Chared 14/2 NM

[evandriel]

Ok I need some help here. I have two fixtures in my kitchen both rated for 2 60w bulbs. Recently one fixture has been burning bulbs at a rate of 2 a week (+/- couple days) and I knew something wasn't right. Upon removal of the fixture not only was the light leg 14/2 burnt but the leg out to the 2nd fixture was charred. So I enter inspect mode. Checked connections in panel, all tight. Breaker rated 15a. Not to many devices being fed of this circuit. What else could I be missing? I haven't done anything new to my kitchen aside from adding a dishwasher to my small appliance circuit.

Specs on house: Mobile home built in 98. Fixtures are flush mount with two 60w Reveal bulbs (maybe these get hotter than normal?) in each fixture. I have taken other fixtures down but all the other fixtures in my house have the bulbs out in the open with a mushroom shell covering them (letting bulbs cool in open air) giving the bulbs about 4 more inches of seperation from the ceiling than the two fixtures in question.

Hopefully that's enough to go off of. Thanks for any and all help.

 

[bennie]

Re: Chared 14/2 NM

Do you mean the fixtures are surface mount, instead of flush mount?

The heat from the lamps are cooking the NM cable.

Only cure is to install flourescent lamps.

Incandescent lamps will reach 2500 degrees C. internally. Wattage is not a factor for this heat.

 

[hurk27]

Re: Chared 14/2 NM

The kitchen is the worst place to find burnt wires in the fixture box's. And the lamp's might be the correct size now but do you know if they were in the past? The reason is most people will want more light in the kitchen and instead of installing a fixture that will allow brighter lamps they will just install higher lamps and the heat only goes one way. And that's up. Now as far as the lamps burning out this can be because of the bulb's or high transient spikes but do a search on bulb's burning out and you will find several threads on this subject. also compact florescent lamps will give you both brighter lite and less heat.

 

[evandriel]

Re: Chared 14/2 NM

Thanks for the responses. After completely removing the box from the ceiling the wire to the switch was like new about 6 inches back from fixture box so I installed a junction box about 2ft back to be on the safe side (they sure do make some perty blank covers these days) and replaced all the wiring and installed two GE 35w florescent fixtures.

Bennie: Yeah I meant surface. Sorry, stress of the situation had my mind going different directions.

Hurk: The 2 fixtures I replaced were both 2 bulb 60w fixtures but they were like all the other fixtures in the house where the bulbs had a good 4" of seperation and open air to keep them somewhat cooler (as well as the outlet box I'd imagine).

Again, thanks for the responses.

 

[bennie]

Re: Chared 14/2 NM

The heat problem is not from the glass envelope.

As I stated, "incandescent lamps will reach an internal temperature of 2500 degrees centrigrade".

This temperature is the same on all incandescent lamps, regardless of rated wattage. The heat is commuted to the lamp base, then to the wires in the box.

 

[rrrusty]

Re: Chared 14/2 NM

I had a service call where the customer was going thru bulbs like crazy; I did a voltage check and found they were getting 175V instead of 120V; The utlity came right out and replaced the transformer right away

 

[charlie]

Re: Chared 14/2 NM

I had a service call where the customer was going thru bulbs like crazy; I did a voltage check and found they were getting 175V instead of 120V

I find that a bit hard to swallow unless you had around 65 volts on the other leg.

 

[templdl]

Re: Chared 14/2 NM

2500 deg C? That is hot! That puts it at 4532 deg F. I didn't realize that. Even though its an internal temperature the heat generated if the heat isn't dissipated is certain to do in the insulation of wire if the insulation doesn't have a high enough insulation rating. It is conceivable that the temperature within the fixure could get as high as 2500 deg C if there was no heat dissipation at all.
As a comparison copper melts at a meer 1981 deg F or 1083 deg C to put the 2500 deg C into perspective.

 

[rong111]

Re: Chared 14/2 NM

there is wire rated for 90c. for terminations at light fixtures or ovens. the internal filament temp of a incan bulb is super high. the actual surface temp of the external glass is hot but much much less. it is in a vacuum. simple physics involved here. also for existing romex there is a teflon sleeve that can be slipped over the cable. many ovens actually come with a cable like this to run from the junction to the appliance. also radiators have this.


ron g.

 

[templdl]

Re: Chared 14/2 NM

Ron, You're right, the actual temperature of the outside of the bulb is much less than the filament but only because the heat is being conducted away from the bulb by the surrounding air either by ventilation or the enclosure which transfers that heat to the exterior of the enclosure.
If this does not occur, such as vent holes being blocked or the enclosure not being able to dissipate the heat to the outside air because of insulation or the enclusure itself is enclosed, you can expect temperature much greater than 90deg C.
We must always consider that heat doesn't magically go away.

 

[iwire]

Re: Chared 14/2 NM

Originally posted by templdl:
We must always consider that heat doesn't magically go away.

I think that is exactly what Bennie is pointing out.

Yes some of the heat the conductor insulation is exposed to is from the ambient temperature around it

However, more heat is brought to the conductor insulation from the copper conductor itself.

You have a filament that is 2500 degrees Centigrade physically connected to the copper conductors by way of the lamp socket, much of this high heat is conducted back through the filament "stands"(?) back through the socket and into the coper conductors.

Any addition of heat insulating sleeves over the standard insulation is going make matters worse in the case of a light fixture, this is different from the protection added to wires where they might touch a hot surface as in a baseboard heater.

Think about it, if you had 18" of 14 AWG bare copper suspended in the air, and you somehow kept the first one inch of it at just 1000 C would you want to touch the other end of this conductor?

 

[evandriel]

Re: Chared 14/2 NM

Bennie: Is this heat the same for florescent lamps? That is what I replaced the fixtures with.

But to get to your point. If the heat of the incan. bulbs were / are the culprit would they not have burned the wire some time ago? I ask this only because I installed these 2 fixtures (the ones that burnt up) some 9 to 10 months ago and the wiring looked good as new back then but then in just this short amount of time the wiring damn near went up in flames. If the fixtures have no bearing on heat dissipation then the only difference was the use of Reveal 60w bulbs rather than standard frosted 60w bulbs.

 

[bennie]

Re: Chared 14/2 NM

The flourescent lamps do not heat much.

Incandescent lamps type B operate in a vacuum. Type C are inert gas filled. Most common lamps available are type C, which burn hotter than the vacuum lamps.

Since lamps were changed to the inert gas filled type, the temperature of lamp bases has increased a large amount.

Hard glass lamps can reach 450 to 510 degrees centigrade externally.

 

[rong111]

Re: Chared 14/2 NM

i actually overlooked the fact the the filament stands are heating the socket and in turn heating the copper. i knew this but wasn't really thinking when i posted. this only occurs if there is not proper ventilation. if an ican bulb is completely suffocated, most surrounding material will ignite and at some point the bulb will actually explode as the gas heats to a point where it evaporates leaving a 'dry' bulb that is basicly a bomb of sorts. not good. as for compact fluorescent they are much cooler as they are not 'burning' a filament at high resistance. cff lamps are exciting an inert gas with ions. in the cheap cff edison base lamps the ballast gets hotter than the tube. but still no where near as hot as incans. the only problems with cff screw base lamps is they do not have great color rendering for the most part and the ballasts can flash at the point where the gas has been exhausted. unfortunatly all this stuff is made in the orient now and is not great quality. i have nothing against the orient but if you can find us or eurpean made lamps they tend to be better quality.

ron g.

 

[iwire]

Re: Chared 14/2 NM

Originally posted by rong111:
i actually overlooked the fact the the filament stands are heating the socket and in turn heating the copper. i knew this but wasn't really thinking when i posted. this only occurs if there is not proper ventilation.

Ron I do not agree that this only happens do to lack of ventilation.

About the only way to keep the temperature of the filament from conducting back into the copper supply conductors is to provide another path for the heat to conduct to.

In an HID fixture this is done with a heavy porcelain socket firmly connected to an aluminum bracket as a heat sink.

Bennie used a technical term for this that I can not remember, but it basically says everything wants to reach the same temperature.

 

[iwire]

Re: Chared 14/2 NM

By the way, I want to thank Bennie and Don for bringing this to my attention.

That is not the case, put a large enough lamp in an open fixture and you can still have conductor insulation damage.

 

[templdl]

Re: Chared 14/2 NM

Thanks. I totally forgot about the "copper conducting heat" issue. Being the breaker guy that I am, I know that the UL temperature rise testing that is done on a UL489 listed breaker with regards to the terminal temperature as well a the breaker's rating is highly dependent upon the cable attached to it. The cable acts as a heat sink in that it actaully removes heat from the breaker. It also assures that the heating that can take place will not exceed the 75deg C insulation rating of the cable.

 

[bennie]

Re: Chared 14/2 NM

Many are under the impression that incandescent lamp filiments operate in a vacuum. This is a misconception. Most lamps are filled with an inert gas, namely nitrogen.

This gas makes the lamp brighter, hotter, and with a shorter life span.

Most rough service lamps are class B, vacuum type.
The filiment is not as hot as a gas filled capsule.

 

[rong111]

Re: Chared 14/2 NM

edison lamp bases get very hot. poor ventilation only hastens the situation. in comercial applications there is a design consideration beyond the fixtures. residential fixtures are designed to first and foremost look nice. with heat management as a secondary consideration.
try to use as small a bulb as you can get by with. and certainly do not overlamp.

also, an intresting tidbit: the better lamps these days have copper bases. the aluminum bases can actually expand from the heat and seize in the socket! thats a lot of heat. most homeowners do not realize how hot incan bulbs get. especially when they go around putting 300 watters in every holder!


ron g.

 

[hurk27]

Re: Chared 14/2 NM

thermal Coalescence I think Bob

[ April 18, 2004, 02:10 AM: Message edited by: hurk27 ]