Running Two Lamps From a 4-lamp Ballast

[Little Bill]

Does it harm either the ballast or lamp to use just two lamps in a fixture that has a 4-lamp ballast, and is wired for four lamps?

If so, would you just leave off one red and one blue wire when wiring the ballast?
This is if you are using a ballast that can run up to 4 lamps.

Almost forgot, this would be a T8 electronic ballast.

 

[GoldDigger]

Does it harm either the ballast or lamp to use just two lamps in a fixture that has a 4-lamp ballast, and is wired for four lamps?

If so, would you just leave off one red and one blue wire when wiring the ballast?
This is if you are using a ballast that can run up to 4 lamps.

Almost forgot, this would be a T8 electronic ballast.

The ballast installation instructions themselves, especially for an electronic ballast, should list the lamp combinations that they are rated for and the wiring to use. Some four lamp ballasts are also rated for three lamps, or for two lamps. Some two lamp ballasts are also rated for one lamp. It varies by manufacturer and model, so there are no generic instructions.

If the wiring of the four lamp fixture (probably a luminaire!) can be made to match the wiring shown in the ballast instructions for two lamp use, (i.e. the particular wires that go to the unfilled lamp holders are marked as unused and insulated in the instructions) then there should not be a short or long term problem for ballast or lamps.
Anything else could result in damage to the ballast or lamps or both. Or not.....
And then you have to figure out what would happen if somebody fills the other two lamp slots and decides the fixtures are defective.

 

[Little Bill]

The ballast installation instructions themselves, especially for an electronic ballast, should list the lamp combinations that they are rated for and the wiring to use. Some four lamp ballasts are also rated for three lamps, or for two lamps. Some two lamp ballasts are also rated for one lamp. It varies by manufacturer and model, so there are no generic instructions.

If the wiring of the four lamp fixture (probably a luminaire!) can be made to match the wiring shown in the ballast instructions for two lamp use, (i.e. the particular wires that go to the unfilled lamp holders are marked as unused and insulated in the instructions) then there should not be a short or long term problem for ballast or lamps.
Anything else could result in damage to the ballast or lamps or both. Or not.....
And then you have to figure out what would happen if somebody fills the other two lamp slots and decides the fixtures are defective.

I don't recall seeing on these ballasts a wiring diagram for anything other than for 4 lamps. But they are listed/labled for "up to 4 lamps".

The customer wants their existing 4-lamp T12 fixtures retrofitted for T8. But after seeing a new fixture I installed in another room and how much brighter it was, they asked if they could just use 2 lamps instead of 4.

I thought I had read somewhere that it could shorten the lamp or ballast life (can't remember which) if only using 2 lamps if wired for 4.

 

[GoldDigger]

I don't recall seeing on these ballasts a wiring diagram for anything other than for 4 lamps. But they are listed/labled for "up to 4 lamps".

Here is an example of part of the manufacturer's description for one T8 electronic ballast:

GE 74465 GE432MVN-DIY, LFL Multi-Volt ProLine Electronic Multivolt Instant Start Ballast. Application: 4 or 3-F32T8 120 to 277 "N".87 BF,

I cannot tell whether both the 4 and 3 lamp configurations are diagrammed on the ballast itself, and in fact, although a wealth of detail about use of the ballast with either 3 or 4 of a variety of lamps is given online, there is no guide on how to wire for 3 lamps.
And we can assume that the ballast is NOT suitable for use with two only 2 lamps.

From the GE catalog sheet for similar ballast types we see that none are rated for both 2 and 4 lamp use.

Other manufacturers may offer different combinations.
Sylvania's catalog shows some ballasts suitable for both 4, 3 and 2 lamp installation and gives instructions for wiring for fewer lamps (page 25, for example.)

Bottom line: Without knowing more about the exact ballast used, your question cannot be answered.

 

[mengelman]

I don't recall seeing on these ballasts a wiring diagram for anything other than for 4 lamps. But they are listed/labled for "up to 4 lamps".

The customer wants their existing 4-lamp T12 fixtures retrofitted for T8. But after seeing a new fixture I installed in another room and how much brighter it was, they asked if they could just use 2 lamps instead of 4.

I thought I had read somewhere that it could shorten the lamp or ballast life (can't remember which) if only using 2 lamps if wired for 4.

I pretty much only use Advance because that is what my supply store carries. Advance has a tech support number right on the ballast. Maybe this helps

 

[MichaelKelley]

The instructions are different for each models and it varies for them. You need to go through the manuals provided with them. I think if you have already setup, then it should not harm now.

 

[TNBaer]

I used to do this a lot. You're going to be running the 2 lamps at a higher current and ballast factor, essentially making them brighter. This is probably fine with most ballasts, I have never seen one where it was not.