE 45W, 130V bulbs (all R20s) and get the same results. Are my fixtures bad or is there a wiring problem or just lousy bulbs? Any ideas? BTW, its a tight fit for the bulbs and while they don't touch the housing, its very close - so could heat be an issue? For the record, the undercounter lighting may be used a couple of hours per day max – most days we don’t need it and they’re rarely used at max brightness. The motion switch is used whenever we go in the pantry which is probably 10 or 12 times per day. Your ideas would be appreciated.

Case 2: We have a series of outside floods from home depot (dual fixture, 150W halogen sticks(?) each). There are two of these dual fixtures on the circuit along with five outlets. The outlets are used at Christmas time. The circuit is timer controlled and of course GFI protected. On one of the duals, one of the lights will burn out very quickly (2 to 3 weeks), the other after about a month. In the other dual fixture, it seems to be more normal with one burning a couple of weeks before the other with a typical life of about 6 months. I’ve used different brands including some 130V from HD (they lasted the shortest time) and get the same results and it tracks the specific fixtures. I also have two other sets of similar fixtures and one set exhibits similar issues and the other set seems to be OK. Is the problem just poor bulbs or something else. Your opinions would be appreciated.

Cheers,
Larry

The halogens in question are slightly off horizonal. The first one's I mentioned point up at about 75 degrees, but are close to horizonal - lighting the trees. The others point down slightly with one being about 60 degrees off horizonal and the other close to horizonal.

For your exterior lights, try using bulbs made by Iwasaki, also sometimes called Eye. At my parents store, they made the difference between calling me once a week to replace all the bulbs that burned out and calling me once a month to replace the only one that burned out! You will only find them at electrical or ligting supply stores. Personally, that's the only place where I buy bulbs. Hope this helps!

A loose neutral would have no affect on bulb life.

Vibration is probably the number one killer.

Overheating is more a problem with the fixture than the bulb.
The filament is at ~3600F, so a few hundred degrees of rise from an overloaded fixture is not going to do much.

The resistance of a filament also increases as it heats up, making them self regulate to their operating point.

Some bulbs are sensitive to mounting position, but it is usually the materials used to attach the base to the bulb that are the problem.

I don't think its a vibration problem, although I'll check when the disposal is on. The dishwashers are very silent and smooth. If heat is a problem - and they do fit very tightly - would a socket extender help because they're very tight fits.

When you say the bulbs fit very tightly, do you mean against the can?

The fixture rating should be on a sticker inside the can.

Voltage transients can affect light bulbs, but you need to factor in how many hours per day the lights are in use.
Transients do no harm to lights turned off.

There are plenty of low price 'bargain' CFLs that are junk.

Some early ones had real problems burning base up.

There is still heat buildup in the electronics, and the cheap ones have little margin before overheating and failure.

If one leg is heavily loaded, it will cause the voltage to rise in the other leg unless it is also similarily loaded. For example, if a 120v leg becomes so loaded that it falls to 110v, and the other leg is lightly loaded, the second leg may rise to 130V. This will definitely shorten the life of incandescent bulbs.

Some things that can heavily load one leg:

1. an arc welder.

2. A group of baseboard heaters run off the same leg.

3. A group of water heaters on the same leg.

An occurences of significant unbalanced legs is not common, yet I expereinced it in my house a couple of years ago. My pole transformer powers 4 residences. A renter next door upset my balance in the winter when he powered a large electric heater on one leg. Some of the lights in my house would brighten when his load hit the line.

Only if the neutral connection feeding the panel is bad somewhere between the pole transformer and the panel.

Just drawing more current does not alter the voltage on the other leg in a correctly operating system.

If you see brightening of lights it is an indication of a bad neutral (not indicated by OP).

Simple failures with no other symptoms are usually vibration or cheap bulbs.

For the same reason, it's bad for halogen lights if the wire section is too small. This causes a voltage drop on the cable, making them burn less bright and reducing their lifetime.