# Using Only 2 Lamps in a T8 4-Lamp Fixture



## papaotis (Jun 8, 2013)

on an energy efficient fixture, why would you want to only run two lamps?:001_huh:


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## Deep Cover (Dec 8, 2012)

You would have to check on the ballast. Some are universal and can be configured for multiple different lamp numbers. Other than one of those, you can significantly reduce the life of the ballast by not following the wiring diagram.


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## HARRY304E (Sep 15, 2010)

Little-Lectric said:


> If you have a T8 4-lamp ballast and you want to only run 2 lamps, do you need to leave off either the red or blue wires, or use 1 of each, depending on which 2 lamps you want?
> 
> Or does it harm either the ballast or lamps if you just leave the wiring and take out 2 of the lamps?


If the ballast is not rapid start ,but trigger start instead you can just take out two lamps and they will run brighter thus burn out faster,,the ballast may fail early too.


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## A Little Short (Nov 11, 2010)

I don't have the ballast here at the moment, but I recall they are labeled as "for up to 4 lamps", but I don't remember seeing but one diagram showing the 4-lamp wiring.

The customer wants to be able to use just 2 lamps if they feel they are too bright.

I installed a new 4-lamp light in another room and they thought that would be too bright for the other room.
In this room, they don't want a new fixture but want me to retrofit the existing lights. I guess I could just buy 2-lamp ballast but am afraid they may want to use all 4 in the future.


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## bobelectric (Feb 24, 2007)

HARRY304E said:


> If the ballast is not rapid start ,but trigger start instead you can just take out two lamps and they will run brighter thus burn out faster,,the ballast may fail early too.


 ???????


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## papaotis (Jun 8, 2013)

Little-Lectric said:


> I don't have the ballast here at the moment, but I recall they are labeled as "for up to 4 lamps", but I don't remember seeing but one diagram showing the 4-lamp wiring.
> 
> The customer wants to be able to use just 2 lamps if they feel they are too bright.
> 
> ...


 if they want to use all 4 later, after having you change it, thats another job!


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## mbednarik (Oct 10, 2011)

Most 4 lamp ballasts I have used are good for 3 or 4 according to the label. IDK about just 2.


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## frenchelectrican (Mar 15, 2007)

The only answer you can find is check the ballast info on the nameplate and wiring diagram and some case you may find a hidden infomation if you go online and check with the manufacter model to see how far they can be delamped.

I know the instant start ( parallel ) verison can delamp without much issue but again check the info before you complety delamp in half due some electronic ballast do have dual connections inside so one side will work off from one side of ballast and the other half on other side of ballast.

So it possible to do that.

Merci,
Marc


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## backstay (Feb 3, 2011)

I put a bunch of 4 light ballasts in a retro job and the owner pulled a few bulbs. Haven't seen any problem yet.


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## Split Bolt (Aug 30, 2010)

HARRY304E said:


> If the ballast is not rapid start ,but trigger start instead you can just take out two lamps and they will run brighter thus burn out faster,,the ballast may fail early too.


Harry, OP is asking about T-8 lamps, which means electronic ballasts. As others said, he needs to read his particular ballast label.


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## jbfan (Jan 22, 2007)

The ballast I use all say for three lamp use, cap one blue wire.
Doesn't say anything about 2 lamp.


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## HARRY304E (Sep 15, 2010)

Split Bolt said:


> Harry, OP is asking about T-8 lamps, which means electronic ballasts. As others said, he needs to read his particular ballast label.


T-8 ballasts are made in instant start where each lamp will work without the other.

Or in a Rapid start ballast where the lamps depend on each other to run.



Differences between Instant Start and Rapid Start Configurations
Over the years, the primary method of operating T12 lamps has been magnetic ballasts using rapid start technology. Rapid start configurations have two wires connected to each side of the lamp. There is a small voltage between the two wires that is used to provide heat to the lamp’s cathodes. An arc forms between the electrical connections in the cathodes, which produces light. When heated, the cathodes release emitter, which is a substance that helps strike the arc across the lamp between the two cathodes. The voltage between each side of the lamp is required to maintain an arc over the length of the lamp tube. Rapid starting is relatively gentle on the cathodes and - when operated on short lighting cycles - the lamp doesn’t sacrifice life. However, due to cathode heating, rapid start systems require several extra watts per lamp more than instant start systems.
T8 lamps are designed as rapid-start lamps, as are T12 lamps, and they are capable of being operated on rapid start, programmed rapid start or instant start ballasts. Electronic T8 ballasts are most commonly instant-start configurations, where there is no cathode heating. With an instant start configuration, a high voltage pulse is used to start the arc between the lamp’s cathodes. The primary advantage of instant start systems is the energy saved since no power is required to heat the cathodes. The main disadvantage is reduced life when instant start systems are installed in applications that are frequently turned on and off.


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## wireman64 (Feb 2, 2012)

Little-Lectric said:


> I don't have the ballast here at the moment, but I recall they are labeled as "for up to 4 lamps", but I don't remember seeing but one diagram showing the 4-lamp wiring.
> 
> The customer wants to be able to use just 2 lamps if they feel they are too bright.
> 
> ...


Too bright ? I have never thought a 4 lamp f32t8 fixture was too bright , even f40t12's


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## Split Bolt (Aug 30, 2010)

HARRY304E said:


> T-8 ballasts are made in instant start where each lamp will work without the other.
> 
> Or in a Rapid start ballast where the lamps depend on each other to run.
> 
> ...



And here I thought you were just replying without reading the OP thoroughly!!!:laughing::thumbsup:


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## 8V71 (Dec 23, 2011)

HARRY304E said:


> T-8 ballasts are made in instant start where each lamp will work without the other.
> 
> Or in a Rapid start ballast where the lamps depend on each other to run.
> 
> ...


Daaaammm Harry. :nerd: :laughing:


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## russellyoungblood (Nov 30, 2018)

u ted talked the **** out of that,gj not being sarcastic either


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## russellyoungblood (Nov 30, 2018)

even if it was just copy and paste its still definitive and enough said,I forgot what I was doing now,was just wondering about 2 lamp F40 ballasts,hate togo to supply house every time its one of those,never have ballasts for those,wondered if any other ballasts work for them,the u-shaped lamps


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## tmessner (Apr 1, 2013)

A Little Short said:


> I don't have the ballast here at the moment, but I recall they are labeled as "for up to 4 lamps", but I don't remember seeing but one diagram showing the 4-lamp wiring.
> 
> The customer wants to be able to use just 2 lamps if they feel they are too bright.
> 
> ...


If you are retro fitting why not use a direct wire lamp, forget the ballast and use as many lamps as you want.


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## A Little Short (Nov 11, 2010)

tmessner said:


> If you are retro fitting why not use a direct wire lamp, forget the ballast and use as many lamps as you want.



Since this is a 5 year old thread I can't remember:wink:
Pretty sure there wasn't any direct wire LED tubes back then.


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## readydave8 (Sep 20, 2009)

A Little Short said:


> Since this is a 5 year old thread I can't remember


Do you remember what you wound up doing, and how it's worked out in the last 5 years?


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## A Little Short (Nov 11, 2010)

readydave8 said:


> Do you remember what you wound up doing, and how it's worked out in the last 5 years?



I had to think about it and look back through my invoices.
The 4-lamp ballast only had options for 3 or 4 lamp, not 2.

I talked them into letting me install 2-lamp T8 ballast after wiring up one to show them the difference between the T12 & T8. Also, with the understanding that if they later changed their mind, I would have to charge them to change them to 4-lamp.
They were/are happy with the 2-lamp T8.


Today, I would try and sell them an LED upgrade!


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