ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Recent Posts
by gfretwell - 06/09/23 11:23 AM
Historical NEC Info
by gfretwell - 06/03/23 02:15 PM
Water heater 208 vs 240
by gfretwell - 06/02/23 06:26 PM
Help Finding Fault
by gfretwell - 05/30/23 10:05 AM
New in the Gallery:
Burger King crown sillyness
Burger King crown sillyness
by wa2ise, December 11
240/208 to a house
240/208 to a house
by wa2ise, October 9
Who's Online Now
0 members (), 14 guests, and 8 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
#163414 05/07/07 08:29 PM
Joined: Jun 2005
Posts: 49
A coworker asked me today what could be blowing the bulbs in his condo. Only the fixtures in his bedroom and closet are unaffected. All the other fixtures will blow bulbs if there are multiple bulbs installed. Two bulbs, it'll blow one. One bulb will work in any outlet in a fixture until a second bulb is put in. They replaced all the fixtures and switches except the fixture in his closet. The close is running the same bulb and fixture since he moved in and the fixture in his bedroom has the same bulb since he replaced the fixture 6 mo. after he moved in. I think he's been there a year.

Any ideas?

Joined: Jul 2004
Posts: 787
1) Does a bulb fail immediately after a second bulb is installed?
2) If not, approximately how long (hours, days, weeks) until a bulb fails?

Excess heat, vibration, and overvoltage all contribute to short bulb life.

3) What is the line voltage when he turns on the lights?
4) Does he use cheap bulbs? (Short bulb life from over voltage and excess heat)
5) What is above him? (Short bulb life from excessive vibrations)
6) Is he running over sized lamps in the fixtures? Fixture is rated for 2 60 W bulbs and he has 75 or 100 W bulbs in them.

Joined: Jun 2005
Posts: 49
He did say the voltage at the fixtures was right around where it should be. I'll check on the rest tomorrow.

Joined: Jan 2007
Posts: 6
Junior Member
Sounds like you are putting a phase to phase short in series when you put in your "second" light bulb. I would put a voltage tester between the screw shell and the inner tab and test for 220 before screwing in the second bulb....Also try turning off all of the other breakers except for the single lighting circuit you should be able to see either a backfeed onto one of the off breakers which will eliminate finding out which one is providing the 220...just a thought...

You can learn a lot from a dummy...buckle up!!
Joined: Mar 2005
Posts: 1,213
I'm thinking along the same lines as Larry. Since it doesn't fail with one bulb, excess vibration doesn't sound very likely.

Could the problem be thermal? If he's exceeding the rating of the fixture, the bulbs could simply be overheating.

Tell him he should be using CFLs anyhow smile

Joined: Jun 2004
Posts: 202
In regards to using CFLs, how resistant are these globes to overvoltage transients? I have worked in installations where the supply voltage swings from 210 to 260 volts due to generator loading & long cable runs.

The customer ended used 250-260 volt globes made by Osram in these installations to get the customers out of trouble.

(the fix to the problem: bad earthing, crook neutral connections & too long runs on too thin wire. Once these three problems were fixed, all worked well smile )

Joined: Aug 2001
Posts: 7,520
I was starting to think along the lines of a bad neutral somewhere, resulting in excessive voltage on one leg with varying loads, but reading the original problem a second time I'm leaning more toward thermal problems.

If a single fixture works fine with one bulb but starts blowing them with two, it could well be excessive heat.

Joined: Apr 2005
Posts: 8
New Member
1.Are all the fixtures that are affected on the same circuit or phase?
2.Are the bedroom and closet fixtures on that circuit?
3.Is any other apartment experiencing the same problem?
4.If other apartments are experiencing the same problem, is the service single or 3 phase?

Remember that an incandescent bulb is basically just a heat element: the higher the applied voltage, the higher the wattage will be, and thus the more heat produced by it. Check for voltage fluctuations and whatever might cause them - specifically a poor neutral connection.

Joined: Oct 2000
Posts: 2,722
Broom Pusher and
Sounds like a thermal issue to me!

Seeing that when an additional Lamp is run in the same fixture, one (or both) Lamps will fail very soon afterwards, points to an issue with ambient temperature.

To the OP:
Verify the Lamp Wattage does not exceed what is listed on the fixtures.
Have your Friend describe what is listed as maximum Lamp wattage, and what is being installed.

Try using two CFLs (Compact Fluorescent Lamps) in one Fixture, to see how long the Lamps run correctly + without noticable decay (hard starting, overheated bases, low light output, blackening at points on the tubes, etc.) or if the Lamps "Cycle" (turn on and off randomly).
Try this in the most problematic fixture.

Another test would be to install Incandescent Lamps with either lower wattage ratings, or higher voltage ratings - or both.

  1. If the Maximum Lamp Wattage rating for a certain fixture is "150 Watts" (maximum of two 75 Watt A17 Lamps), try using two 40 Watt Lamps. See how long they last, as compared to prior Lamp replacements.
  2. Try using Lamps rated at 130 VAC (Volts, Alternating Current) in the same fixture, with a combined Wattage not exceeding the fixture's rating.
  3. Try both "1" and "2" above in the same fixture - lower combined wattage and higher voltage rating.

These Fixtures likely do not include Thermal Protection, so instead of a Fire, the Lamps are "Comitting Suicide", in an attempt to reduce heat concentration.

As others have mentioned, how fast do the Lamps fail, are the Lamps the "Cheap-O" brand, are they + fixture sextremely hot when running, and is there any possibility for failures due to vibrations?

Lastly, verify running nominal voltages at the Fixtures during operation, and observe pecular Lamp operation due to Voltage sags and spikes.
If the Voltage at the Lamps is > 10% of the Lamp's rating, the drawn power (wattage) will be higher than what is rated - and results in higher heat + light output, with shorter life.
If Voltage is above rating & combined wattage exceeds the fixture's maximum, this will kill standard duty 120 VAC rated medium base Incandescent Lamps really fast.

Good luck!


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

Link Copied to Clipboard

Tools for Electricians
Tools for Electricians

* * * * * * *
2023 National Electrical Code (NEC)
2023 NEC Now Available!
* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Member Spotlight
California ,Long Beach
Posts: 28
Joined: May 2007
Top Posters(30 Days)
BigB 6
Popular Topics(Views)
307,238 Are you busy
235,208 Re: Forum
219,301 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5