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capt2 09-05-2009 06:24 PM

T8 vs. T12 Ballasts
I've got a 4 light 48" floresc fixture that won't light after replacing the bulbs . (It had the fatter T12 tubes in it) So I figure the ballast is bad. I checked the ballast number on the fixture--the part # reads QTP4x32T8/UNV.

So my questions are:
Is this ballast/fixture is actually meant for T8 tubes rather than the T12s that were in it?
Would T12 tubes cause a T8 ballast to fail??
OR....are T12 and T8 tubes interchangeable with this type of ballast? Does the /UNV on the part number signify universal?

Thanks in advance.

EBFD6 09-05-2009 06:49 PM

You should always use compatible lamps and ballasts.

Using T8 lamps with a T12 ballast or T12 lamps with a T8 ballast will cause the lamps and ballast to fail prematurely.

The unv in your part number refers to the fact that the ballast is a universal voltage ballast, meaning it will work on both 120 volts or 277 volts.

Ranger31 09-05-2009 09:20 PM

WARNING: Fluorsecent Ballast, High Voltage

My advice is to hire an electrician and not play with the fluorescent ballast, unless someone with such experence has show how to replace one safely.

That being said, I'll supply you with information that shouldn't get you kill.

Before moving on, try the lamp, in a light fluorscent light fixture you know
to be working. if the lamp lights, then it's three things.

1) Bad fluorscent lamp sockets. 2) bad starter 3) bad ballast.

If, and I say if, you already have remove the cover, go back to you
electrical panel and turn off the circuit breaker that supply power to
the light in question.

After making sure all power to the light question is OFF. Take a flashlight
and maybe a magnifier and read what is printed on the ballast located in-
side the light.

Univeral brand ballast have printed the type of Lamps that can be use
with UNIVERAL WATT REDUCER Rapid Start Ballast.

There two columuns

The first columun: USE WITH LAMP TYPE * underneath that heading
is written the types of lamps they allow for use with their ballast.

(1) F40T12ES Qty. 2
(2) F40T12/U Qty. 2
(3) F40t12 Qty. 2
(4) F40T10 Qty. 2

Now, the replacement fuorescent lamps you purchase, look at their ends.
Written on their end, words such as: PHILIPS, F40T12, F = type: fluor-
scent. 40 = 40 watts. T = tubular in shape. 12 = 1-1/2" in dia.

Today buy a fluorescent lamp is no walk in the part, there are many lamps to chose from. Grab just any lamp, that like the one your holding in your hand, bring it home and there a good chance it will not work, or not work properly. That way I recomemd that you go to a retailer
The retailer should be able to find you the right lamps.
This new lamp, should work with your existing light fixture ballast.

Also you may have a light that uses a Fuorescent Starter, you'll see this
sticking thru the cover, that conceals the light ballast. This is something you can replace by twisting so it let loose, after you have remove it, take this to the hardware store.

But, in my experience, the lamp not lighting, is usual cause by wore-out
fluorescent bi-pin lamp sockets, in which case, call an electrician to have then replace.

One last thing, handle your fluorescent lamp like you would a new born
baby. Don't drop it, drop it and it will explode. Be sure to take it to a lamp recycling center. Or ask the retail store where you purchase your replacement, if they will take cae of it.

kbsparky 09-05-2009 10:33 PM


Originally Posted by capt2 (Post 324183)
...-the part # reads QTP4x32T8/UNV....

That part number tells me that the ballast was rated for (4) T8 bulbs.


Is this ballast/fixture is actually meant for T8 tubes rather than the T12s that were in it?
Would T12 tubes cause a T8 ballast to fail??
OR....are T12 and T8 tubes interchangeable with this type of ballast?.....
Yes, only T8 bulbs will work properly with this ballast. Using T12 bulbs can cause burnout for the bulbs and/or the ballast.

As someone else stated the UNIV refers to the input voltage, not the type of bulbs you can use.

kbsparky 09-05-2009 10:35 PM


Originally Posted by Ranger31 (Post 324230)
...handle your fluorescent lamp like you would a new born baby. Don't drop it, drop it and it will explode....

Actually implode is the more accurate term when dealing with intact fluorescent lamps. :wink:

nap 09-05-2009 11:24 PM


Originally Posted by kbsparky (Post 324259)
Actually implode is the more accurate term when dealing with intact fluorescent lamps. :wink:

but what happens with a glass shard involved in an implosion that does not hit anything to stop it's travel?

it flies outward like in an explosion.:thumbsup:

ultimately the same result:

glass all over the fricken place.:yes:

just a bit of trivial information concerning lamps;

when you see a number such as the 12 in T12, that refers to the diameter of the lamp in terms of 1/8ths of an inch. So, as Ranger stated, a T12 would be 12/8 inch diameter which equals 1 1/2 inch. A T8 would be 1 inch and a T5 5/8 inch diameter.

You will also see similar markings with lamps such as flood lamps and spot lamps. Common designations include 30 and 38 which would translate to 30/8 or 3 3/4 inches and 38 which would translate to 4 3/4 inches.

spark plug 09-06-2009 12:17 AM

debate about T8 vs. T12 Ballasts!
Capt... (Poster #1) Just on the far out chance that you decide to replace the ballast yourself. Be forewarned NOT to follow the color of the wires blindly. Read the diagram of the NEW Ballast carefully. And be certain that you interpret it correctly! There is a difference (in connecting the wires to POWER and to the sockets) between a MAGNETIC and ELECTRONIC Ballst! (Now more than ever):yes::no::drink:Don't Drink and Drive!!!

nap 09-06-2009 01:11 AM

and along with sparkplugs info; there is also a difference in the connections between instant start and rapid start ballasts. as well, the tombstones (end terminals) can have differences. There are shunted and non-shunted. In the shunted, the two contacts are internally connected and the non-shunted, obviously the opposite.

some ballast connections require one or the other. Can't remember which needs what at the moment though but I think the instant start ballasts use the shunted tombstone. You can convert a non-shunted to a shunted tombstone by simply connecting the two terminals using a piece of wire.

the wiring diagram on the ballast will indicate of one or two wires are required to connect to the tombstone.

Ranger31 09-06-2009 05:36 AM

T8 vs T12

Here are the facts, as I've come to know them.

You have a four lamp, light fixture, surface or recess.

You took out all the fluorsescent Lamp and removed the cover, which exposed one interiorior mounted lamp ballast.

If it was truly a T12 fixture with four lamp, it would had come with two
ballast not the one.

4XT8 = four T8 Lamps

So the lamp you removed. Am I right in saying you didn't bring it with you
to the lamp store, for the replacement.

There is a sizeable different between T12 and T8 that you should had notice.

Me wonders, did someone else purchase the lamps, wrong type, T12, and installed then in your light.

The T8 is a completetly different animal then the T12, but they share the same size Bipin, on each end of the individual lamp.

One last thing; your T8 ballast runs hotter then a T12 ballast.

WARMING: make sure the circuit breaker is off before you attempt to make any repairs, if in doubt, call the ELECTRICIAN.

micromind 09-06-2009 11:12 AM

Good guess on the tombstones nap. (Though I think that years of experience might have slanted the guess a bit!)

The T8s are shunted, the T12s are not.

If the ballast connections are one wire to each tombstone (T8), then it must be shunted. If two wires to each tombstone (T12), then it cannot be shunted.

The reason being, the T8s are a 'cold cathode' design. High voltage and high frequency are used to start the arc inside the tube.

The T12s have a small heating element at both ends of the tube. Once it is hot enough, the arc will start at 60HZ.

The two types of ballasts are of different operating designs, hence the different connections and different lamps.


capt2 09-06-2009 02:09 PM

Thank you everyone for all the replies--I learned a few things.
KInd Regards

nap 09-06-2009 03:12 PM


The T8s are shunted, the T12s are not
. r u sure? I was thinking there was s difference between instant start and rapid start, even with the T8's. It's been awhile since I have done service. I see a stain on my pillow every morning where the knowledge has drained out.

Now I'm going to have to go and look. Dang this OCD:censored:


Ranger31 09-06-2009 03:29 PM

A T8 ballast has leads of two blues and one red wires.

The red wire, wires in parallel to set of medium Bi-Pin socket at one end of
each fluorescent lamp. The two blue, each is run individual to a medium
Bi-Pin socket at the other end of each fluorescent lamp.

A four lamp, T8 ballast wires almost the same, only this time you will have two extra wire, colors are; red and yellow. Only this time the yellow wire, wires like the red in the first example. The red wire is wire just like the blue blue in the first example.

PLEASE NOTE: I used Sylvania leads color code, for my answer, I am not sure if all MFG, use the same color
code for their ballast wires.

micromind 09-06-2009 06:04 PM

There are two types of T8 ballasts that I know of; instant-start and programmed-start. They wire differently, and use different tombstones.

The instant-start (by far the most common around here) has one wire to each end of each tube. It uses shunted tombstones. This type simply applies a high voltage (around 600 or so) at high frequency (usually more than 40KHZ) to the tube. This will fire the arc across the inside of the tube, the voltage will be reduced, and the ballast simply maintains current across the tube.

The programmed-start has two wires to each end of each tube. This type obviously uses non-shunted tombstones. Just like the older magnetic ballasts with T12s. It starts by applying a low voltage across the cathodes at the ends of the tubes. After a second or so, it applies the high voltage across the tubes. When the arc is established, the ballast switches to constant-current, just like the instant-start.

If the tubes are lit continuously, there's virtually no difference between these two types. If they're switched on and off a lot, the tubes will last much longer with a programmed-start ballast. Instant-start ballasts are very hard on tubes when they start, but they are less costly to produce and are (in my experience) more reliable.


kbsparky 09-06-2009 06:54 PM


Originally Posted by micromind (Post 324557)
...Instant-start ballasts are very hard on tubes when they start, but they are less costly to produce and are (in my experience) more reliable.


My experience has been the opposite. The electronic ballasts are much more prone to failure than the older, magnetic type ballasts ever were. A simple surge can zap the electronics, while the older ones would keep humming along for years.

Most of the ballast failures I have seen are as follows:

Electronic: less than 5 years old, many of these less than 3 years old. :furious:
Magnetic: 10-20 years old. It's rare to see a bad magnetic one that is less than 7 years old. :whistling2:

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