T8 lamp run by a T12 magnetic ballast

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I've seen it done once or twice, although I don't remember whether it was T-8 tubes on a T-12 ballast or vice versa, but it didn't work for even a moment, and it wasn't pretty. I remember the tube being instantly damaged.
 
Just had a customer last week call and said he had a fixture that was acting weird and he already replaced the lamps. When I got there, I found he had relamped it with T-8's. The fixture had two 2 lamp T12 magnetic ballasts, in which both were toasty. His lamp supplier had told him they did not make the T12 lamps anymore, and the T8 lamps would work fine in the old fixtures. Went down to the orange box and bought him a case of T12's. Changed the two toasted ballast to one four lamp T8, as eventually he will have to replace all of the ballasts.
 
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I Wonder Why?

I Wonder Why?

I am trying to figure out why they are not compatible. Besides size are the lamps made differently? What is the difference between a T8 and a T12 lamp besides the size?

Thank You,
 
I found this on the net so...

Since ballasts limit current flow in the circuit, an improper (t12) ballast would cause more current to flow through the t8 lamp than the lamp is rated to handle.
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My Theory of Electricity must be skewed.

My Theory of Electricity must be skewed.

Dennis Alwon said:
I found this on the net so..."Since ballasts limit current flow in the circuit, an improper (t12) ballast would cause more current to flow through the t8 lamp than the lamp is rated to handle".
Click to expand...

Is my thinking wrong?

1) I think of current as the amount of watts being pulled by a lamp. How can a lamp pull more watts than it is designed to pull?

2) Is something forcing wattage into it?

2.1) Does voltage have anything to do with it?

3) Is the output voltage of a t12 ballast higher than the output voltage of a t8 ballast?

4) What principle does the electronic ballast that feeds both t12 and t8 lamps operate under?

Thank You,
 
Ravenvalor said:
Is my thinking wrong?

1) I think of current as the amount of watts being pulled by a lamp. How can a lamp pull more watts than it is designed to pull?

2) Is something forcing wattage into it?

2.1) Does voltage have anything to do with it?

3) Is the output voltage of a t12 ballast higher than the output voltage of a t8 ballast?

4) What principle does the electronic ballast that feeds both t12 and t8 lamps operate under?

Thank You,
Click to expand...

think of the arc in the tube as a short (or very low resistance) the ballast limits the current flow to keep the lamp operating at its rated wattage.
 
Ravenvalor said:
Is my thinking wrong?

1) I think of current as the amount of watts being pulled by a lamp. How can a lamp pull more watts than it is designed to pull?
Click to expand...

That may work for simple incandescent lamps, where the voltage is always 120 volts.

But you should be thinking of watts as the product of voltage x current.

For fluorescent lamps, the lamps are designed to handle a certain amount of voltage and current. That gives them a wattage rating. The ballast regulates the current that flows into the fixture by adjusting the voltage it applies across the lamp.

So T12 lamps are normally designed to run at about 40 watts. You can get high power factor ballasts that might run them 10% over that, at 44 watts. You can also get Low power factor ballasts that might only run them at 34 watts.

T8 lamps are made to run at about 32 watts. Again, ballasts can be selected to run them a little higher, or quite a bit lower.

So when you connect a T8 lamp into a T12 fixture, you will probably overdrive it.

Also, fluorescent lamps have different starting methods - preheat, instant start, rapid start, programmed start, etc. The difference in starting between a T12 and a T8 might be the first think to kill a lamp when you plug it in.
 
steve66 said:
That may work for simple incandescent lamps, where the voltage is always 120 volts.

But you should be thinking of watts as the product of voltage x current.

For fluorescent lamps, the lamps are designed to handle a certain amount of voltage and current. That gives them a wattage rating. The ballast regulates the current that flows into the fixture by adjusting the voltage it applies across the lamp.

So T12 lamps are normally designed to run at about 40 watts. You can get high power factor ballasts that might run them 10% over that, at 44 watts. You can also get Low power factor ballasts that might only run them at 34 watts.

T8 lamps are made to run at about 32 watts. Again, ballasts can be selected to run them a little higher, or quite a bit lower.

So when you connect a T8 lamp into a T12 fixture, you will probably overdrive it.

Also, fluorescent lamps have different starting methods - preheat, instant start, rapid start, programmed start, etc. The difference in starting between a T12 and a T8 might be the first think to kill a lamp when you plug it in.
Click to expand...

Hey thanks,

If I can find a f34t12 magnetic rapid start ballast do you think that I can effectively operate a 4' t8 lamp on it?

I appreciate the time you have spent explaining this to me.

J.
 
I'm pretty sure that Workhorse ballasts operate T8 & T12's. I do alot of service work and I carry three or four or each WH ballast because they cover over a hundred different lamp combinations of linear, cf, u-bends, dulux, you name it type fluorescents.
 
HotConductor said:
I'm pretty sure that Workhorse ballasts operate T8 & T12's. I do alot of service work and I carry three or four or each WH ballast because they cover over a hundred different lamp combinations of linear, cf, u-bends, dulux, you name it type fluorescents.
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Very interesting, I have never heard of the term work horse ballast. Can you recommend a model? I am using a magnetic ballast that is rated for 40 and 34 watt fluorescent t12 lamps. Do you think it would run a f32t8 lamp?
 
HotConductor said:
I'm pretty sure that Workhorse ballasts operate T8 & T12's. I do alot of service work and I carry three or four or each WH ballast because they cover over a hundred different lamp combinations of linear, cf, u-bends, dulux, you name it type fluorescents.
Click to expand...

I carry three "Workhorse 5" ballasts on my truck. I like them for display lighting fixtures like anthony or such. The only drawback to the Workhorse ballast is that they are typically single voltage.
 
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